18v electric mistress trouble

[01redcobra]

I've got an old 18v Electric Mistress pedal here that I can't seem to hit a happy medium with. The pedal was working, but had a noticeable click each time the flange cycled. On checking, I had 16+ volts on pin 8 of the 4558 and the joining pins on the sad1024(5,7,9,11) and no amount of adjustment on the bias pot would change that more than a few tenths of a volt if that. The schematic shows that there should only be 12v on those pins. I changed out the lm741 and nothing changed. In the pedal I'm working on, there are 2 bc309's which I see nowhere in the schematic. One ties into pin 6 of the 741. I checked it out of circuit with a vom and it shows bad. I didn't have any bc309's so I substituted a sn3906 and after that I have around 6 volts on the pins that should have 12v and the effect is not working anymore. Am I on the right track here? Where can I get a bc309 as that seems to be the problem. I'm not well versed on transistor circuits and I could really use some help here. I'm pretty lost at this point, but I don't want to throw in the towel. Thanks in advance!

[Enzo]

The bias will never adjust the power supply voltage.

If the 741 is new, that 2N5037 is suspect for being shorted. And the 2N3563 "diodes" as well, those are what your BC309 are I'll bet.

If you replaced the BC309 with a 2N3906, make sure the leads are in the right order, 2Nxxxx types are EBC across the front, so if the BC is different, then the new transistor must be turned sideways or backwards to fit.

Your two mystery transistors are being used as diodes - the ones around the 741. Note the E and the BC next to the diode leads? That means the emitter is beinf used as the cathode or our "diode" and the base and collector are wired together as the anode. SO if it shows as shorted while in circuit, that is because the two legs are wired together.

[01redcobra]

Thanks so much Enzo! It was the 2n3906 I replaced, the c and e are the opposite of the c and e on the bc309. I switched it around and we are back in business. I'm getting 13.3 at the pins that are supposed to get 12, but it sounds good and I don't have the clicking anymore. I think 13 v is still within tolerance. Once again, I thank you!

[Steve Conner]

It's worse than that, the two mystery transistors are being used with their base-emitter junctions reverse biased, as Zener diodes

This actually works, but the "Zener" voltage varies between different brands, part numbers and lots. The transistor maker doesn't bother controlling it, because they don't expect you to use transistors like that. It's usually around 5 to 7 volts, IIRC.

So this is probably why you're getting 13V instead of 12: because your ghetto Zener reference has a higher voltage than the 2n-whatever that EH originally used. If you wanted to change it, you'd have to mess with the 18k or 30k resistor in the bottom left-hand corner, which set the gain of the feedback loop for the voltage regulator. But if it's working, why bother

[01redcobra]

Thanks Steve, My question now is, is it ok to let it run with the 13v instead of 12v? Could any damage result from that? I sounds much better at this voltage than when I first got into it and it had 16+ volts there (clicking and other oddities). Just wondering about longevity.

[Steve Conner]

I can't think of any component in there that wouldn't be happy at 13V indefinitely. Even the SAD1024 is specced up to 15V.

[01redcobra]

Enzo and Steve, thank you for all your help. You guys are great!