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Need help now

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5/22/1999 4:30 AM

Need help now

OK,

Now I need help on trying to make this circuit work. I guess this means you guys:

(Gus, Jack, GFR, R.G. etc...)

http://members.xoom.com/aronnelson/mystery.JPG

Please help me with this pedal... The voltage going into what I assume is the base of the transistor is supposed to be 1/2 V but I don't get this. It seems like there should be a voltage divider like the TS series going to the base of the transistor and I assume the 10K resistor is supposed to do this but it doesn't.

This is a pretty famous and great sounding pedal if I could make this trace work.

I'm very close to giving up and sending the whole pedal to R.G. :-(

:-(

5/22/1999 4:39 AM
aron	BTW: the dotted line across the 10K resistor is an actual trace on the board connecting the + pin on the op amp directy to the diode/base of the transistor. The 10K resistor is connected across the trace (to the + pin and the diode/base junction) on the copper side of the board. Aron

5/22/1999 7:09 AM

At first glance, it appears that the thing may be working like a clipping diode at the active load of the input differential amplifier inside the opamp. It appears that the current limited load inside is clamped to swing only one diode above and one diode below its static value bo D2 and the current from the output being fed back into the offset node by the collector of the PNP transistor.

I'm not sure what the copper trace around the 10K is about. That doesn't make sense - yet. If there is only a 10K in there, I think it operates as I've outlined.

Have you used a meter to actually verify that the trace is continuous?

5/22/1999 12:19 PM
aron	Yes I did, and you are right. The trace is broken. So ignore that copper trace around the 10K resistor. I went through every pad and checked for continuity to verify that what should be connected is connected. This thing just refuses to bias correctly. The 83C2V7 could be a BZX83C2V7, which cross references to a NTE5002A. A zener diode. http://nteinc.com/linear_web/5000to5099.html Aron

5/22/1999 2:06 PM
Mike Burgundy	Okay, so I am probably outclassed by the other guys in this thread, but I do have an idea on the measurements on that diode; my DMM measures resistance on voltage levels low enough not to turn on most semiconductor junctions (way handy!). diodes measure infinite resistance either way. Combine this with zenerdiodes; the 3M sounds plausible... I´m curious; what famous pedal are we talking about?

5/23/1999 9:16 PM
Dave Chun	>Im curious; what famous pedal are we talking about? My guess is the Hot Cake pedal, two controls with the mid lift switch...?

5/22/1999 9:35 AM

Is there no tone control pot? You can see the Big Muff-type tone network connected to the output.

Pins 1 and 5 of a TL071 are used to trim the balance of the current mirror connected to the input differential stage to control Vos (input offset voltage). It might be possible to limit both positive and negative swings by controlling these pins.

Maybe I can put this into EWB and see what makes it tick.

-Jack

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