Peavey Delta Blues repair

[six el six]

Has anyone worked on this type of construction before that can advise me on troubleshooting the tremelo circuit in this amp? The amp works fine/sounds good with the exception of the tremelo which produces no effect whatsoever. As you can see from the attached photo, accessibility is an issue. Testing the circuit via signal tracing, voltage readings, e.t.c. appears to be impossible. Schematic and photo attached.

Also, if anyone has time to give me a quick lesson on what is going on with that tremelo circuit with reference to the schematic that would be swell. I've got the "gist" of how it functions but what is the deal with the LDR designation in the upper right hand corner of the schematic where it looks the switching occurs? Come to think of it, scratch what I said about having the "gist" of the tremelo circuit. I'm clueless. Little help/explanation please?

[sluckey]

Q3 and Q4 are the trem oscillator. They produce a low frequency (2 - 10Hz typical) signal. The frequency is controlled by the speed pot and the amplitude is controlled by the intensity pot. This trem signal at the intensity pot wiper is applied to optoisolator/optocoupler U2. U2 has 2 sections, a LED light emitting diode) and a LDR (light dependant resistor).

The optoisolator works like this... When the LED is not illuminated, the LDR resistance is verrrry high (many megohms). When the LED is illuminated, the LDR resistance will be low.

Since the trem signal is applied to the LED side of U2, the LED will flash on/off at the trem speed and the brightness will be controlled by the intensity pot. The LDR resistance will also change IAW the LED flashing.

The LDR side of U2 is connected between ground and the guitar signal path headed to V1b grid and will shunt more or less of the signal to ground, depending on it's resistance. A high resistance shunts little guitar signal to ground and a low resistance shunts more guitar signal to ground.

Troubleshooting should be fairly easy since Q3 and Q4 are on the top board underneath the intensity pot. Look for a changing voltage (trem signal) on the positive side of C57 with the intensity pot turned max CW. An analog meter or scope would be nice to have. If there's no trem signal here, troubleshoot Q3/Q4. Make sure you have correct power supply voltages feeding this circuit. If you do have trem signal at C57, the problem is likely a bad connection somewhere between C57 and U2 (U2 is on the bottom board) or U2 may be bad.

Hopefully you'll find a loose connector. If not, it's gonna require some patience to dissassemble the boards to replace a component.

[jojokeo]

Hopefully you'll find a loose connector. If not, it's gonna require some patience to dissassemble the boards to replace a component.

Generally speaking, SS components rarely go bad in normal circuit useage since they have built-in reverse voltage protection, heat protection, etc. If it's not a loose connection, 9 times out of 10 it's poor solder joints that are the culprit. Closely inspect all solder joints and touch up as necessary. They're easily seen by little "rings" around the solder pad.

[dpm309]

I recently worked on a Peavey Classic 20 with the same type of folded "U" shaped circuit board.  Many amp techs refer to this board construction as the "Tunnel Of Hell".  The wire jumper connections between the boards can easily come loose etc.  Check these carefully also.  On the Classic 20, it turned out to be a bad Master Volume pot.

Dan

[six el six]

Troubleshooting should be fairly easy since Q3 and Q4 are on the top board underneath the intensity pot. Look for a changing voltage (trem signal) on the positive side of C57 with the intensity pot turned max CW. An analog meter or scope would be nice to have. If there's no trem signal here, troubleshoot Q3/Q4. Make sure you have correct power supply voltages feeding this circuit. If you do have trem signal at C57, the problem is likely a bad connection somewhere between C57 and U2 (U2 is on the bottom board) or U2 may be bad.

Hopefully you'll find a loose connector. If not, it's gonna require some patience to dissassemble the boards to replace a component.

1. Q3 and Q4 Vdc aren't correct. (Q3 -11, -11, -24), (Q4 -.6, -1, -11).

2. There is 0 Vdc on the positive side of C57.

3. -36 Vdc is good at the power supply measured at diode side of R66.

4. I have a good scope. If you ever think I should use it to check something please pardon me in advance for asking novice questions about it's operation.

5. What does this U2/LED/LDR combo thing look like? I know what a "roach"/optocoupler looks like in a Blackface Fender but I don't know what I'm looking for here. I can just barely peer under part of the board and I don't see anything like what I'm familiar with (or anything else that looks like it would be one).

Looks like I'm going to be disassembling this amp and checking component by component. Validation on my findings?

Thanks everyone for the help.





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[sluckey]

1. Q3 and Q4 Vdc aren't correct. (Q3 -11, -11, -24), (Q4 -.6, -1, -11).

They look OK to me.

2. There is 0 Vdc on the positive side of C57.

Back up to the wiper of the intensity pot and look again.

5. What does this U2/LED/LDR combo thing look like?

Hard to say. It's shown as a square with a notched corner on the layout so it may be a DIP chip. Concentrate on the oscillator.

[Jack1962]

I have repair a few of these amps and if it where on my bench I would suspect the LDR , that has been the bad component on most of the Delta amps with trem problems I have worked on.

[billcreller]

I worked one one of those for my son's buddy.  Found some cracks in the PC board.  I don't envy anyone working on that amp....

[tubeswell]

I worked one one of those for my son's buddy.  Found some cracks in the PC board.  I don't envy anyone working on that amp....

Ditto, those PV DB/C30/C50 amps are a real PITA to work on, let alone try to modify. The jumper leads between the boards flex and crack at the slightest provocation

[ThermionicEngine]

Designed to be manufactured, not repaired.  Story of most current production tube amps, excluding the boutiques and high-end no PCB amps.  Wave solder process...  Now I'm seeing stuff with SMD's.  I can't afford the soldering station used to remove or install SMDs.  I actually ordered an op amp from Mouser (BA4560- I ran out.)  It showed up in an SMD package.  My bad for not checking the data sheet.  I can't add anything to the trem repair issue- all of the ones I've dealt with were either power supply issues or reverb. "Tunnel of Hell" -LOL

[LooseChange]

I have repair a few of these amps and if it where on my bench I would suspect the LDR , that has been the bad component on most of the Delta amps with trem problems I have worked on.

Ditto.

[six el six]

1. Q3 and Q4 Vdc aren't correct. (Q3 -11, -11, -24), (Q4 -.6, -1, -11).

They look OK to me.

2. There is 0 Vdc on the positive side of C57.

Back up to the wiper of the intensity pot and look again.

5. What does this U2/LED/LDR combo thing look like?

Hard to say. It's shown as a square with a notched corner on the layout so it may be a DIP chip. Concentrate on the oscillator.

Good to know I can have negative voltage on the transistors. PRR posted a book title I recently purchased on transistor theory. I guess I need to begin digesting it but 1st semester calculus is consuming most of my thinking energy.

My vtvm shows ac trem signal on one leg of the intensity pot (facing controls right). I don't know if that's the wiper or not.

If I understand correctly I'm looking at disassembly in search of a bad connection or bad optoisolator/coupler/LDR/U2 (possible IC)?

I am loathe to pull the board out of this amp. I'm considering bailing on it and informing the owner that he may want to live without onboard trem.

[PRR]

> Good to know I can have negative voltage on the transistors.

You can have negative voltage on tubes. It is un-conventional because it usually leads to complications.

Transistors, unlike tubes, come in two polarities, NPN and PNP. When PNP were cheaper, we did everything positive ground. When NPN got cheaper, negative ground was popular. In between we sometimes had the preamps positive ground and the power stage "upside down" on the same negative power rail.

In this case: they "had to have" some negative DC for the EL84 grid bias. The reverb chip isn't fussy. The trem oscillator can be designed either way. Since PNP and NPN are the same price now (it is all epoxy and handling costs, not the guts), it was simplest to run all the miscellaneous off the same negative supply needed for bias.

--

I think you either refuse the job, or take out ALL the screws and nuts and re-work ALL the solder joints. If one is bad, the batch is bad. Aside from any additional failures as you unfold, there will be more failures down the road which will be blamed on the last guy to touch it. So a serious re-work. Or decline it.

[six el six]

Update for those interested. I hated this repair. It was the optocoupler/resistor VTL 5C3/2. It was necessary to bend the bare wires between the boards (three of them) to gain access to the old unit that failed.

Thanks to everyone.

[worth]

I'm curious... what will you charge the customer for your Peavey Hell repair ?

[plexi50]

Hell repair jobs suc. But it is a learning experience. So for me when i do find a problem that takes a long time to find it's worth the hell you go through to find it. No pain / No gain  

[six el six]

I limit my repair fees in advance i.e. if it takes forever to do the repair because I don't know exactly what to do and I end up making less than minimum wage on it then oh well, at least I learned something.