Question about Fisher 400 Tone Control bypass ckt. For 500B

[1rebmem]

I'm studying Dave's Tone Control bypass circuit on the 400 modified schematic and it looks to me when the High Filter Switch is in OFF, it inserts a 1.2m resistor and 8pF (?) capacitor between the two amplifier stages.

I don't see the 1.2m and 8pF on the original schematic so do these need to be added in lieu of the PC3 tone circuit when the tone circuit is bypassed?

The reason for my question is I'd like to draw a modified 500B schematic with Tone Control bypass and submit for review on the forum.




Full schematic is in post 128:
http://audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/page-7

Thanks for any help,
Jef

 

[dcgillespie]

The PC5/PC6 packaged component pieces make up the HF filters, and are removed since the high filter function itself is removed. The HF Filter switch is then repurposed as a "Tone Control On/Off" switch, so that when the old HF Filter switch is turned on, the tone controls are engaged, and when off, they are bypassed. Since the PC3/PC4 tone control packages and general design of the tone control stages are basically the same between both receiver models, the new compensation value I determined for C79/C80 will be similar as well. The new bypass components are the 1.2 Meg resistor and 8 pF cap, and will remain the same.

Dave

 

[thornev]

That's an interesting idea. I never use the HF switch and I now wonder what the 500-C would sound like with tone controls bypassed. It's not the same as putting them full counter-clockwise or at 12 o'clock, right? Without tone controls is there an extra "umph" to the overall frequency range? Thorne

 

[1rebmem]

In making the following schematic proposed changes, I had two goals:
1) Convert the "Tape Monitor Input" jack to another Aux input.
2) Delete the high filter PEC (PC5) and convert the "High Filter" switch to a tone circuit (PC3) bypass switch.

Please correct and comment on my work, thank you. Jef

Some notes:
- These changes are on a 500B schematic. I used Dave G's Fisher 400 Improvement thread/schematic for basis.
- Dave electrically relocated his Tape Monitor Switch for amplifier stage phase correction. I don't need to do that I believe?
- The ability to bypass the receiver tone circuit allows for a flat frequency response.
- I previously rewired the Reverb In/Out jacks to Preamp Out/Amp In jacks so you will see the wiring on these jacks deleted.
- As currently drawn, the "Tape Monitor Switch" is completely unused. This is due to the Reverb In/Out jacks repurposed and the "Tape Monitor Input" jacks being converted to Aux inputs. If you have suggestions for better alterations please let me hear them.
- The changes are only shown for the Channel A.
- The gray shaded portions shows where the original schematic has been changed. Text in red shows added/altered text.
- I used "Preview" on my Apple Macbook Pro for this work for those wondering. First time using it, quite the challenge for me.





Fisher 500B Service Manual with schematic:
http://akdatabase.com/AKview/albums/userpics/10004/Fisher 500-B Service.pdf

Daves 400 thread. Post 128 has the 400 modified schematic.
http://audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/page-7

 

[dcgillespie]

Quick comments:

1. While electrically changing the location of the Tape Monitor switch in the circuit does correct the absolute phase issue with it, the primary reason for moving the switch was so that the Tape Monitor input operated with the same sensitivity as the other high level inputs. As designed, the Tape Monitor inputs have a lower sensitivity that the Aux/tape inputs do. Make this move only if it suits your needs.

2. The Tone Control On/Off switch makes for easy switching between a preset tone control setting and an instant flat setting - or -- comparison between a tone control flat setting and forced flat setting. On the pieces I've installed this modification on, I've always been impressed with just how close the tone control flat setting, and the bypassed setting (forced flat) setting sound. Assuming the tone controls and their PCs are in good shape, the original design is actually pretty darn good.

3. Were a (specifically) B unit mine, I think I would leave the Monitor Inputs connected to the Tape Monitor switch -- this so that this input can still act as Loop for external processing units (EQ and the like) that some might like to use - or still work with a three head deck if desired. Then, I'd add a set of new jacks somewhere to act as the new high level inputs for the old Tape Head position.

Just some thoughts to consider.

Dave

 

[thornev]

Thanks, Dave G. #2 answers my earlier question. Thorne

 

[1rebmem]

Dave,
Please bear with my questions while I try and work this out, what will best suit my needs.

the primary reason for moving the switch was so that the Tape Monitor input operated with the same sensitivity as the other high level inputs

Your modified 400 Tape Mon jacks act like an Aux input when you turn the Tape Monitor switch to on........... correct?

this so that this input can still act as Loop for external processing units (EQ and the like) that some might like to use

Couldn't I perform this function with my Preamp-In / Amp-Out jacks (repurposed Reverb jacks)?

Thanks a bunch,
Jef

 

[dcgillespie]

The modified Tape Mon circuit has the sensitivity of the Aux jacks, but is still otherwise a Tape Mon input, allowing the monitoring of a three head deck that is recording any source appearing at the Selector switch that is selected. If you are not using the Tape Mon input for true monitoring use, then yes, the Tape Mon input becomes another high level input like the Aux input is -- but you can never record any source plugged into the Tape Monitor inputs, because that signal never appears at the Tape Output jacks.

You can in fact use the Pre Out/Power In jacks to insert an external effects unit in the signal chain if that unit is capable of a 2.0 volt rms output level.

Dave

 

[1rebmem]

Dave,
I've redrawn my 500B schematic.
Hopefully it will accomplish:
1) Tone bypass using the High Filter switch.
2) Tape Monitor Input jack functioning as a tape monitor AND a an aux input with the same sensitivity as the factory aux input.
3)

Does R26/C17 (and R24 at the switch wafer) remain connected between the Tape Monitor Input jack and selector switch?
I think they need to be jumpered out?
I am still fuzzy on the tape monitoring function/use.

The 500B service manual/schematic link is above in post 4.

I would appreciate your time Dave looking over the circuit before I implement it.
Thank you..... Jef

 

[dcgillespie]

In the best tape decks of the day, they employed three heads that the tape passed over. They were all mounted in line and right next to each other, so that any point on the tape passed all three heads in a fraction of a second at high quality tape speeds (7.5 and 15 IPS). The first head the tape came to was an erase head, so that any previous information on the tape was removed before anything new was recorded on it during the recording process. The second head was the record head that put new information on the tape. The third head was the playback head. In medium quality decks, they only used two heads -- erase and a combined record/play head. But the better decks separated the record and playback functions into two separate heads so that each head could be optimized for its purpose. When two physically separate heads were used for recording and playback, the best external equipment used with such a deck had Tape Monitoring facilities like your 500B does. The purpose of the Tape Monitor facilities in the Fisher, is that when used during the recording process with a three head deck, whatever signal was selected at the selector switch (such as say FM Stereo MPX) appeared at the Recording Output jacks, and then went to the tape deck to be recorded on the tape by the record head. But an instant after a given signal was recorded on the tape, the tape then passed by the playback head, whose output was connected to the Tape Monitor input. In this way, when the Tape Monitor switch was turned off, you were hearing the signal that was being set to the tape deck to be recorded. But when the Tape Monitor switch was turned on, then you were hearing the actual signal recorded on the tape, delayed by the fractional second that it took for the tape to pass from the record head to the playback head. Today, many younger folks new to vintage equipment have no clue what the Tape Monitor input was for, because the whole idea of "tape" recording was before their time. But back in the day, it meant you had the best of the best setup.

In the 500B, with a single input jack serving both the Tape Monitor input function and the Selector switch Tape input function, it is not possible to have the jack serve both of these functions with the same sensitivity as that of the Aux input. This is unique to the B series of receivers and why Fisher had the Tape monitor function appear later in the circuit. This reduced the sensitivity of the Tape Monitor input function, and then a reduction network (R24, R26, C17) was used to connect the same jack to the Tape input setting on the selector switch -- which appeared earlier in the circuit -- but the reduction network made that setting have about the same sensitivity as the Tape Monitor function did. The reduction network was necessary however as when the Tape setting was not selected, the Selector switch shorts out that input. Without the reduction network, then Tape Monitor function would also be shorted out if the Selector switch was turned to anything but Tape -- making the whole idea pointless as then you wouldn't be able to monitor anything.

In the B receivers, it would be best to leave the Tape Monitor switch where it is in the circuit if you plan on using just the one pair of input jacks to serve both Tape and Monitor functions. The best answer would be to add an additional set of input jacks so that separate Tape and Tape Monitor inputs can be had. That would allow the Tape and Tape Monitor input jacks to have the same sensitivity as the Aux input does. However, when the Tape Monitor switch is electrically moved, it should appear on the Selector switch side of R27/R30 -- not after them as you have shown. If you add the additional jacks, then the reduction networks would be removed as they would no longer be needed.

Dave

 

[1rebmem]

Dave...... thanks for taking the time for that great explanation on tape recording and tape monitoring. I am coming up on 60 years of life so I recall little use of actual tape. 8-Track and cassette's yes, tape......no.
This thread, if nothing else, is helping increase my knowledge on not only the 500B but on other Fishers.

Some more questions:
1) You mention in an above post how impressed you have been with the fact that with the tone controls set flat compared to the tone control bypassed, they match pretty well. So I'd like your opinion as to whether I should implement the tone control bypass switch mod. Pro's and con's?
2) What components in the 500B are the permanent low filter? Pro's and con's of eliminating it?
3) From what I see, the 400 and 500C circuitry are closer and your 400 mods would integrate into the 500C easily compared to a 500B True?
I ask this for 500C owners (like Thorne) who are interested in the 400 to 500C mod's.

My 500B currently has the Tape Head input converted to an Aux input for a total of two. I will probably convert the Tape Monitor Input to an Aux by soldering a jumper across R26/C17 and lifting one end of R24. Minimal and reversible. Then when the selector switch in placed in Tape position, the Tape Mon jacks will serve as an Aux input. This will give me 3 Aux inputs.
I will draw modified schematics of any mods I implement based on this thread and post here so other 500B users can use them as desired.

Thanks again Dave,
Jef

 

[dcgillespie]

1. The pros are that you can set the unit for flat performance in an instant, and by measurement, the circuit is in fact quite flat compared to when the tone controls are in the circuit. The cons are that there is hardly a huge audible difference so it doesn't transform your Fisher into something it never was.

2. C27 and C30. Short them out to remove the filter. This should never be done in a 500C or 800C, but can be in the 500B and 800B.

3. All of the mods can be implemented on all of the models -- but some will require more work in some areas than others, like the B series having just one pair of input jacks to serve two separate input functions, where as the later models use two pair of jacks. Therefore, extra jacks would need to be installed to do the Tape Monitor modification correctly in the B models.

The one exception regards implementing the phase inverter work with the 500C. With the modified power amplifier for the 400 late series receivers, the noose was removed, and the B+ supply for the AF Amplifier stage was moved to the next lower B+ supply point. In the 400 and even the 500B/800B, this measure produces improved performance. But the procedure sort of morphed all on its own into a standard mod to do for all receivers and amplifiers that included a noose -- but the modification was never published for anything but the 400. In fact, in the 500C such a move will often create problems more than not, because Fisher chose to bias the AF Amplifier stage much differently than it did in the 400 and B series receivers. But while leaving the noose in in C receivers is much better than removing it and reducing B+ to the previous stage, leaving the noose in presents its own problems when foreign manufacture output and driver tubes are used. I've got new information ready to go for the C receivers, but just have not had time to publish it yet. I hope to get to it soon.

Your plan for the Tape Monitor jacks will work fine. It's not ideal, but IMO makes for better use of the jacks in today's audio environment.

Dave

 

[thornev]

Jef - I did remove the noose in my 500-C. The problem created was that I couldn't get the "ohmage" on pins 1 and 3 of V12 and V13 to match (the procedure for each pot is to adjust so that the pin 1 value matches the static value at pin 3). In order to generate more "headroom" so that I could make the ohms match, I had to replace the 68K resistors (R114 and R116) with 45K values. Then I was able to get the ohm values to match on pins 1 and 3.

As far as if it made a difference, if I remember correctly, the benefit was to reduce the amount of distortion and increase the power at high volumes. I don't know what "high volume" means. I never have the volume control past about 11 o'clock.

Thorne

 

[1rebmem]

Dave...... Again, thanks for your time in providing information and answers in this thread!
Very helpful to me and other 500B users no doubt. I intend to add to this thread some more marked up schematics.

Jef

 

[1rebmem]

I did remove the noose in my 500-C. The problem created was that I couldn't get the "ohmage" on pins 1 and 3 of V12 and V13 to match (the procedure for each pot is to adjust so that the pin 1 value matches the static value at pin 3). In order to generate more "headroom" so that I could make the ohms match, I had to replace the 68K resistors (R114 and R116) with 45K values. Then I was able to get the ohm values to match on pins 1 and 3.

Thorne,
Dave will be publishing more on the Noose modification soon as he stated and I think you are going to read about additional testing he has been doing on the noose. It might lead you to reinstall the noose on your 500C, of course I can't say for sure.
I also had to change resistor values on my 500B, R89 and R92 from 33K to 56K, in order to implement the noose mod.
Jef

 

[chuck802]

1. The pros are that you can set the unit for flat performance in an instant, and by measurement, the circuit is in fact quite flat compared to when the tone controls are in the circuit. The cons are that there is hardly a huge audible difference so it doesn't transform your Fisher into something it never was.

2. C27 and C30. Short them out to remove the filter. This should never be done in a 500C or 800C, but can be in the 500B and 800B.

3. All of the mods can be implemented on all of the models -- but some will require more work in some areas than others, like the B series having just one pair of input jacks to serve two separate input functions, where as the later models use two pair of jacks. Therefore, extra jacks would need to be installed to do the Tape Monitor modification correctly in the B models.

The one exception regards implementing the phase inverter work with the 500C. With the modified power amplifier for the 400 late series receivers, the noose was removed, and the B+ supply for the AF Amplifier stage was moved to the next lower B+ supply point. In the 400 and even the 500B/800B, this measure produces improved performance. But the procedure sort of morphed all on its own into a standard mod to do for all receivers and amplifiers that included a noose -- but the modification was never published for anything but the 400. In fact, in the 500C such a move will often create problems more than not, because Fisher chose to bias the AF Amplifier stage much differently than it did in the 400 and B series receivers. But while leaving the noose in in C receivers is much better than removing it and reducing B+ to the previous stage, leaving the noose in presents its own problems when foreign manufacture output and driver tubes are used. I've got new information ready to go for the C receivers, but just have not had time to publish it yet. I hope to get to it soon.

Your plan for the Tape Monitor jacks will work fine. It's not ideal, but IMO makes for better use of the jacks in today's audio environment.

Dave

Hi Dave and everyone,
I am working on my 500B which already has a lot of work done - I am interested in point 2 above (2. C27 and C30. Short them out to remove the filter. This should never be done in a 500C or 800C, but can be in the 500B and 800B). A few questions:
Question (1): would replacing C27 and C30 with a much larger value do much of the same thing (like replace .001uf with 01uf) and would it preserve anything of value rather than shorting them, and
Question (2): I may have missed this in a prior post of this or other Fisher receiver but what does eliminating this permanent low filter accomplish - or put another way what was that low filter accomplishing? Was it some kind of permanent rumble filter?
Question (3): Why should we not do this to a 500C? I believe you I just want to understand it.
Question (4): Has anyone replaced any of the rca jacks with something of better quality (gold or whatever)? I am interested in others' take on this topic. I realize it detracts the unit from original but if it's of value who woudn't want it? I've been thinking of replacing a few of the most used jack pairs - have not figured it out yet, or even if it's do-able without making a new panel - I will probably would not go to that length.
Interested in others' take on these - and thanks so much to Dave, Jef, and all, these are great group discussions, very insightful. -Chuck

 

[dcgillespie]

Hi Chuck -- Great questions! I'll answer them in the order presented:

1. Using larger caps would accomplish nearly the same thing as shorting them out, but since doing so gains nothing, there is no advantage to changing the cap to a larger value. Shorting the cap out looses nothing, and removes the filter completely.

2. The filter is basically a garbage filter, helping to prevent rumble, wow and flutter, and other LF noises from the mechanical sources of the day from needlessly wasting power on non-musical signals. In doing so, it prevents output stages from becoming overloaded, and also prevented the resulting IM distortion that such LF garbage might create. It also acts as an additional rumble filter of sorts -- this since so many of Fisher's pieces ended up in a console setting -- either by Fisher themselves, or by individual owners after purchase. In that setting, it helps to prevent LF feedback from the speakers to the turntable. Today however, while there are still many console junkies around (a term of affection!), most who still play vinyl typically have far superior TTs than the changers that were so often used with Fisher equipment back in the day, which greatly minimize the garbage noise from the decks and changers of old. Plus, modern sources simply don't have the garbage noise issues associated with them, so the filter is an anachronism today.

3. The reason that the filter should not be removed in a 500C or 800C receiver, is because in those units, Fisher employed their electronic Stereo/Mono switching in the FM section, which changed state based on whether an FM Stereo MPX station was being received or not. Earlier units either did not have the feature, or early units that did, employed a mechanical relay to perform the Stereo/Mono switching. It was found however that the relays were not real dependable in switching audio signals, because the micro arcing that normally occurs between the contacts in a properly sized relay doesn't happen in that setting. The result was that channel dropout happened all too frequently due to contacts that became compromised. Fisher's answer was to develop a diode matrix electronic switching circuit that is utterly dependable in operation. The only drawback to the circuit is that when the diodes change state, there is a healthy pulse associated with it that overloads the line amp input stage if the filter is removed. So, in those units, the filter must stay in place to prevent momentary overload as the matrix circuit does it's thing.

4. If you do pursue replacing the jacks, just remember that some of them are mounted so as to be isolated from the chassis -- this so that the jacks can be grounded at the point of lowest noise for the circuit served.

I hope this helps!

Dave

 

[chuck802]

Hi Chuck -- Great questions! I'll answer them in the order presented:

1. Using larger caps would accomplish nearly the same thing as shorting them out, but since doing so gains nothing, there is no advantage to changing the cap to a larger value. Shorting the cap out looses nothing, and removes the filter completely.

2. The filter is basically a garbage filter, helping to prevent rumble, wow and flutter, and other LF noises from the mechanical sources of the day from needlessly wasting power on non-musical signals. In doing so, it prevents output stages from becoming overloaded, and also prevented the resulting IM distortion that such LF garbage might create. It also acts as an additional rumble filter of sorts -- this since so many of Fisher's pieces ended up in a console setting -- either by Fisher themselves, or by individual owners after purchase. In that setting, it helps to prevent LF feedback from the speakers to the turntable. Today however, while there are still many console junkies around (a term of affection!), most who still play vinyl typically have far superior TTs than the changers that were so often used with Fisher equipment back in the day, which greatly minimize the garbage noise from the decks and changers of old. Plus, modern sources simply don't have the garbage noise issues associated with them, so the filter is an anachronism today.

3. The reason that the filter should not be removed in a 500C or 800C receiver, is because in those units, Fisher employed their electronic Stereo/Mono switching in the FM section, which changed state based on whether an FM Stereo MPX station was being received or not. Earlier units either did not have the feature, or early units that did, employed a mechanical relay to perform the Stereo/Mono switching. It was found however that the relays were not real dependable in switching audio signals, because the micro arcing that normally occurs between the contacts in a properly sized relay doesn't happen in that setting. The result was that channel dropout happened all too frequently due to contacts that became compromised. Fisher's answer was to develop a diode matrix electronic switching circuit that is utterly dependable in operation. The only drawback to the circuit is that when the diod

Hi Chuck -- Great questions! I'll answer them in the order presented:

1. Using larger caps would accomplish nearly the same thing as shorting them out, but since doing so gains nothing, there is no advantage to changing the cap to a larger value. Shorting the cap out looses nothing, and removes the filter completely.

2. The filter is basically a garbage filter, helping to prevent rumble, wow and flutter, and other LF noises from the mechanical sources of the day from needlessly wasting power on non-musical signals. In doing so, it prevents output stages from becoming overloaded, and also prevented the resulting IM distortion that such LF garbage might create. It also acts as an additional rumble filter of sorts -- this since so many of Fisher's pieces ended up in a console setting -- either by Fisher themselves, or by individual owners after purchase. In that setting, it helps to prevent LF feedback from the speakers to the turntable. Today however, while there are still many console junkies around (a term of affection!), most who still play vinyl typically have far superior TTs than the changers that were so often used with Fisher equipment back in the day, which greatly minimize the garbage noise from the decks and changers of old. Plus, modern sources simply don't have the garbage noise issues associated with them, so the filter is an anachronism today.

3. The reason that the filter should not be removed in a 500C or 800C receiver, is because in those units, Fisher employed their electronic Stereo/Mono switching in the FM section, which changed state based on whether an FM Stereo MPX station was being received or not. Earlier units either did not have the feature, or early units that did, employed a mechanical relay to perform the Stereo/Mono switching. It was found however that the relays were not real dependable in switching audio signals, because the micro arcing that normally occurs between the contacts in a properly sized relay doesn't happen in that setting. The result was that channel dropout happened all too frequently due to contacts that became compromised. Fisher's answer was to develop a diode matrix electronic switching circuit that is utterly dependable in operation. The only drawback to the circuit is that when the diodes change state, there is a healthy pulse associated with it that overloads the line amp input stage if the filter is removed. So, in those units, the filter must stay in place to prevent momentary overload as the matrix circuit does it's thing.

4. If you do pursue replacing the jacks, just remember that some of them are mounted so as to be isolated from the chassis -- this so that the jacks can be grounded at the point of lowest noise for the circuit served.

I hope this helps!

Dave

Thinking out loud - for the 500C/800C maybe a larger cap value would be a good compromise providing some benefit - since you cannot short the cap.
es change state, there is a healthy pulse associated with it that overloads the line amp input stage if the filter is removed. So, in those units, the filter must stay in place to prevent momentary overload as the matrix circuit does it's thing.

4. If you do pursue replacing the jacks, just remember that some of them are mounted so as to be isolated from the chassis -- this so that the jacks can be grounded at the point of lowest noise for the circuit served.

I hope this helps!

Dave

VERY helpful Dave, thank you! And now I understand about the 500C/800C.

 

[chuck802]

Dave,
I've redrawn my 500B schematic.
Hopefully it will accomplish:
1) Tone bypass using the High Filter switch.
2) Tape Monitor Input jack functioning as a tape monitor AND a an aux input with the same sensitivity as the factory aux input.
3)

Does R26/C17 (and R24 at the switch wafer) remain connected between the Tape Monitor Input jack and selector switch?
I think they need to be jumpered out?
I am still fuzzy on the tape monitoring function/use.

The 500B service manual/schematic link is above in post 4.

I would appreciate your time Dave looking over the circuit before I implement it.
Thank you..... Jef

View attachment 1338866

Hi Jef and Dave, can you help me out with something - I'm trying to implement Dave's tone bypass using your (Jef's) nicely drawn schematic(s). I have a question on the schematic dated Nov 24. Is that the final? Because it looks like R56 connects direct to the plate of V9, pin 6 (left side of C44). Shouldn't it instead connect to the right side of C44? I am probably missing a later version or misunderstanding something but wanted to check first, hopefully you can help me with this. Thanks very much for the work you put into these schematics, and to Dave for the tone bypass. -Chuck

 

[dcgillespie]

Chuck -- It looks like the original schematic is blotted out in such a way as to make it look like R56 connects directly to the plate terminal, but of course as you point out, this is not correct. Were the blotted out section visible, you would surely see R56 go on across the top of C44 so as to be able to connect to the other side of it, versus how it appears to be shown.

To further "amplify" this point (pun intended), look at the other section of this tube. If the blotting were applied to the schematic of that tube section the same way it is applied to the section in question, then it would look like R37 is directly connected to the plate terminal pin 1 of that tube section as well -- but of course, it's not either. It surely would have helped if Fisher had used a horseshoe symbol to show when one lead crosses over another on the schematic to clearly identify that there is no connection between the two leads. Without such an aid, you are left to your own understanding of circuit design to determine when wires cross with a connection between them, and when there is no connection.

I hope this helps!

Dave