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Rough TX-300

Discussion in 'Fisher' started by calman46, Apr 29, 2018.

  1. fred soop

    fred soop Super Member

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    The 100 Ω resistors will dissipate approximately 3.5 watts. However, if the test time is kept short, 2 watt resistors will be ok.

    The capacitor change looks like it is a factory job. The purple wires are the -35 V terminal and it appears that R222 was removed. Note the end of a cut wire. This looks like a change to the bias of Q206 and the added capacitor is power supply decoupling.
     

     

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  2. calman46

    calman46 AK Subscriber Subscriber

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    I increased C212 & C215 from 0.5uf /70v to 1uf /100. The film caps a have are 50v . Thanks Fred
     
  3. calman46

    calman46 AK Subscriber Subscriber

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    I'm ready , 100 ohm resistors in place DBT and varriac ready . I did notice that the diagram for 100 ohm resistors does not match my unit in that how the output transistors are marked on the diagram . ,Q1 & Q5and Q3 &Q7 have the 1 ohm resistor on my TX-300 . Any suggestions before I power up ?
     
  4. fred soop

    fred soop Super Member

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    The test diagram showing the 100 Ω resistors appears to have the transistors numbered differently than the actual schematic. Makes no difference as ALL 4 transistors in each channel get the same 100 Ω between collector and emitter.
     
  5. calman46

    calman46 AK Subscriber Subscriber

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    I'm going with the test diagram and voltages are correct and I will used the correct output transistor numbering for my TX300.
     
  6. calman46

    calman46 AK Subscriber Subscriber

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    OK power on ,passed DBT and no smoke. Voltage is high on SR5 new silicon rectifier. I have -46vdc should be -38vdc . I need to knock it down 8volts . Would 2watt resistor be adequate ? 10ohm to start. When I set the bias (no output transistors) I set it to 0.6v across the pots and check for 0.12v-0.15v across the 1 ohm emitter resistor ? . It appears that the top half of the test diagram has more errors than the bottom half.
     

     

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  7. fred soop

    fred soop Super Member

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    Can't determine the exact value for the dropping resistor because some of the load currents can't be determined from the schematic but it appears that the load is slightly over 170 mA. I'd start with a 39 Ω resistor. You may need to experiment with the value. A 2 watt resistor would be adequate but I prefer to double the power ratings on resistors. A 3 watt would be good.

    The bias setting on the output using the substitution resistors will be approximate. It will need adjustment after the transistors are installed. Have you checked the pots with an analog ohmmeter and determined that the resistance changes smoothly when adjusting? If there is any hiccup and the wiper goes open momentarily, there will be damage.
     
  8. calman46

    calman46 AK Subscriber Subscriber

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    18 ohm got the voltage right. Luckily I had some 18 ohm 5w ceramic resistors on hand . Set the bias next …..
     
  9. fred soop

    fred soop Super Member

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    I misspoke in post #67. The bias can NOT be set, even approximately, using the 100 Ω resistors because there is nothing to set. The voltage across the emitter resistors will simply be the ratio of them to the 100 Ω resistors, just a simple voltage divider.
     
  10. calman46

    calman46 AK Subscriber Subscriber

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    as I understand the test diagram the 100 ohm resistors simulate the output transistors in circuit. So adjust trimpots to 0.12v to 0.15v across the emitter resistor . The 0.6v across the trimpot is a "double check". I may have to tweak the bias once the output transistors are installed . The bias is 0.12v to 0.15v across the emitter resistors with the OP transistors install. I will use the DBT on first power up OP transistor installed . That's my plan ,what do you think ?
     
  11. fred soop

    fred soop Super Member

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    With the 100 Ω resistors, the trimpots will not adjust anything other than the voltage across the trimpot. The path through the transistors is not connected to anything other than power and emitter resistors.The 0.12 to 0.15 V across the emitter resistors is ok. It should be 1/201 of the supply voltage.

    After the transistors are installed, the voltage across the emitter resistors (assuming they are 1 Ω as specified) should be approximately 30 mV and this is the best you can do without an IM distortion analyzer. Then, check for DC at the output and readjust the trimpots slightly for 0 V out. Split the difference and adjust each trimpot slightly in the opposite direction. Again, be SURE that the trimpots have smooth transition when adjusting.
     

     

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  12. calman46

    calman46 AK Subscriber Subscriber

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    I get it now, once you said set bias for 30mV it came to me . The voltages on the test diagram is what should be there with 100 ohm resistors in. I took the voltage measurements from the output transistors sockets and matched them to the test diagram and corrected numbering Q1,Q2 etc. That was confusing me.. I set the bias 29mV for now I want to see it does . Q1 was erratic at first but settled down to 29mV. New trimpots are working nice , on to the DC...…..
     
  13. fred soop

    fred soop Super Member

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    29 mV is close enough. The final adjustment for 0 V output should be a very small adjustment.
     
  14. calman46

    calman46 AK Subscriber Subscriber

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    Well the unit passes a signal but there are problem(s). The left side volume weak the right side is better but has hum until you turn the volume up a little. The right side volume pot and right side balance pot only measure 20k ohms across the pot , worn out? or maybe short in the right side? . I deoxit them again Few quick pics
    20180710_195209.jpg 20180710_195402.jpg
     
  15. fred soop

    fred soop Super Member

    Messages:
    2,028
    I assume you disconnected the pots before measuring. 20K would be a defective pot.

    Disconnect the wiper leads (center terminal) of the volume pot and swap channels at that point. If the problems switch channels, it's ahead of the volume control. If the problems continue on the same channel, it's after the control.

    As for the hum, it could be in the output stage itself. I assume (again) that it does not change with volume level. Disconnect one side of the driver transformer primary. If the hum is still present, it is in the output stage and if it's in both channels, it may be the power supply. Those 50 year old electrolytic capacitors could probably stand to be replaced. The other possibility is, did you rewire anything? Ground is not really ground. Ground wires are to be treated as resistors and if grounds are connected in the wrong order, hum will result.
     
  16. fred soop

    fred soop Super Member

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    Looking at the photos, it appears that you are using the wrong capacitors for main filters. Those small caps, even if they are of sufficient capacitance, will not be able to handle the ripple current at high current draw and there is a lot of series resistance, both inside the capacitors and the leads.

    Ripple current is the charging and discharging current in and out of the capacitor. It must charge during the few milliseconds that the transformer voltage is higher than the capacitor, then discharge to supply power over the next half cycle. Ripple current will be 3-4 times the load current and the capacitor must be large enough physically to dissipate that power.

    Series resistance isolates the capacitor from the line that it is attempting to filter. In the first drawing below (hope the system doesn't mess up the spacing), the capacitor is connected to the line with a length of wire. Some of this resistance is in the actual external wire, some is internal to the capacitor. In the second drawing, the external wire resistance is eliminated, placing the capacitor electrically closer to the line it is filtering. In addition, capacitors intended for main filtering will have the internal resistance minimized. That also reduces heat caused by the ripple current. Yep, the multiple spaces were chopped. Now edited with dots replacing the spaces. Ignore the dots as they are only used to get the lines spaced properly.

    ______o_______......____......______
    ............|..............................|....|
    ............|..............................|....|
    ............|...............................\.../
    ...........= C...........................== C


    Ground wiring is important also. The 2 main filter capacitors should be connected with a heavy wire and the only connections to this wire are the transformer center tap and the ground to the rest of the unit. These connections should be as close together as possible and near the center of the wire.

    1414
    [​IMG]


    Note also that the positive and negative supply leads are treated similarly. They are connected directly as close to the capacitor as possible with a single wire in and a single wire out.

    After the main ground connection, the other grounds must also be connected in the proper order. This is all necessary to prevent ripple current in the ground circuit from getting into small signal stages where it can get amplified. The order is:


    1 - Main filter capacitor(s)

    2 - Auxiliary circuits such as speaker relays, etc. (not present in the TX-300)

    3 - Zoebel networks (also not present)

    4 - Speaker return leads. Separate wires from each speaker terminal to the ground wire.

    5 - Return leads of any secondary filter capacitors.

    6 - Return leads of bypass capacitors, though these may already be tied to signal ground on the circuit board.

    7 - Small signal connection. This is a good place to ground to the chassis which must be at one point only.

    8 - Input selector switch, if that switch grounds unused inputs.

    9 - Separate wires from this point for each channel, to control preamp signal grounds.

    10 - Separate wires to phono preamp.

    11 - Selector switch, wire for each channel to the ground side of the high level input jacks.

    12 - From phono preamp to ground side of low level input jacks.

    Jacks are to be insulated from chassis ground but a small ceramic capacitor should be connected from jack ground to chassis ground as close to the jack as possible, to suppress any RF that gets in the system.
     
    Last edited: Jul 11, 2018

     

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  17. calman46

    calman46 AK Subscriber Subscriber

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    When you say " proper order" for ground do you mean ? Don't used the same ground point for power and signal ? I'm self taught so the terminology and be confusing to me .
    I didn't change any of the power supply wiring from the original config. I used Nichicon VR 85C caps for the main filter caps ,ripple current rating is 2500mA. I'm sure there are better caps but $$ is factor ,especially on this unit. I thought if you used the cap within its rated voltage it was ok.

    Anyway I found one of the speaker terminals bare ground wire was the culprit . I passed a 1K signal through it's in both sides evenly. The low freq, hum not volume control is still in the right. When I play music through its very staticky in both sides . The music is clear just a lot of static. I will go through your check list , reflow some joints . Thanks for your all your help Fred it is appreciated .
     
  18. calman46

    calman46 AK Subscriber Subscriber

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    The original film caps are still in place on the preamp and amp boards .How do those EROMET caps age ? I did lift one when I recapped and its value was dead on . If cleaning and soldering doesn't fit it I'll have to replace them . I'm cleaning the push buttons again .
     
  19. fred soop

    fred soop Super Member

    Messages:
    2,028
    The grounds are all to be connected together, but in the proper order such that high currents are "resolved" ahead of small signal grounds. The order can be thought of as a bus with ground connections made at different points on that bus. They need to be in the correct order. For example, if a ground for a small signal stage is connected closer to the main filters than the speaker returns, the higher current and voltage drop caused by the speakers would "contaminate" the signal ground with a small portion of the speaker voltage and would cause distortion and possibly feedback.

    If your hum is only in one channel, then it would not be the power supply. Might be a bypass capacitor. Easiest thing to try first is to turn on Tape Monitor and connect cables to REC out and TAPE MON in, reversing the channels. If the hum does not change channels, it is after this point. Otherwise, it's ahead of this point. Also, set the MODE switch to MONO and see if the hum now appears in both channels. If so, the problem is ahead of this switch.
     
  20. KentTeffeteller

    KentTeffeteller Gimpus Stereophilus!

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    Also, one of the best electronics instructors and broadcast engineering instructors I ever had used Rectumfrier also.
     
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