1. tonalvision

    tonalvision Supporting Member

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    Can anyone direct me to the best place to get help fixing my Blues Jr III

    Reverb not working. subbed a 9fb2c1b to see what would happen and I got some reverb.

    Bought a new MOD pan, no good
    Went to billm to read up on it and got some direction.
    I did the test where you put your finger on the jack for recovery and listen for the hum/buzz and could hear it get louder as I turned the reverb pot.
    Test point voltages are off and on pins 4 and 5 of the op amp I have -15v on 4 and 0 (nothing) on pin 5.
    Replaced op amp with a TL074 and no good.
    Now I do not get the hum on the black recovery lead to the reverb tank.
    Amp kind of barks when you hit the switch to off position. (don't know if this might be related to my reverb problem)

    Not sure where to go next. Any help?
     
    Last edited: Dec 7, 2017 at 8:24 PM
  2. 66 Kicks

    66 Kicks Member

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    Pin 4 should have -15Vdc and pin 8 should have +15Vdc.

    How did you install the 14-pin quad op amp in place of the TL072?
     
  3. TimmyP

    TimmyP Member

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    Did you "Ohm out" the new pan to make sure it is good? It should be a 8EB2C1B.

    Most tank failures are broken wires or dirty Molex connectors, and easily fixed.
     
  4. tonalvision

    tonalvision Supporting Member

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    I did ohm out the new pan. It reads within spec.
    Ok I should have said tlO72. It is not a tl074. I installed a tl072
    Billm says that it is directly interchangeable with the 4560 that came out of the amp.
    Also the tank I tried that worked was a long spring type like a type 1 or 4 not an 8. I did work though
    Bill says this about the voltages on the op amp "Check the voltage on pins 4 and 5, which should be -15VDC and +15VDC respectively."

    http://billmaudio.com/wp/?page_id=46

    Thanks for your comments.
     
  5. 66 Kicks

    66 Kicks Member

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    With all due respect for the departed, he was wrong. Pin 5 in the non-inverting input for the second op-amp and it should be at 0Vdc. TL072 and 4560 are interchangeable.

    Do you presently have -15Vdc on pin 4 and +15Vdc on pin 8?
     
    pdf64 likes this.
  6. tonalvision

    tonalvision Supporting Member

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  7. 66 Kicks

    66 Kicks Member

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    I'm not real clear on the return hum test you did. Try removing the return cable from the output jack on the tank and touch the hot pin on the cable while the Reverb Control is up and the Master Volume is up. Another test you could try would be to connect your guitar to the disconnected return cable and see if it plays with Reverb and Master up.
     
  8. tonalvision

    tonalvision Supporting Member

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    I removed the return cable from the output jack and touched the hot pin with reverb up and MV up. you can hear a difference when the pin is touched. The intensity of the sound goes up and down with operation of the reverb control. I connected my guitar output to it and cannot hear the guitar through the amp.
     
  9. 66 Kicks

    66 Kicks Member

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    There should be a loud hum when you touch the hot pin and it doesn't sound like there is. The failure of the guitar to play confirms that the return is not working. The first link in the return chain is the return cable. With the power off, use an ohmmeter to test the resistance from the cable hot pin to pin 5 on the TL072 -- it looks like it should be 10K (if you installed a socket, it would probably be a good idea to remove the TL072). Also test the resistance from the sleeve portion of the return cable plug to ground on the board.
     
  10. tonalvision

    tonalvision Supporting Member

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    I did not socket the chip. I had a Texas instruments TL072 sitting around and was too impatient to wait for as socket to come in the mail. The 4560 most likely is good. anyway...

    There is a loud hum when I touch the hot pin. The sleeve of the return cable checks out. It seems good to me.
    Now comparing my board to schematics I was puzzled for a while. Most schematics I saw had the 10k ohm resistor as R48.
    On my board it shows up as R42 and goes to pin 3 on the TL072 and not 5.
    Check out the second page here:

    http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-Schematic.pdf

    It is apparent to me that the reverb was updated from the other schematics I saw. and page 2 matches my board.

    Upon checking, I have 10k ohms from the pin of the return cable to pin 3 on TL072.

    I was not thorough when puttiing things back together after a previous tear down and forgot to screw ground lead to chassis when testing for guitar signal through return pin of reverb wire. Having corrected that, I tested again for guitar signal and get just the faintest of change in the static coming out of the speaker when I pick the strings of the guitar.
    Nothing that I would call a guitar signal coming through.
    Your help is greatly appreciated.
     
  11. tonalvision

    tonalvision Supporting Member

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    Also tested pin 3 of TL072 for voltage. 0 volts. As far as I can tell it should be +15vdc.
     
  12. pdf64

    pdf64 Member

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    Pin 3 is the opamp's non-inverting input; if that was clamped up at 15V, then the pin 1 output would also be stuck up there, unable to pass signal (cleanly). Given that the circuit is on +/- supplies, about 0V is good for the inverting and non-inverting opamp inputs, and for the output.
     
  13. tonalvision

    tonalvision Supporting Member

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  14. pdf64

    pdf64 Member

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    How about using a signal generator app in your smartphone, whatever adaptors etc to feed it into the reverb recovery cable, and your meter set to Vac to check that the signal is getting in and out of the opamp?
     
  15. 66 Kicks

    66 Kicks Member

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    So now you have a loud hum for the pin test, but no guitar. What if you reconnect the return cable to the tank output and gently stroke the springs?

    Presuming that the tank still doesn't work, we want to apply a signal to the return cable and see what's going on at pin 3 and pin 1 of the op-amp. A guitar will work for this if you have no other means of generating a signal. Set your DMM to AC for this test. The same signal should be at pin 3 as at the guitar. The signal should be around one hundred times larger at pin 1.
     
  16. tonalvision

    tonalvision Supporting Member

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    I can hear the sound of the springs when I stroke the springs, like you hear when you bump the amp when it is on.
    I connected the guitar output to the return pin on reverb cable, no ac voltage that I can read.
    I don't have as much confidence in this meter. My old autoranging meter quit on me and I ran out and got a decent sears meter but you have to adjust the range manually.
    I know the meter should be fine, it is just and apprehension that I have. I am testing on the 200 ac range.
     
  17. tonalvision

    tonalvision Supporting Member

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    Ok enough angst about the meter... I did not mention that I tested at pin 1 and pin 3 of the op amp, 0 voltage on each.
     
  18. 66 Kicks

    66 Kicks Member

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    We seem to be going about this in a very haphazard way. I would ordinarily start by kicking the amp to see if the reverb crashes. If it does, I would then put the return cable plug into the tank input jack and kick the amp again to see if the reverb crashes. If it does, I would then check the signal coming out of the reverb send cable. If there is adequate signal, I would then reconnect the whole reverb and see if it works. If it doesn't, some foul language might erupt.

    Would you say that the reverb crash is what you would expect when you bump the amp when it is on?
     
  19. tonalvision

    tonalvision Supporting Member

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    Placed return pin into input jack and kicked the amp. I can hear the crash!
    There is a signal coming out of the send cable.
    I put it all together and no reverb.
    The crash sounds like I would expect when I bump the amp and it is on.
     

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