Morley WVO repair

[Nele]

I have a Morley WVO in for repair. It is working (turning on) but the wah-part isn't working (volume is). Instead of producing a wah-effect there is no top-end when engaged and only the volume decreases when moving the pedal.
First I suspected the LDR but it turns out to be working ok. Then I tested the transistors and they tested ok (standard diode-test).
Voltages coming from the power supply check out as well.

Given the simplicity of the circuit (and pcb) it's very annoying that I can't seem to fix this.
Any suggestions?

 

[Gus]

Check your voltages

Check the transistor voltages in circuit B, C and E. This will help figure out what is wrong.

You have three probably old electrolytic caps(schematic is from 76) in circuit C1, C6 and C11. Check them with a cap meter.

If you do get it to work check all the electrolytic caps.

 

[Nele]

Thanks for your replay. Forgot to mention that I already replaced those caps.

 

[Gus]

Check the switch it could have high resistance at the contacts.

 

[Nele]

Voltages:

Q1 (MPSA12)
E: 7
D: 6,8
C: 27

Q2 (MPSA06):
E: 0,07
B: 0,7
C:29

Q2 was already replaced by a new one.  Switch measures ok.

 

[Khron]

Are C8 and/or C10 ok?

 

[Bo Deadly]

I assume there is some kind of mechanism that allows more / less light reach the LDR?

If yes, then you cannot test it with the cover off. Or maybe cover it with something but the LDR can be very sensitive. You need to really make it dark around it.

 

[Nele]

Are C8 and/or C10 ok?

Yes, all film caps measure ok

I assume there is some kind of mechanism that allows more / less light reach the LDR?

Yes. I've tested by covering the ldr slowly and also by closing the case. Makes no difference.

 

[Khron]

Cracked / cold solder joints somewhere?

An oscilloscope might come in quite handy here, though...

 

[Nele]

Cracked / cold solder joints somewhere?

Nope

An oscilloscope might come in quite handy here, though...

Can I use a regular signal from a signal-generator/laptopoutput for tracing? Or do I need something special to emulate the output of a guitar?

 

[Khron]

Any audio signal will do, although something like a white / pink noise signal would be optimal, so you can see on a spectrum analyzer (as part of RoomEQ Wizard, or a DAW plugin or whatever) how the whole audio range looks. You couldn't see that with just a sinewave.

 

[Nele]

Some updates:

- I've replaced the LDR with a 5 Meg potmeter for testing and the Wah effect is only kind of working when the potmeter is between 0 and 60 ohms

- I've traced the signal with a scope and the output waveform is phase- reversed compared to the input.

- I disconnected the feedback resistor R6 and there is no loss of frequency,  just an overall boost.

- I measured all of the resistors (by disconnecting one end) and they seem fine

- transistors have been replaced

- electrolytics have been replaced

- I can't find any broken traces or cold joints

- wiring is ok.

I'm stumped.. Still not working. It looks like the only solution is rebuilding the circuit on a piece of vero-board and replacing it completely? Am I missing something here?

 

[Bo Deadly]

I don't even understand this circuit. Is the LDR dual? If there's no series resistance with the input the first one will have no effect. I guess the circuit assumes there would be a guitar there. Are you testing with a git or are you just running some noise from the PC in? If the latter you need to add 10K series resistance to simulate a git.

 

[Nele]

I don't even understand this circuit. Is the LDR dual? If there's no series resistance with the input the first one will have no effect. I guess the circuit assumes there would be a guitar there. Are you testing with a git or are you just running some noise from the PC in? If the latter you need to add 10K series resistance to simulate a git.

It's a dual pedal: a volume pedal in mode 1 with LDR-2 and a wah-wah pedal in the other mod with LDR-1. In wah-mode LDR-2 is switched out of circuit.
I tested it with a guitar and with a signal. No difference.

 

[Nele]

Well, finally I got it working again. It turned out to be the LDR. Although the original LDR was working (this threw me off) it's value was way too high.
After building the circuit on a breadboard with new components and substituting a 5M pot for the LDR I got the same results: no Wah effect except near zero-value of the pot. But using a 100K pot got better results.

So in the end I replaced the LDR with the lowest value LDR I could find (R=On 400R, R=Off >1MOhm) and also replaced one of the 2.2nF caps with a 1nF one for a useable Wah effect.