800-C Restoration
- Thread starter audmod01
- Start date
audmod01
Super Member
baconbadge, rufleruf and the group;
OK, I hear the requests for alignment and will try to provide decent pictures and information. For starters, I have several pieces of test equipment that I plan to use: A Heathkit IM-11 Vacuum Tube Voltmeter (VTVM) which has an analog display meter; a Heathkit IG-5257 Post Marker Sweep Generator which has a 10.7mHz crystal controlled oscillator; an HP 5382A Frequency Counter with digital display; a Tektronix 2335 oscilloscope and probes good to 100mHz; plus the Fisher 300 FM MPX signal generator. An analog meter display is preferred because it is visually far easier to see a tuning peak rather than interpreting changing digital display segments.
The Fisher 300 MPX signal generator has not yet been checked out. I did clean it up and deoxed the controls and switches, but power has not been applied until this morning. I am slowly bringing it up with my 500W variac to give the filter capacitors time to reform from sitting for years. I have no way to know when it was last used or turned on. I do not want to damage its power transformer by turning it on without knowing the condition of the electrolytic capacitors in its power supply.
The approach I take to the IF alignment of the 800-C is similar to that which I have used in the past. On any modern FM receiver or tuner that uses a 10.7mHz IF frequency, it is very important that the first IF circuit in the receiver/tuner be tuned to exactly 10.7mHz. That first circuit is the output of the Mixer in any receiver/tuner. All the rest of the IF and Detector alignment depends upon that circuit being tuned correctly. If it is off, all IF subsequent adjustments will not be correct. For that reason I prefer to use my Heathkit IG-5257 Post Marker Sweep Generator's 10.7mHz crystal RF output capacitor coupled to the mixer circuit in the unit under test (UUT). One of the easiest ways to couple the signal into the mixer circuit of a tube type receiver is to use a tube shield or other suitable insulated conductive sleeve to slip down over the outside of the Mixer tube or Mixer/Oscillator tube if a multipurpose single tube performs both function. Between the output of the 10.7mHz RF signal source and the capacitor coupling I use an RF switch selected variable 75 ohm attenuator to reduce the 10.7mHz signal to the lowest level possible that will still provide an IF signal that can be detected at the output of the UUT IF detector. In the case of Fisher UUTs, it is going to be a ratio detector in the majority of cases. The output level of the detector is available at Test Point 3 (TP3) in the Fisher 800-C. You may have to look at an early serial number sequence service manual on the 800-C to find the actual Fisher FM alignment instructions. Some of the later SN sequence service manuals omit the AM & FM alignment instructions.
The general instruction for aligning Fisher and any brand of UUT is to use the minimum RF signal input needed to produce a usable detected signal voltage at the Detector test point. It is important to keep the input RF signal low enough to prevent the Limiter circuits of the receiver from going into limiting action. As each IF circuit from IF amplifer no. 1 through the last which includes the Limiter(s) is adjusted you will need to go back to the RF input signal and reduce the amount of signal getting into the Mixer circuit to prevent the Limiter(s) from going into limiting. The reason for this is that when a Limiter goes into limiting, it produces full signal output to the Ratio Detector which will prevent you from being able to see the true signal peak of the last stages of the IF circuitry. The peak will instead be quite broad and show little variance as you rotate slugs in the IF coils or transformers. Dave recommends that you look at the instructions for aligning a Fisher KM-60 kit type FM tuner to get a good idea of the overall approach to aligning an FM UUT. The more IF stages a UUT has, the more gain there is in the IF strip, making limiter action occur at weaker and weaker RF input signal levels.
In an IF strip with good IF transformers and coils the 10.7mHz IF signal will produce sharp tuning peaks of the measured DC voltage at the output of the Ratio Detector. If you have a good broadband oscilloscope you can also see a similar peak-to-peak signal level visually at each IF amplifier stage. If you are going to use that method, any connection to a particular tube's signal grid or plate will need to be made through at least a 470K ohm 1/2 watt resistor in series with the scope's or meter's signal probe tip. It may also be necessary to add a .01uF disc ceramic capacitor in series with the 470K ohm resistor if you are connecting to the plate of an IF amplifier or limiter tube. The purpose of the resistor is to keep from loading down the tuned circuit that you are trying to peak. If you load down a signal grid circuit or the plate circuit resonant circuit by connecting either a meter or scope probe with no resistor for isolation from the circuit, the tuning response of the circuit will be compromised and finding the real tuning peak for the circuit will be seriously degraded. If you are looking at the plate circuit of an IF tube in order to align IF transformers or coils ahead of the plate of a given stage, only concern your adjustments to the circuits prior to or before the output of the stage you are connected to. The need for a series .01uF capacitor at a plate circuit is to avoid allowing any high voltage DC to cause an arc-over or damaging the input of an oscilloscope. The same is true if you are using a voltmeter at that point instead of an oscilloscope.
It is important to keep in mind that aligning IF circuits in any UUT is a bit of an art. Experience is the best teacher and it does take time to learn how to do a good alignment. There are some other points that are of importance in order to avoid damage and personal injury. Here we assume you are aligning a Fisher UUT which is equipped with its own internal AC power input transformer. I do not advise anyone in this group to work on an AC/DC hot chassis UUT due to possible severe shock hazards that are present in those types of UUT. Such units require the use of an isolation power transformer to avoid shock hazards to yourself and to your test equipment. Oscilloscopes can be particularly susceptible to damage if connected to a hot chassis design without using an isolation transformer. Another issue that often presents itself within older equipment is that tuning slugs in coils and transformers can become frozen in position. Attempts to turn a slug that is frozen in place can result in cracking the slug or tearing loose the fine hair-like wires to the coils inside the transformer or coil housing. Slugs or cores can sometimes be found to replace a cracked slug, but it has to be the right type and size of ferrite material for the frequency range of the IF circuitry. Sometimes frozen slugs can be freed by applying heat, but sometimes not. Some may be freed by applying various solvents, but such chemicals have to be chosen very carefully so as to not use one that causes internal damage to the rest of the components inside a coil or transformer assembly. Many IF circuit coils and transformers have plastic coil forms or other pieces inside that can be damaged by solvents. Always use the correct type of alignment tool. There are both hex-tools and screw-driver tip types. Do not use metal screwdriver tips (they often de-tune circuits). There are specialized plastic hex tools (of several working end sizes) and plastic or fiber-glass screwdriver types for doing alignments in IF and even Multiplex circuits. Make sure you have the correct type of tool to use before you think about doing alignments on any UUT. Using the wrong tool and breaking something in the process can be very expensive to repair when you have to take it to a good technician for repair. In these days, finding a good technician who can work on vacuum tube units is not easy. Replacement IF coils and transformers for these units are not normally going to just be found sitting on a shelf ready to be installed in a unit.
More later. I have to give the Variac treatment of the Fisher 300 MPX generator time to work.
Joe
OK, I hear the requests for alignment and will try to provide decent pictures and information. For starters, I have several pieces of test equipment that I plan to use: A Heathkit IM-11 Vacuum Tube Voltmeter (VTVM) which has an analog display meter; a Heathkit IG-5257 Post Marker Sweep Generator which has a 10.7mHz crystal controlled oscillator; an HP 5382A Frequency Counter with digital display; a Tektronix 2335 oscilloscope and probes good to 100mHz; plus the Fisher 300 FM MPX signal generator. An analog meter display is preferred because it is visually far easier to see a tuning peak rather than interpreting changing digital display segments.
The Fisher 300 MPX signal generator has not yet been checked out. I did clean it up and deoxed the controls and switches, but power has not been applied until this morning. I am slowly bringing it up with my 500W variac to give the filter capacitors time to reform from sitting for years. I have no way to know when it was last used or turned on. I do not want to damage its power transformer by turning it on without knowing the condition of the electrolytic capacitors in its power supply.
The approach I take to the IF alignment of the 800-C is similar to that which I have used in the past. On any modern FM receiver or tuner that uses a 10.7mHz IF frequency, it is very important that the first IF circuit in the receiver/tuner be tuned to exactly 10.7mHz. That first circuit is the output of the Mixer in any receiver/tuner. All the rest of the IF and Detector alignment depends upon that circuit being tuned correctly. If it is off, all IF subsequent adjustments will not be correct. For that reason I prefer to use my Heathkit IG-5257 Post Marker Sweep Generator's 10.7mHz crystal RF output capacitor coupled to the mixer circuit in the unit under test (UUT). One of the easiest ways to couple the signal into the mixer circuit of a tube type receiver is to use a tube shield or other suitable insulated conductive sleeve to slip down over the outside of the Mixer tube or Mixer/Oscillator tube if a multipurpose single tube performs both function. Between the output of the 10.7mHz RF signal source and the capacitor coupling I use an RF switch selected variable 75 ohm attenuator to reduce the 10.7mHz signal to the lowest level possible that will still provide an IF signal that can be detected at the output of the UUT IF detector. In the case of Fisher UUTs, it is going to be a ratio detector in the majority of cases. The output level of the detector is available at Test Point 3 (TP3) in the Fisher 800-C. You may have to look at an early serial number sequence service manual on the 800-C to find the actual Fisher FM alignment instructions. Some of the later SN sequence service manuals omit the AM & FM alignment instructions.
The general instruction for aligning Fisher and any brand of UUT is to use the minimum RF signal input needed to produce a usable detected signal voltage at the Detector test point. It is important to keep the input RF signal low enough to prevent the Limiter circuits of the receiver from going into limiting action. As each IF circuit from IF amplifer no. 1 through the last which includes the Limiter(s) is adjusted you will need to go back to the RF input signal and reduce the amount of signal getting into the Mixer circuit to prevent the Limiter(s) from going into limiting. The reason for this is that when a Limiter goes into limiting, it produces full signal output to the Ratio Detector which will prevent you from being able to see the true signal peak of the last stages of the IF circuitry. The peak will instead be quite broad and show little variance as you rotate slugs in the IF coils or transformers. Dave recommends that you look at the instructions for aligning a Fisher KM-60 kit type FM tuner to get a good idea of the overall approach to aligning an FM UUT. The more IF stages a UUT has, the more gain there is in the IF strip, making limiter action occur at weaker and weaker RF input signal levels.
In an IF strip with good IF transformers and coils the 10.7mHz IF signal will produce sharp tuning peaks of the measured DC voltage at the output of the Ratio Detector. If you have a good broadband oscilloscope you can also see a similar peak-to-peak signal level visually at each IF amplifier stage. If you are going to use that method, any connection to a particular tube's signal grid or plate will need to be made through at least a 470K ohm 1/2 watt resistor in series with the scope's or meter's signal probe tip. It may also be necessary to add a .01uF disc ceramic capacitor in series with the 470K ohm resistor if you are connecting to the plate of an IF amplifier or limiter tube. The purpose of the resistor is to keep from loading down the tuned circuit that you are trying to peak. If you load down a signal grid circuit or the plate circuit resonant circuit by connecting either a meter or scope probe with no resistor for isolation from the circuit, the tuning response of the circuit will be compromised and finding the real tuning peak for the circuit will be seriously degraded. If you are looking at the plate circuit of an IF tube in order to align IF transformers or coils ahead of the plate of a given stage, only concern your adjustments to the circuits prior to or before the output of the stage you are connected to. The need for a series .01uF capacitor at a plate circuit is to avoid allowing any high voltage DC to cause an arc-over or damaging the input of an oscilloscope. The same is true if you are using a voltmeter at that point instead of an oscilloscope.
It is important to keep in mind that aligning IF circuits in any UUT is a bit of an art. Experience is the best teacher and it does take time to learn how to do a good alignment. There are some other points that are of importance in order to avoid damage and personal injury. Here we assume you are aligning a Fisher UUT which is equipped with its own internal AC power input transformer. I do not advise anyone in this group to work on an AC/DC hot chassis UUT due to possible severe shock hazards that are present in those types of UUT. Such units require the use of an isolation power transformer to avoid shock hazards to yourself and to your test equipment. Oscilloscopes can be particularly susceptible to damage if connected to a hot chassis design without using an isolation transformer. Another issue that often presents itself within older equipment is that tuning slugs in coils and transformers can become frozen in position. Attempts to turn a slug that is frozen in place can result in cracking the slug or tearing loose the fine hair-like wires to the coils inside the transformer or coil housing. Slugs or cores can sometimes be found to replace a cracked slug, but it has to be the right type and size of ferrite material for the frequency range of the IF circuitry. Sometimes frozen slugs can be freed by applying heat, but sometimes not. Some may be freed by applying various solvents, but such chemicals have to be chosen very carefully so as to not use one that causes internal damage to the rest of the components inside a coil or transformer assembly. Many IF circuit coils and transformers have plastic coil forms or other pieces inside that can be damaged by solvents. Always use the correct type of alignment tool. There are both hex-tools and screw-driver tip types. Do not use metal screwdriver tips (they often de-tune circuits). There are specialized plastic hex tools (of several working end sizes) and plastic or fiber-glass screwdriver types for doing alignments in IF and even Multiplex circuits. Make sure you have the correct type of tool to use before you think about doing alignments on any UUT. Using the wrong tool and breaking something in the process can be very expensive to repair when you have to take it to a good technician for repair. In these days, finding a good technician who can work on vacuum tube units is not easy. Replacement IF coils and transformers for these units are not normally going to just be found sitting on a shelf ready to be installed in a unit.
More later. I have to give the Variac treatment of the Fisher 300 MPX generator time to work.
Joe
baconbadge
Active Member
A superb explanation Joe! -- If I may however, the only comment I would make at all is one of clarifying: Other than for the description of "us(ing) an RF switch selected variable 75Ω attenuator........", as long as the input signal to that attenuator is 10.7 mHz, and it's output is being injected directly into the IF circuits, then all references to that signal in the remainder of the text would be as an "IF" signal, as opposed to the term RF, that crept into the text at times. For clarity to those following along, the term RF would be reserved as identifying those signals received or injected into the antenna terminals of the unit, which is done when aligning the "front end" circuits of the UUT.
Dave
Dave
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Sorry for the hijack.
Thanks audmod01,
This is just so timely for me. I'm tidying up my refurb of a rat rig Fisher KM-60 tuner and I'm almost at the point where I have to determine if I really need a professional alignment. While I am not likely to do anything more than the touch-up alignment in the assembly manual I still would like to learn about how a real alignment is done. I will never invest in the equipment for a professional alignment and I would still send it out if I felt it really needed it.
Somebody pass the popcorn.
P.S. I have a Fisher 500B in the queue in rat rig condition. So, I won't stop at the KM-60.
Thanks audmod01,
This is just so timely for me. I'm tidying up my refurb of a rat rig Fisher KM-60 tuner and I'm almost at the point where I have to determine if I really need a professional alignment. While I am not likely to do anything more than the touch-up alignment in the assembly manual I still would like to learn about how a real alignment is done. I will never invest in the equipment for a professional alignment and I would still send it out if I felt it really needed it.
Somebody pass the popcorn.
P.S. I have a Fisher 500B in the queue in rat rig condition. So, I won't stop at the KM-60.
audmod01
Super Member
Dave;
Good point about the RF versus IF signals. Both are RF and I tend to use the term RF for most signals above human hearing or above the MPX L-R sideband signal or HD1 digital signal in FM receivers. It does help most people to differentiate the two areas of circuitry.
Tim;
Unless your tuner still has some hidden faults in its IF circuitry, the procedure in the KM-60 manual will allow you to do as good an alignment as most professional technicians. I read through the KM-60 alignment instructions last night and found them to be quite complete.
In the case of the Fisher 800-C, the signal level meter in the unit may be used in the same manner as in the KM-60. The only variation will come at the end of the IF where it is desired to look at the output at the Ratio Detector TP-3 or TP-4. There you will have to use a separate DC meter to measure the voltages there. Note that in the 800-C the signal grid circuit of the last or Limiter stage passes through an isolation resistance R116, a 47K ohm resistor, then to C88 and R118 in parallel to chassis ground. The G1 reference point connects to the signal level meter via G2 through S1 Rear lug 10 as long as you have an FM reception function selected with the front Selector switch. The KM-60 has a built-in switch that allows you to select that last IF signal grid circuit for alignment of most of the IF strip or to the TP3 point for final Ratio Detector alignment.
For everyone contemplating an alignment of a 500-C or 800-C it will be worthwhile to download the manual for the KM-60 and study that before attempting alignment of the 500-C or 800-C. Print out the KM-60 schematic and the schematic sections of the 500-C or 800-C to include the FM Front End and its entire IF Strip and Ratio Detector. Read though the KM-60 alignment procedure. Compare the two schematics of the FM circuitry. You will notice that they are quite similar. There are some minor differences. Both have a 56K ohm resistor bridged across the secondary winding of the Mixer output transformer. At the output (plate circuit) of the 1st IF amplifier, the KM-60 has a 47K ohm resistor, R5, bridged across the primary of T2 while the 500-C and 800-C have a 39K ohm resistor bridged across the equivalent primary. Both the KM-60 and the 500-C or 800-C use a 47K ohm resistor bridged across the primary of the next IF transformer. The next IF stage in a KM-60 uses a 27K ohm resistor, R13, bridged across the L1 coil in the plate circuit. The equivalent circuit in a 500-C or 800-C has a 22K ohm resistor bridged across the equivalent coil in its plate circuit. The purpose of those bridging resistors is to broaden the IF frequency response curve so that it will pass all of the desired bandwidth of signal required for best Multiplex decoder circuitry to work well. Dave has mentioned elsewhere that the older Fisher tuners/receivers produced right after the initial FCC approval of the existing multiplex broadcasting method had IF and Ratio Detector circuitry that did not have as broad an IF response curve to guaranty the best results with many MPX decoders. These bridging resistors help the IF circuits response to be broad enough to deliver good MPX detection after proper alignment is done.
In general an RF coil or transformer winding with no resistor bridged across it has its highest Q factor and narrowest response curve as built. Using high resistance wire or adding a bridging resistor across a winding will reduce the Q of a circuit and broaden the response curve. There are various other design methods that affect Q and IF response curves which you may want to investigate as a point of curiosity if you are interested. There are plenty of sources on the internet which discuss the topic.
While I am using a crystal controlled oscillator to provide a 10.7mHz signal, in most cases you will deal with, the chances are that the output transformer of the Mixer circuit in the FM Front End will not need to be adjusted. You can usually assume that its adjustment is correct. The one caveat is that if someone ahead of you did mess with its adjustment it could be incorrectly tuned. Fisher and other manufacturers of Tuner or Receive kits, always aligned this critical section at the factory and gave instructions to the kit assembler and owner doing their own alignment to leave that adjustment alone. I would suggest that you do likewise unless you do have access to a 10.7mHz signal that is guaranteed to be on frequency to permit accurate alignment of the Mixer output transformer.
I began the alignment of the 800-C by shorting across the FM antenna input terminals with a short piece of wire. If you have any indication of interference from a strong FM station in your area, another way to eliminate the effects of an FM station interfering with your IF alignment effort would be to un-plug the RF amplifier tube. That will usually guarantee that you will not get any interference from a local station. If you want to complete alignment of the UUT later to include the RF circuitry, reinstall the RF amplifier tube and follow the instructions for FM RF alignment in the KM-60 manual. As a general rule RF alignment uses trimmer capacitors to peak the signal response at the high frequency end of the dial and the coil slugs peak the response at the low frequency end of the dial. The KM-60 instructions are quite good on this aspect of alignment. In the case of the AM section of the 800-C, the same approach works well. AM band RF trimmer capacitors peak signal response at the high frequency end of the dial and the coils peak response at the low frequency end of the dial.
Note that the KM-60 has the exact same number of IF amplifying circuits as the 500-C or 800-C, that is four gain stages. The best part of the situation is that if you follow the KM-60 instructions you will most likely achieve the best results your tuner or receiver is capable of. Use of the more elaborate equipment such as what I have is likely to add more complexity to your efforts and not deliver any better results. There exists some very expensive professional alignment equipment that allows alignments that deliver the absolute lowest residual distortion in the detected signal, but most of us are not going to have access to that type of equipment. We are talking about equipment that costs thousands of dollars. From a practical aspect most of us cannot hear the difference.
I am debating with myself about what to show in respect to pictures. My digital camera can take videos, however, it lacks ability to produce a picture with good focus of small items. I doubt I could show the movement pointer of an analog voltmeter. I do believe I can take some pictures of the underside of the chassis and add pointers to the appropriate test points the alignment procedure mentions. I will try to get some of that done after lunch.
Joe
Good point about the RF versus IF signals. Both are RF and I tend to use the term RF for most signals above human hearing or above the MPX L-R sideband signal or HD1 digital signal in FM receivers. It does help most people to differentiate the two areas of circuitry.
Tim;
Unless your tuner still has some hidden faults in its IF circuitry, the procedure in the KM-60 manual will allow you to do as good an alignment as most professional technicians. I read through the KM-60 alignment instructions last night and found them to be quite complete.
In the case of the Fisher 800-C, the signal level meter in the unit may be used in the same manner as in the KM-60. The only variation will come at the end of the IF where it is desired to look at the output at the Ratio Detector TP-3 or TP-4. There you will have to use a separate DC meter to measure the voltages there. Note that in the 800-C the signal grid circuit of the last or Limiter stage passes through an isolation resistance R116, a 47K ohm resistor, then to C88 and R118 in parallel to chassis ground. The G1 reference point connects to the signal level meter via G2 through S1 Rear lug 10 as long as you have an FM reception function selected with the front Selector switch. The KM-60 has a built-in switch that allows you to select that last IF signal grid circuit for alignment of most of the IF strip or to the TP3 point for final Ratio Detector alignment.
For everyone contemplating an alignment of a 500-C or 800-C it will be worthwhile to download the manual for the KM-60 and study that before attempting alignment of the 500-C or 800-C. Print out the KM-60 schematic and the schematic sections of the 500-C or 800-C to include the FM Front End and its entire IF Strip and Ratio Detector. Read though the KM-60 alignment procedure. Compare the two schematics of the FM circuitry. You will notice that they are quite similar. There are some minor differences. Both have a 56K ohm resistor bridged across the secondary winding of the Mixer output transformer. At the output (plate circuit) of the 1st IF amplifier, the KM-60 has a 47K ohm resistor, R5, bridged across the primary of T2 while the 500-C and 800-C have a 39K ohm resistor bridged across the equivalent primary. Both the KM-60 and the 500-C or 800-C use a 47K ohm resistor bridged across the primary of the next IF transformer. The next IF stage in a KM-60 uses a 27K ohm resistor, R13, bridged across the L1 coil in the plate circuit. The equivalent circuit in a 500-C or 800-C has a 22K ohm resistor bridged across the equivalent coil in its plate circuit. The purpose of those bridging resistors is to broaden the IF frequency response curve so that it will pass all of the desired bandwidth of signal required for best Multiplex decoder circuitry to work well. Dave has mentioned elsewhere that the older Fisher tuners/receivers produced right after the initial FCC approval of the existing multiplex broadcasting method had IF and Ratio Detector circuitry that did not have as broad an IF response curve to guaranty the best results with many MPX decoders. These bridging resistors help the IF circuits response to be broad enough to deliver good MPX detection after proper alignment is done.
In general an RF coil or transformer winding with no resistor bridged across it has its highest Q factor and narrowest response curve as built. Using high resistance wire or adding a bridging resistor across a winding will reduce the Q of a circuit and broaden the response curve. There are various other design methods that affect Q and IF response curves which you may want to investigate as a point of curiosity if you are interested. There are plenty of sources on the internet which discuss the topic.
While I am using a crystal controlled oscillator to provide a 10.7mHz signal, in most cases you will deal with, the chances are that the output transformer of the Mixer circuit in the FM Front End will not need to be adjusted. You can usually assume that its adjustment is correct. The one caveat is that if someone ahead of you did mess with its adjustment it could be incorrectly tuned. Fisher and other manufacturers of Tuner or Receive kits, always aligned this critical section at the factory and gave instructions to the kit assembler and owner doing their own alignment to leave that adjustment alone. I would suggest that you do likewise unless you do have access to a 10.7mHz signal that is guaranteed to be on frequency to permit accurate alignment of the Mixer output transformer.
I began the alignment of the 800-C by shorting across the FM antenna input terminals with a short piece of wire. If you have any indication of interference from a strong FM station in your area, another way to eliminate the effects of an FM station interfering with your IF alignment effort would be to un-plug the RF amplifier tube. That will usually guarantee that you will not get any interference from a local station. If you want to complete alignment of the UUT later to include the RF circuitry, reinstall the RF amplifier tube and follow the instructions for FM RF alignment in the KM-60 manual. As a general rule RF alignment uses trimmer capacitors to peak the signal response at the high frequency end of the dial and the coil slugs peak the response at the low frequency end of the dial. The KM-60 instructions are quite good on this aspect of alignment. In the case of the AM section of the 800-C, the same approach works well. AM band RF trimmer capacitors peak signal response at the high frequency end of the dial and the coils peak response at the low frequency end of the dial.
Note that the KM-60 has the exact same number of IF amplifying circuits as the 500-C or 800-C, that is four gain stages. The best part of the situation is that if you follow the KM-60 instructions you will most likely achieve the best results your tuner or receiver is capable of. Use of the more elaborate equipment such as what I have is likely to add more complexity to your efforts and not deliver any better results. There exists some very expensive professional alignment equipment that allows alignments that deliver the absolute lowest residual distortion in the detected signal, but most of us are not going to have access to that type of equipment. We are talking about equipment that costs thousands of dollars. From a practical aspect most of us cannot hear the difference.
I am debating with myself about what to show in respect to pictures. My digital camera can take videos, however, it lacks ability to produce a picture with good focus of small items. I doubt I could show the movement pointer of an analog voltmeter. I do believe I can take some pictures of the underside of the chassis and add pointers to the appropriate test points the alignment procedure mentions. I will try to get some of that done after lunch.
Joe
audmod01
Super Member
I completed the IF alignment of the 800-C. I used a VTVM along with the 10.7mHz crystal oscillator. When I completed that procedure, I disconnected and did a final touch-up of the FM front end at about 89mHz and at about 104mHz using just interstation noise - just adjust for maximum noise or you can look at the output voltage on TP3 at the output of the Ratio Detector. With my VTVM I was seeing about -34VDC. If you use a Volt-Ohmmeter, use a series 270K ohm resistor with the lead connected to TP3. That provides enough isolation that the meter circuitry will not load down the test point and degrade the results. Again, you are looking for greatest negative voltage that adjustment provides.
The MPX decoder in this 800-C works quite well. I used a variable control across R104 the 1.0M ohm resistor in the bias for the 19kHz amplifier circuit in the MPX circuit while selecting a number of weaker but still decent signal stations and adjusted so that the receiver would produce stereo with a small amount of noise which could be filtered out using the FM Stereo Filter position of the function selector knob. The value which satisfied me was about 34.6K ohms, so I had two resistors which in series measured 34.5K ohms and I installed those two across R104 to set the bias where I prefer it to work. Some people might not want to listen to any stereo signal with any degree of noise, but I do. The stereo filter does a good enough reduction of noise such that I can tolerate it. Your own preferences should be your guide on this. I live in a fringe area and for certain stations that have weak signals here, it is either listen with some noise or not listen at all. That is primarily on the college educational stations which operate with low power to begin with and are slightly off the main direction beam of my outside antenna. I keep the antenna pointed straight south to Dallas which has the station I listen to the most.
The service manual does not show the FM or AM Front End adjustments, so I will show a picture of each and identify them for everyone.
I need to verify correct operation of the Fisher 300 MPX signal generator and then I will go through the MPX-65 alignment and show pictures of that.
Joe
The MPX decoder in this 800-C works quite well. I used a variable control across R104 the 1.0M ohm resistor in the bias for the 19kHz amplifier circuit in the MPX circuit while selecting a number of weaker but still decent signal stations and adjusted so that the receiver would produce stereo with a small amount of noise which could be filtered out using the FM Stereo Filter position of the function selector knob. The value which satisfied me was about 34.6K ohms, so I had two resistors which in series measured 34.5K ohms and I installed those two across R104 to set the bias where I prefer it to work. Some people might not want to listen to any stereo signal with any degree of noise, but I do. The stereo filter does a good enough reduction of noise such that I can tolerate it. Your own preferences should be your guide on this. I live in a fringe area and for certain stations that have weak signals here, it is either listen with some noise or not listen at all. That is primarily on the college educational stations which operate with low power to begin with and are slightly off the main direction beam of my outside antenna. I keep the antenna pointed straight south to Dallas which has the station I listen to the most.
The service manual does not show the FM or AM Front End adjustments, so I will show a picture of each and identify them for everyone.
I need to verify correct operation of the Fisher 300 MPX signal generator and then I will go through the MPX-65 alignment and show pictures of that.
Joe
audmod01
Super Member
Do any FM RF adjustments with the bottom shield in place. If you remove the shield you will change the tuning of the circuits.
The AM Front End alignment is best done from the top as that is where access to the tuning capacitor trimmer access is found and the AM Front End coils can be adjusted from the top also.
Joe
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audmod01
Super Member
I have spent the last three days installing a new Pioneer AVH-2440NEX digital radio in my 2007 Dodge Ram pickup. All the wonders of working under the dash and under the truck to implement all the features including a backup camera! It will probably take another three days to recover from all this.
This last picture shows the microphone for phone hands free communication. I now have access to my iPhone navigation app on screen plus backup camera display.
Now I can get back to my Fishers!
Joe
This last picture shows the microphone for phone hands free communication. I now have access to my iPhone navigation app on screen plus backup camera display.
Now I can get back to my Fishers!
Joe
A TESLA?!?!?!?!?!?!?!?!
Get rid of it, it doesn't have any tubes in it! 
Don't count on the Camera except as an ADJUNCT to the Mirrors. Keep yer head on a SWIVEL! And don't forget that 36 pack of DURACELLS "D" Cells!
You can keep yer Tesla and I'll raise you a 1936 AUBURN Roadster. Or a '32 "A" Roadster or 5 window coupe. But I'll keep my '13 Yukon XL for Towing......... And spewing all those noxious fumes that CARB has a Tizzy about. 
Hell my '2014 Honda Mower puts out more crap than my Yukon. And uses about the same amount of gas. 
Get rid of it, it doesn't have any tubes in it! Don't count on the Camera except as an ADJUNCT to the Mirrors. Keep yer head on a SWIVEL! And don't forget that 36 pack of DURACELLS "D" Cells!
You can keep yer Tesla and I'll raise you a 1936 AUBURN Roadster. Or a '32 "A" Roadster or 5 window coupe. But I'll keep my '13 Yukon XL for Towing......... And spewing all those noxious fumes that CARB has a Tizzy about. Hell my '2014 Honda Mower puts out more crap than my Yukon. And uses about the same amount of gas.
audmod01
Super Member
Today after much delay doing other tasks, I finally got around to checking the Fisher 300 MPX Generator. I did a cursory check using its 100mHz RF output and was not seeing consistent results and so decided it was time to go through the MPX generator following its service manual maintenance section. I may have to look back at some of the early parts of the manual too. So far I am finding the typical issues such as switches needing cleaning, followed by minor tweaks on the peak output of the 19kHz Oscillator and the 38kHz doubler. Both of those only took very minor tweaks to reach peak output. I used my oscilloscope on X10 probe setting to view and adjust for peak output from both circuits. The manual says the 38kHz output should be no less than 22VAC. I measured close to 60VAC. When I reached the setting for 2.5VAC output at the rear 1kHz/8kHz output jack, I had trouble with both the 1kHz and the 8Khz level settings. Both of the internal controls that set those voltages had developed bad spots from sitting for no telling how long - likely many years. That happens due to signals being present and from humidity and air contaminants which cause an electrolysis process that cause bad spots under the moving contact where it touches the carbon trace. Some application of contact cleaner and running the control back and forth many times clears the problem most of the time. Stubborn cases sometimes happen that do not respond to treatment. In those cases to avoid replacing a control you can often reverse the end connections (swap the leads at each end of the control so that ground becomes the signal application lug and the other gets connected to ground). Then the control will accomplish its intended purpose, but just operates in reverse rotation. Fortunately both the 1kHz and 8kHz controls responded to treatment. In this particular generator the 8kHz control and circuit was the worst offender. The 8kHz level was wildly high compared to the 1kHz level when switching with the front function control between the two frequencies.
The composite output level control was exhibiting erratic levels on the meter as it was turned, so it also had to be cleaned with contact cleaner. The function switch also received treatment with contact cleaner. Step by step the generator's output signals are getting better.
I had to stop for the day and will resume tomorrow. Earlier activities in the day have me tired.
Joe
The composite output level control was exhibiting erratic levels on the meter as it was turned, so it also had to be cleaned with contact cleaner. The function switch also received treatment with contact cleaner. Step by step the generator's output signals are getting better.
I had to stop for the day and will resume tomorrow. Earlier activities in the day have me tired.
Joe
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audmod01
Super Member
Larry wrote earlier: "Don't count on the Camera except as an ADJUNCT to the Mirrors. Keep yer head on a SWIVEL!"
That is very true, and especially if you are towing a travel trailer or fifth-wheel camper. All you can see in the rear view mirror mounted on the windshield is the front of the camper. Even a backup camera does not help except while hitching up. The left and right outside mirrors are essential for safe towing of any camper and most utility trailers.
I am just glad that I finished the new radio install and everything is working as it should. My daughter had a chance to use the truck with her camper for several days last week and spent time hiking trails and just enjoying nature. She was much more relaxed when she came home.
Joe
That is very true, and especially if you are towing a travel trailer or fifth-wheel camper. All you can see in the rear view mirror mounted on the windshield is the front of the camper. Even a backup camera does not help except while hitching up. The left and right outside mirrors are essential for safe towing of any camper and most utility trailers.
I am just glad that I finished the new radio install and everything is working as it should. My daughter had a chance to use the truck with her camper for several days last week and spent time hiking trails and just enjoying nature. She was much more relaxed when she came home.
Joe
audmod01
Super Member
I began working through the maintenance instructions in the Fisher 300 MPX generator service manual beginning on page 18. I have gone through nearly asll of that section, taking pictures of the oscilloscope screen at each step. I downloaded the images and learned only about 15% of them were any good. I will have to go back through the process again and place the camera on something solid to steady it before snapping a picture. In the meantime the camera battery needs recharging.
So finishing this process is delayed. I am not able to get the full output peak-to-peak levels for both left and right channels on the meter at 1kHz so there is something not right.
Joe
So finishing this process is delayed. I am not able to get the full output peak-to-peak levels for both left and right channels on the meter at 1kHz so there is something not right.
Joe
audmod01
Super Member
OK, after several false starts on checking the Fisher 300 MPX Signal Generator, I finally hit upon a distance to take pictures from of the oscilloscope screen that delivered usable results.
This is the 19kHz signal at the front 19kHz jack. Probe is set to direct and V/Div, is set to 2V, so the output is about 4.5V Peak-to-Peak. The manual says the minimum acceptable is 1.5V.
This is the output of the 38kHz Doubler amplifier. The 300 service manual says the minimum signal level should be 22V. I have the probe set to X10 (for low capacity loading) and the V/Div. set to 20V/Div. The level was measured at the end of R23, a 33K ohm 1/2 watt resistor, same as T2-4, the secondary winding of the transformer. The level is about 64V.
After several tries I was finally satisfied with the degree of Carrier Suppression at the Composite Out jack on the front of the generator. The adjustments are two variable controls, R30 and R31 across the 4-diode matrices on the secondary of the T2 38kHz Doubler. The controls' moving tap finds the setting that most perfectly balances the signals applied across the diode bridges. When both controls are set correctly the sine wave displayed is symmetrical and at minimum peak-to-peak level. This is with the probe set to direct and connected to the Composite Out jack of the front panel. The scope input is set to 5mV/Div. (the most sensitive on the scope) and is somewhat fuzzy, but it looks like I have achieved the stated goal of not more than 4mV. The trace was displayed with some erratic vertical position bounce I was unable to stop, so the picture is not aligned well to get a really good measure of the peak-to-peak voltage. Since the distance between horizontal grids represents 5mV I think I achieved the desired result.
The next step in the generator checks is somewhat ambiguous as to how it is to be measured, so I am waiting on a reply from Dave.
Joe
This is the 19kHz signal at the front 19kHz jack. Probe is set to direct and V/Div, is set to 2V, so the output is about 4.5V Peak-to-Peak. The manual says the minimum acceptable is 1.5V.
This is the output of the 38kHz Doubler amplifier. The 300 service manual says the minimum signal level should be 22V. I have the probe set to X10 (for low capacity loading) and the V/Div. set to 20V/Div. The level was measured at the end of R23, a 33K ohm 1/2 watt resistor, same as T2-4, the secondary winding of the transformer. The level is about 64V.
After several tries I was finally satisfied with the degree of Carrier Suppression at the Composite Out jack on the front of the generator. The adjustments are two variable controls, R30 and R31 across the 4-diode matrices on the secondary of the T2 38kHz Doubler. The controls' moving tap finds the setting that most perfectly balances the signals applied across the diode bridges. When both controls are set correctly the sine wave displayed is symmetrical and at minimum peak-to-peak level. This is with the probe set to direct and connected to the Composite Out jack of the front panel. The scope input is set to 5mV/Div. (the most sensitive on the scope) and is somewhat fuzzy, but it looks like I have achieved the stated goal of not more than 4mV. The trace was displayed with some erratic vertical position bounce I was unable to stop, so the picture is not aligned well to get a really good measure of the peak-to-peak voltage. Since the distance between horizontal grids represents 5mV I think I achieved the desired result.
The next step in the generator checks is somewhat ambiguous as to how it is to be measured, so I am waiting on a reply from Dave.
Joe
audmod01
Super Member
I found the answer to my questions to Dave in the specifications listed on page 3 of the 300 MPX Generator manual. The front banana jacks have a 300 ohm impedance and the rear RCA jack has an impedance of 5K ohms. The audio out at the rear RCA jack is an RMS measurement, so I can use my Fluke DVM there. I will go back and use the correct load across the jacks to make the measurements. The RF output is stated to be 60 ohms, about 1/2 way between 50 and 75 ohms. The Composite Output should be adjustable to the range of 0-6V P-P.
Joe
Joe
audmod01
Super Member
Here are a few more scope pictures of the Fisher 300 being adjusted for proper output levels, linearity and phasing.
This is representative of the 1kHz signal output at the rear RCA out connector of the 300. The actual setting was set to about 7V peak-to-peak with no load other than the scope probe. I first thought that I needed to have a 5K load on the port before setting the level, but after communicating with Dave Gillespie I learned that was not necessary. 7.072V peak to peak is equivalent to about 2.5VRMS. So the actual volts/division setting when the correct peak to peak voltage was achieved was the 2V/Div. setting of the vertical gain control of the scope. I took an actual picture of that with my camera but the picture was so blurry that I had to discard it. In spite of having the camera braced on a chair arm, I still managed to move the camera while taking the picture. I was unable to get a decent RMS measurement with my Fluke 8500A DVM due to the presence of very high-energy RF signals from an FM station transmitter 4.5 miles from our house.
Here is the 7V peak to peak measurement of the 8kHz output at the rear RCA connector as above. Both oscillator outputs need to be set as close as reasonably possible to each other.
Here is a picture of the 20kHz linearity after adjustment.
Here is the linearity at 20Hz. The linearity at this frequency is not as perfect as at 20kHz, but is normal for the generator. Dave pointed out that it is much more difficult to achieve good linearity at the extreme of 20Hz than at higher frequencies. At 50Hz or 60Hz th linearity is significantly better. FM specifications were originally stated for a low end response of 50Hz with early discriminator and ratio detectors and stations did not guarantee good linearity or flat response below 50Hz.
This image shows the 19kHz phase with the 1kHz audio signal from the Left channel.
Here is the 19kHz phase with the 1kHz modulation from the right channel.
I did have to replace the 12AT7 that served as the audio oscillator. The original tube was somewhat weak. A new tube took care of the issue. Other tubes appear to be working OK. The unit still has its original selenium bridge rectifier in the power supply and it works fine. Dave says that his 300 still has its original selenium bridge rectifier too. They were conservatively rated and do not fail often.
I may find a few more pictures to take and post later today.
Joe
This is representative of the 1kHz signal output at the rear RCA out connector of the 300. The actual setting was set to about 7V peak-to-peak with no load other than the scope probe. I first thought that I needed to have a 5K load on the port before setting the level, but after communicating with Dave Gillespie I learned that was not necessary. 7.072V peak to peak is equivalent to about 2.5VRMS. So the actual volts/division setting when the correct peak to peak voltage was achieved was the 2V/Div. setting of the vertical gain control of the scope. I took an actual picture of that with my camera but the picture was so blurry that I had to discard it. In spite of having the camera braced on a chair arm, I still managed to move the camera while taking the picture. I was unable to get a decent RMS measurement with my Fluke 8500A DVM due to the presence of very high-energy RF signals from an FM station transmitter 4.5 miles from our house.
Here is the 7V peak to peak measurement of the 8kHz output at the rear RCA connector as above. Both oscillator outputs need to be set as close as reasonably possible to each other.
Here is a picture of the 20kHz linearity after adjustment.
Here is the linearity at 20Hz. The linearity at this frequency is not as perfect as at 20kHz, but is normal for the generator. Dave pointed out that it is much more difficult to achieve good linearity at the extreme of 20Hz than at higher frequencies. At 50Hz or 60Hz th linearity is significantly better. FM specifications were originally stated for a low end response of 50Hz with early discriminator and ratio detectors and stations did not guarantee good linearity or flat response below 50Hz.
This image shows the 19kHz phase with the 1kHz audio signal from the Left channel.
Here is the 19kHz phase with the 1kHz modulation from the right channel.
I did have to replace the 12AT7 that served as the audio oscillator. The original tube was somewhat weak. A new tube took care of the issue. Other tubes appear to be working OK. The unit still has its original selenium bridge rectifier in the power supply and it works fine. Dave says that his 300 still has its original selenium bridge rectifier too. They were conservatively rated and do not fail often.
I may find a few more pictures to take and post later today.
Joe
Great job Joe!! The 300 is such a neat piece of equipment -- I still marvel that at 55 years old, it still does a superb job of aligning time aligned decoding units.
Dave
Dave
