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Old 4th November 2002, 01:24 AM   #1
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Default amp problem

i just got an old pioneer m-90 amp and when i turn it on the right channel hs distortion but it went away after a few minutes and sounded great after that. it seems to only happen to the right channel after it has been powered down for a couple of hours or longer. .....two days later i noticed that the area over the heat sink for the right channel is much cooler than the area over the left side. (after power up) and now the distortion is not going away. can some one help? im opening it up tonight.
shallenn
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Old 4th November 2002, 04:23 AM   #2
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llooks like i am going to be talking to myself! this thing is built like a tank. i have opened it up and have been trying to track wires and what not to not much success. i belive that the problem is power related and not signal related. i have left it powered up and the left channel heat sinks are warm to the touch and the right channel sinks are cold.im having a little trouble tracking power to the right channel amp board. it does have power <--on the points that i actually put my meter leads on---compareable to the left channel board.there is a symetrical bank of very large caps and diodes towards the front of the unit , is this used to clean up input power?? i dont really know how to test transisters
but i figure that if the left side transisters are warm when in "idle" then so should the right.-----------------facts that i do know.. signal does get through but at lower levels and badly distorted, heat sinks of the affected side are cold to touch, and all external wireing is good. please someone help!!! shallenn
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Old 4th November 2002, 05:13 AM   #3
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ok, im back replying to myself. as a final "technical touch" i gave the wires and board for the right side a good "wiggle" and right about that time i noticed that the heat sink was getting warm! now i cant say that the "wiggle" may suggest a loose connection due to the fact that it takes a little time for the transisters to warm the sink...the warming is not instant.my question is this...is there any part of an amplifier that would or could act this way if it is "going out"? ie: lower volume, bad distortion, cool transisters then all of a sudden every thing is fine.....ill return later if i need to reply to myself! shallenn
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Old 6th November 2002, 07:51 AM   #4
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Quote:
Originally posted by shallenn
i dont really know how to test transisters
If you have a multimeter with a diode tester you have to measure a diode between the base and the emitter, and between the collector and the base (see pic.), if your multimeter detects a short circuit (beep) or your multimeter cannot detect a diode the transistor must be replaced.

It sounds like a bias problem to me, lots of distortion when playing at low levels and less distortion at higher levels, and the transistors not getting warm. Check the wiring and the parts of the bias circuit.
BTW, do you have a schematic??

Best regards,

Hugobross
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Old 6th November 2002, 07:57 AM   #5
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Quote:
Originally posted by shallenn
there is a symetrical bank of very large caps and diodes towards the front of the unit , is this used to clean up input power??
i guess that's the power supply you're talking about, it rectifies (diodes) the AC of the transformer and the caps makes the voltage at a stable, constant level.

HB.
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Old 11th November 2002, 10:35 PM   #6
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i do not have a schematic but the 110 ac seems to go directly into one of the two xformer and prob. on to the large caps to smooth the voltage. the distortion gets worse when you turn it up. i just gave it a "technical tap" on the front left area of the unit, and the distortion went away. manny of the connections inside are wire wraped. since the tapps seem to "fix" the problem i believe its a loose contact. i have a fluke 85 multimeter but i dont know if it will measure diodes, its not in front of me. the craftmenship of this unit are very nice ie: copper clad chassy ,audio section isolated from xformers by way of heat sink ect. im somewhat scared i will mess it up!!
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Old 12th November 2002, 12:25 AM   #7
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Judging from the fact that when you gave it a technical "tap" on the right side the distortion went away I would hazard a guess that the volume potentiometer on the right side of the amp has a loose connection. See the link below for an image of the amp and some specs.

http://www.retroaudio.ru/pioneer/ampl/M-90.shtml

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Old 17th November 2002, 02:23 AM   #8
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i concure that there is probably a bad connection but not the pot. it is absolutely clean when turned!
shallenn
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