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Need Johnson Marquis SM120 Schematic
I was given a dead Johnson SM120 amp, and I'd really like to get it going, as it is much nicer than the amp I'm now using. The signal is getting thru the tube stage, but never making it to the amp chip(s). I probed around quite a bit, but without a schematic it is really hard to figure out what is going on. I searched the web but had no luck; could someone please provide a schematic for this amp? The SM60 is probably he same amp with only one 60 watt chip, so that schematic would also help me. Thank you.
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SM120? Or JM120?Education is what you're left with after you have forgotten what you have learned. -
Have you plugged a cord from FX send to FX return? ANy help?
HAve you verified the speaker itself works? Have you tried plugging an external speaker into the jack on the rear panel?Education is what you're left with after you have forgotten what you have learned.Comment
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Also checking the "speaker on/off" and also seeing if there is sound at the headphone output. Might as well check the easy stuff off first.Originally posted by Enzo View PostComment
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Thank you so much for your reply. My mistake: it is indeed a JM120. Yes, I tried all the "low hanging fruit" you mentioned, with no luck. I looked that the input to the amp chip(s) with a 'scope, and verified that there is nothing there when I apply a 1000 Hz signal to the input of the amp. I see the signal at the plate of the first stage tube, but from there it disappears. . .Comment
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OK, so plug a signal into the FX return, does THAT come out the speaker? Assuming for now that it does, and we have a preamp issue. Just to verify, signal into the front, and nothing comes out the FX send?
All knobs centered, look for signal. I see one little tube, so assuming the rest of it is op amps? IN which case, verify both 15v rails (presumably, might be 12v or some other reasonable amount) get to the ICs, then go down the ICs and check the output pins on each for unwanted DC and for signal.Education is what you're left with after you have forgotten what you have learned.Comment
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Great primer on general troubleshooting -- I appreciate that! Yes, signal into FX return comes out speakers, signal into front does not come out FX send or speakers. Circuit is one tube, op amps (mostly LF353) and other multi-pin ICs. I'm starting the voltage and signal verification you suggested. . .Comment
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