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Ok... So this amp came to me yesterday witb only the clean channel working... and the LED wasnt lighting up... I troubleshot it down to the IC... reverb wasnt working either so i changed that one as well....fixed... I thought the amp was repaired until I turned it off then back on... now, intermittently, when I switch it back to the clean channel,the LED flashes between off and yellow and the relay chatters quite rapidly.... Both states of the LED work... I measured the voltages at the test points.... all are well.... The -16/+16 are steady, even when the relay is chattering... all the diodes test ok.... I have hit every solder joint on the back of the board.... Changed Q3 as I thought it may be the culprit... nope... So I'm suspecting a bad relay... Any other suggestions are more than welcome.... Thanks for lookin... Oh, when the amp is not switching rapidly between clean and drive, it sounds very good...
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Which IC did you narrow it down to and what did you replace it with?Originally posted by Danglin' Fury View PostExperience is something you get, just after you really needed it. -
U3... I cross referenced it to an MC33078....Comment
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Try fitting the correct part NJM4560. The output drive is higher and it's critical in this application. If that doesn't do it then we can dig further.Originally posted by Danglin' Fury View PostExperience is something you get, just after you really needed it.Comment
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I will certainly try that... Had the other on hand...Comment
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What Im not understanding is it only happens when the switch is off... as in going from the drive channel back to the clean channel... It never happens going from clean to drive......Comment
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Perhaps a bit of hysteresis in the circuit?Education is what you're left with after you have forgotten what you have learned.Comment
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As in the threshold is too near the edge?Comment
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Not sure, but I do note they used TL072 in the audio circuits, but the 4560 in the channel switching.Education is what you're left with after you have forgotten what you have learned.Comment
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Indeed they do.... i will certainly get,a 4560 tomorrow to replace the MC33078 I put in... I still dont understand why it only freaks out when I change from drive to clean... Never from clean to drive...Comment
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You might get some insight if you were to check the voltages you have with the table on the schematic under "Footswitch Operation".Experience is something you get, just after you really needed it.Comment
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No joy on that....I already used said table and checked the voltages... they are all extremely comparable to said table... That's what has me buggered... And the + and - 16 volts are stable when its tripping out... the voltages on Q3 trip out when it happens but i changed that already... all the diodes are good as are the resistors...Comment
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I've had chattering relay problems that came down to C46 or C47 being open or disconnected.Comment
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Thank you very much sir....I will most certainly check those today!Comment
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How old is this amp? There was an early problem with some of these amps where the relays would float. There was a TSB about it, change a resistor in the feedback loop of the drive opamp (I think).
I'm not in the shop until Saturday, but when I am I'll look up the models and dates involved. Unless you solve the problem before I can look it up and post it here.Comment
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