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I now get a hum at TP8 (pin 7) with the probe and a bit of the 1K signal mixed with hum at pin 6. I can hear the tone faintly when injecting it at J14
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Wha??? Why would we have the reverb drive and return connected together???Education is what you're left with after you have forgotten what you have learned.Comment
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I mentioned it yesterday in post #15. I rationalized it as completing that part of the circuit for testing. It's disconnected now, and I still have DC on pin 7 of IC4.
Originally posted by Enzo View PostComment
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It wasn't going to have been the source of the DC, but it also didn;t belong.
I am still with my suggestions of post #27.Education is what you're left with after you have forgotten what you have learned.Comment
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Agreed, I plan to replace both IC4 and Q25 while I have the board up. I can't get my hands on a J112 without ordering one, and Radio Shack doesn't use that part number. An internet search indicated a 2N4391 is a suitable replacement. What is a suitable substitute that Radio Shack might have in stock?Originally posted by Enzo View PostComment
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Find out if you need it, just remove Q25. Does the amp now work? If not, the IC is probably bad, and if it does, then leave it out until you can get a new one, the amp will work fine without Q25.
The type of JFET is not at all critical, but be aware the legs of a sub may not be in the same order, you'd have to determine that and install a sub part so the legs went into the proper holes on the board.Education is what you're left with after you have forgotten what you have learned.Comment
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OK, I have some 2N5458 N-Channel JFETs on hand and the legs are in the same order, so I'll drop one of those in there if need be.Comment
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As I said, find out, don;t just throw parts at it.Education is what you're left with after you have forgotten what you have learned.Comment
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Pulled Q25, and the amp did not work. Still had DC on pin 7 output. Replaced IC4 and can inject a signal at TP8 and at pin 6 of IC4 and get strong throughput.
Moved back to TP7 and found DC on pin 7 output of IC3. Pulled Q8 and still have DC on pin 7 and no throughput. I am replacing IC3.Comment
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I wonder what happened to this amp.
You very well may find that all of the opamps are bad.Comment
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Great question. The power cable was missing its ground lug, and I think it may have been exposed to a less-than-desirable power source when it quit passing signal. I dunno. At least we have it working from IC4 downstream.Comment
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OK, done deal. Boy, you fellows sure took me to school on this one. I feel like I've just come up on the semester break. Thanks all. Thank g-one for making me make sense of the process.
Enzo, I only replaced the bad parts: R42 (22 ohm), Q8 (j112), IC4 (TL072), and IC3 (TL072). I put Q25 back where it belonged.Comment
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