Help with V2/V3 Electric Mistress

[Maharg]

Come one, come all, and join me on my latest vintage EH repair project...a V2/V3 Electric Mistress:

https://drive.google.com/file/d/1HhYhx12NI-EmJcj84o2i3V5XTm4NwQve/view?usp=sharing

This looks to me as though it has had the power section upgrade/fix already completed, and likely long ago.  The effect does not pass any signal, and I have admittedly not gotten too far deep into it.  For as long as I have owned it, it has not worked.  I suspect the power section may have been damaged, given the following voltages, just to kick things off:

LM471
1. 16.88V
2. 18.28V
3. 16.88V
4. 16.88V
5. 16.88V
6. 18.00V
7. 18.46V
8. negligible

4558
1. negligible
2. 16.5V
3. negligible
4. negligible
5. 16.5V
6. 16.5V
7. 16.5V
8. negligible

The pedal is being powered via a Voodoo Labs Pedal Power 2 (18v positive tip).

Now, I'm far from an expert, but that's a lot of "16V" going on.  Am I right to think that step 1 is to change out the LM741, or might it be OK and I should move straight to replacing the 4558?  As always, much appreciation for the community support.

-M.

[jimilee]

Finding a schematic would be super helpful.


Sent from my iPhone using Tapatalk

[Maharg]

D'oh - of course:

http://www.metzgerralf.de/elekt/stomp/mistress/images/1976-electric-mistress-v2-schematic.gif

[Zerro]

Power values at pins of both 741 and 4558 are everywhere about 16.18V. Seems that ground pole is interrupted for a lot of parts if not for all. Controll pcb tracks.

[Maharg]

Nice catch, Zerro - I did, in fact, have a loose ground.  I should mention that the effect passes bypassed signal, as well as wet signal though there is no noticeable effect (it has the volume drop but sounds dry/clean).  The clock, though, is audible - ticking away in the background and responding to the "Rate" pot.  I am still suspicious of the 4558; these updated voltages, with the ground problem resolved, may show why:

LM471
1. 20mV
2. 3.3V
3. 3.3V
4. 20mV
5. 20mV
6. 3.3V
7. 18.4V
8. 10mV

4558
1. 2V
2. 2V
3. 2V
4. 20mV (goes to ground)
5. 2V
6. 2V
7. 2V
8. 18V

EDIT
Got less lazy and checked the voltages at the 2N4354.  Power comes into the Emitter at 18.4V, but leaves the Collector at the same 18.4V, with a modestly different 17.76V at the Base.  It seems the 2N4354 isn't doing its job...

[Zerro]

 Yes, that transistor can be damaged, though don't know when it hapenned. Divider 22k-30k must give at middle point cca 0,5 of V+. So, in ideal case cca 6V, with 18V at V+ even more, cca 9V. But at pin 3 of 741 you have only 3V.

Then you can play with VR1. At pins 3 and 5 of 4558 there should be something about 6V (if V+ is 12-13V).

That divider 22k-30k is wrong, or input 3 at 741 (not 471 :-) is broken.

Honestly, nowadays are simplier ways how to get 12V stab for those circuits - simple 78L12 (100mA) or 78M12 (500mA). Depending on current consumption.