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Thread: Universal Guitar Amp with Vibrator and Reverberator

  1. #36
    Old Timer J M Fahey's Avatar
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    Ok, the main point is that they work

    You were very lucky

    Now you basically need to clean (and eventually replace) "mechanicals" , such as switches, jacks, pots, and connectors, but that stuff is visible/understood without schematics.

    As of reverb and vibrato, move their controls and check, also might need some footswitch to enabvle them ... is there a footswitch jack anywhere?

    Again, best News is that main ampbasically works.

    GTest it both with generator and a real Guitar.

    Curious about power supply rail voltages .

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Ok, the main point is that they work


    Curious about power supply rail voltages .
    I'll check voltages tonight. The footswitch jack is in pieces. I have a new one to install. The reverb cable grounds have pulled from the board. I'll reattach them and connect the reverb pan. I'll re-clean the pots and jacks and see if any of the crackly noises go away.

    It does seem I may be out of the worst of the woods. Thanks for all your support and advice Juan!

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    Back at it. Had my son play his guitar through the amp and I thought it sounds pretty good, but he's not convinced. Still some low level noise that needs to be resolved.

    I reattached the reverberator cables to the board and attached the reverberator tank. Reverberator still not working. I have signal to the tank, but not out of it.

    I didn't install the new footswitch jack to maintain access to the underside of the board until I get a handle on creating a schematic. I tried different combinations connecting the wires to the jack to see if I could get it to work. I did see some variation in signal when I change the speed of the vibrator, but nothing from the intensity pot.

    I measured voltages off the transformer and got 21.6vac on each leg of the secondary and 43.9vac across the secondaries. I measured 28.6vdc at first filter cap and -29vdc at second filter cap. I had 23.2 and -24.8vdc at the other two filter caps.

    Attached is a pic of the reverberation tank.

    Click image for larger version. 

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  4. #39
    Old Timer J M Fahey's Avatar
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    Thanks.
    I expected around +/-25V rails since 40251 is rated 50V.

    Transistors typically stand 5, sometimes even 10 extra volts, but not beyond.

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    Juan Manuel Fahey

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    Getting the amp to power up and pass signal appears to be the easy part. The amp is noisy. Noise is unaffected by volume. Still no reverberator or vibrator. Reverb tank seems bad so I'll test with another.

    I drafted a layout of the pcb. It shows component values and measured resistor values. Next I'll shine a light from under the board and try to draw a schematic.

    Click image for larger version. 

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    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by tigerzilly; 02-22-2019 at 01:39 PM.

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    I'm having trouble staying focused with this one. I started the schematic, but have a long way to go. So I started desoldering pots to test and clean. The resistive paint on a number of them has flaked off in spots. One pot labled 100k only measured 10k. Looks like I need to find replacements.Click image for larger version. 

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  7. #42
    Senior Member trobbins's Avatar
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    Were you able to check how close your speaker line is to 0Vdc - that will be a simple indicator that your output stage is biasing up ok.

    Was a signal on your scope looking fairly clean for mid-tone controls, and could you up the volume (perhaps with a resistor load instead of speaker) to show symmetric clipping?

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    Quote Originally Posted by trobbins View Post
    Were you able to check how close your speaker line is to 0Vdc - that will be a simple indicator that your output stage is biasing up ok.

    Was a signal on your scope looking fairly clean for mid-tone controls, and could you up the volume (perhaps with a resistor load instead of speaker) to show symmetric clipping?
    When I get the pots back in, I'll power back up and check!

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  9. #44
    Supporting Member The Dude's Avatar
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    Quote Originally Posted by tigerzilly View Post
    ......One pot labled 100k only measured 10k......
    Are you sure? I've not seen resistors or pots lower in value.

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    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Old Timer J M Fahey's Avatar
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    Now I can believe itīs actually 10k, you measured 10k, and maybe you misread it as 100k.

    Suggest you do not pull parts ar random, and even less having no schematic.

    An accident waiting bto happen.

    Absolute worst case, leave it aside for now.

    Better unrepaired but original than *destroyed*.

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    Juan Manuel Fahey

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    Quote Originally Posted by The Dude View Post
    Are you sure? I've not seen resistors or pots lower in value.
    Yes Im sure. I measured multiple times. The pot says 100k but measures 10k. I was thinking perhaps the pot was improperly assembled when manufactured.

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    Quote Originally Posted by trobbins View Post
    Were you able to check how close your speaker line is to 0Vdc - that will be a simple indicator that your output stage is biasing up ok.

    Was a signal on your scope looking fairly clean for mid-tone controls, and could you up the volume (perhaps with a resistor load instead of speaker) to show symmetric clipping?
    Well I replaced four pots and re-powered. I was getting .18 volts at the speaker output. Sinewave looked good, but spiked with max treble....

    ...then my speaker output wires shorted. I smelled burning near the reverb pot. Now I have 29 volts dc at output and no sinewave. Ugh!

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    Quote Originally Posted by J M Fahey View Post
    Now I can believe itīs actually 10k, you measured 10k, and maybe you misread it as 100k.

    Suggest you do not pull parts ar random, and even less having no schematic.

    An accident waiting bto happen.

    Absolute worst case, leave it aside for now.

    Better unrepaired but original than *destroyed*.
    It definitely reads 100k and measures 10k.

    Too late! My speaker output wires shorted while testing. Something fried!

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  14. #49
    Lifetime Member Enzo's Avatar
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    Are you measuring the pot disconnected from the circuit? Or are you measuring it while it is soldered in place? While connected, there will be parallel current paths.

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    Quote Originally Posted by Enzo View Post
    Are you measuring the pot disconnected from the circuit? Or are you measuring it while it is soldered in place? While connected, there will be parallel current paths.
    Hey Enzo. You're my hero! I started with a couple Peavey amps and reading threads with your recommendations have been instrumental!

    I measured the pot both in and out of circuit. Its currently out of circuit now. See pic. I have a 100k replacement pot in circuit now. Its the reverb pot.
    Click image for larger version. 

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  16. #51
    Old Timer J M Fahey's Avatar
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    When you do complicated things angry/extremely_tired/in_a_hurry (or all together) shit happens, thatīs why I suggested leaving it aside, at least for some time.

    Too late

    Pity is that it uses a huge "everything in a single PCB"; IF it had a separate power amp you might junk just that and mount a fresh LM3886 amplifier board, which matches that power supply (+/-25V) but now ...

    Oh well.

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    Quote Originally Posted by J M Fahey View Post
    When you do complicated things angry/extremely_tired/in_a_hurry (or all together) shit happens, thatīs why I suggested leaving it aside, at least for some time.

    Too late

    Pity is that it uses a huge "everything in a single PCB"; IF it had a separate power amp you might junk just that and mount a fresh LM3886 amplifier board, which matches that power supply (+/-25V) but now ...

    Oh well.
    Well I wasn't angry, tired, or in a hurry...just careless.

    So you're saying everything is fried? Is it time to trash it?

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    Last edited by tigerzilly; 03-20-2019 at 04:46 PM.

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    Quote Originally Posted by tigerzilly View Post

    So you're saying everything is fried? Is it time to trash it?
    You won't know until you assess how much damage has been done. one step forward, three steps back .

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  19. #54
    Old Timer J M Fahey's Avatar
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    No, I say **the power amp** is fried and given the lack of schematic plus your lack of experience combined that **for now** you canīt repair it.

    Leave it aside, maybe on your free time you properly draw then full schematic , no matter how long it takes .

    Then in 1/3/6/12 months you post it here and we go on.

    Just donīt hurry or get frustrated, that amp is nice and repairable, but we need to go through some intermediate steps.

    What I said is we might have saved some time if power amp were on a separate board, because then full replacement was possible; if not then we have no shortcut and must walk the full path.

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    Juan Manuel Fahey

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    I wired the vintage Jensen speakers for 8 ohms and installed a 1/4" jack. It makes an awesome sounding 2x12 speaker cabinet!

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    Last edited by tigerzilly; 06-20-2019 at 02:24 AM.

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