Dunlop MC404 CAE Wah Pedal - No Power Help

[misterc57]

Use Ohmmeter to measure resistance (no battery, no power supply, phone plug inserted)

- from battery clip minus to TL032 pin 4,
- from battery clip plus to TL032 pin 8,
- from battery clip plus to clip minus,
- from TL032 pin 4 to pin 8.

- from battery clip minus to TL032 pin 4, 0.7 ohm
- from battery clip plus to TL032 pin 8, open
- from battery clip plus to clip minus, open
- from TL032 pin 4 to pin 8. 30K to 70K and higher, keeps rising slowly

[Helmholtz]

- from battery clip plus to TL032 pin 8, open

Is this with red meter lead to pin 8? Please measure again with red lead to battery plus and black lead to TL032 pin 8.
Also measure again between battery clip contacts with reversed meter leads.

It seems that the diode is in series with the positive supply, so meter polarity matters.

[misterc57]

Is this with red meter lead to pin 8? Please measure again with red lead to battery plus and black lead to TL032 pin 8.
Also measure again between battery clip contacts with reversed meter leads.

It seems that the diode is in series with the positive supply, so meter polarity matters.

5M resistance

[Helmholtz]

5M resistance

Referring to which of the 2 tests?

[misterc57]

Referring to which of the 2 tests?

Sorry.

5M to pin 8
open on battery clip contacts, either orientation of probes.

[Helmholtz]

5M to pin 8
open on battery clip contacts, either orientation of probes.

Ok, so if this was all done with a mono jack plug inserted, you have an interruption in the positive supply path.

You said you saw a 0.3V drop "at" the diode. What does "at" mean as the diode has 2 ends? A voltage drop would be measured from end to end of the diode.

Also not clear what actual voltage you saw on TL032 pin 8 wrt minus/ground.

[misterc57]

Ok, so if this was all done with a mono jack plug inserted, you have an interruption in the positive supply path.

You said you saw a 0.3V drop "at" the diode. What does "at" mean as the diode has 2 ends? A voltage drop would be measured from end to end of the diode.

Also not clear what actual voltage you saw on TL032 pin 8 wrt minus/ground.

with DC power supply plugged in, and jack insert to instrument input...

seeing 0.38 V drop across D4 (measured at both legs of D4)

with DC power supply plugged in, and jack insert to instrument input, black probe at board ground screw hole...

seeing 0.7 mV at pin 8

seeing 9.4 V at DC plug ring pin

there is a Q3 next to D4, with continuity to the DC plug ring pin. I see 9.4 V at one leg, but only seeing 11 mV and .5 V at the other legs.

[Helmholtz]

seeing 0.38 V drop across D4 (measured at both legs of D4)

I understand that one of D4 pins is 0.38V more positive than the other one when measured wrt negative/ground.
But what is the absolute voltage at either pin wrt ground?
Is one of the pins grounded?

there is a Q3 next to D4, with continuity to the DC plug ring pin. I see 9.4 V at one leg, but only seeing 11 mV and .5 V at the other legs.

Identify transistor leads.

You really need to trace out the power in wiring. An Ohmmeter is useful here. Also a flashlight sometimes helps.

Of course it could simply be a broken solder joint or pcb trace around the input jack or the power supply socket.

[misterc57]

D4 anode is 1.5M wrt ground. Measure 0.53 V with PS
D4 cathode is 32K wrt ground. Measure 0.15 V with PS

will try to draw PS circuit

[misterc57]

Just found this thread https://www.thegearpage.net/board/in...power.2192673/

Same issue, blames Q3. There is a way to jump it to test?

"Q3 is a MMBT5087 (PNP, BJT, SOT23), according to a DIY forum. If you can find a schematic or e-mail MXR to verify, that would be wise.
It is (according to another source) a part of a power protection circuit, together with the diode D4. It is there to protect the main circuit if someone plugs in an incompatible power adapter."

[Helmholtz]

Simply jumpering Q3 leads without knowing how it is wired might make things worse.

[Helmholtz]

Q3 wiring might be like Figure 5 here: https://sound-au.com/appnotes/an013.htm
In this case jumpering emitter to collector would restore power.

[misterc57]

C13 and R25 are connected to ground.

Reading 0.5 V at Q3 Base, 11 mV at Q3 Collector

[Helmholtz]

As the collector is the voltage output, there must be other things connecting here.

[misterc57]

This probably is not drawn 100% accurate but this is what I am seeing. What is the purpose of Q3 here? Is it possible to jump the 9V supply to the cathode of D4?


C13 and R25 are connected to ground.