Parasound 2125 2 channel power amp problem - diyAudio
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Old 3rd July 2013, 10:42 AM   #1
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Default Parasound 2125 2 channel power amp problem

Seems I was wrong, I do have normal sound out of both A & B speaker outputs after replacing the two op amps on the input board. The only way I can turn the amp off though, is to turn the volume down on the preamp and unplug the power amp. Could it be all I need to do is replace the relay because it's probably stuck in the on position? The OP Amps I replaced were LF353P what I found on hand, was some OPA2134pa amps so I used them as I have installed sockets once before, all I had to do was just plug them in. Should these be alright for replacements? if not I think I have some more LF353P amps somewhere. I also, installed extra decoupling caps along with bypass caps when I installed the DIP sockets. Since the power switch is a momentary push button type, could the IC chip that's also close to the relay be bad or, the electronic on/off switch cause this amp to come on by it's self just by plugging in the ac power cord? Even with the a and b speaker push buttons turned off, when I plug the ac power cord in, the small green L & R channel indicator lights up as if I had pushed the on off power button. The amp actually sounds better now with the OPA2134PA op amps in it. Thanks for all opinions/suggestions and possible places to get a service manual would be very appreciated.

Last edited by crippledchicken; 3rd July 2013 at 10:45 AM.
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Old 3rd July 2013, 04:33 PM   #2
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Default schematic

I received this schematic showing the standby section of my amp from Parasound. Can anyone maybe, tell what component could be causing instant turn on when the ac power cord is plugged into the amp? Parasound 2125
Attached Files
File Type: pdf SCHEMATIC 275-2125_ POWER SUPPLY_STANDBY.pdf (56.3 KB, 59 views)

Last edited by crippledchicken; 3rd July 2013 at 04:36 PM.
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Old 3rd July 2013, 06:54 PM   #3
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I suppose you have checked that standy-by momentry switch S204 (loc A-5) is not stuck closed? And then there's the relay K501 to check. Sorry that's not much help, just repeating what you said already....
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Old 3rd July 2013, 08:38 PM   #4
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I haven't checked either one yet, but I plan to. Thanks!
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Old 4th July 2013, 11:48 AM   #5
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Quote:
Originally Posted by Andersonix View Post
I suppose you have checked that standy-by momentry switch S204 (loc A-5) is not stuck closed? And then there's the relay K501 to check. Sorry that's not much help, just repeating what you said already....
I noticed with the amp on no input/sound, that if I push the button like trying to turn on or off the amp I hear a light thump each time I push the switch. Should I assume, that the momentary on/off switch is not stuck closed? Could CD40135 at U1 also, be suspect as well as the relay at K501? Thank You!
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Old 4th July 2013, 05:26 PM   #6
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Seems a reasonable assumption (that the switch is not stuck closed). A light thump, like a relay coil energizing? Give the relay a tap and maybe it will let go if that's actually the problem.
Of course U1 could be suspect, but check the simple mechanical stuff first.
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Old 4th July 2013, 08:28 PM   #7
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I've tried tapping the relay but no luck. I have been searching the net for a replacement relay just in case it turns out to be stuck closed but, can't seem to find one anywhere any ideas? I'm going to take my ohm meter and check the relay and momentary switch soon for closed contacts. I hope it's not the IC chip I hate having to solder those things in especially, the tiny surface mount chips my eyes are not that good any more. Thanks Again!
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Old 4th July 2013, 08:47 PM   #8
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If it's the IC (and then you're lucky it's stuck 'on'), then perhaps just bypass the whole stand-by arrangement with a high-current on-off switch?
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Old 4th July 2013, 08:52 PM   #9
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If it's the IC (and then you're lucky it's stuck 'on'), then perhaps just bypass the whole stand-by arrangement with a high-current on-off switch?
That sounds like a lot easier way to go about fixing it if that's the culprit. Thanks!
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Old 4th July 2013, 08:53 PM   #10
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Hi,
Check the voltage at the base of Q214 should be positive to turn off K501. If it is zero then Q214 it is enable grounding the base of Q214. K501 it is control by the CD4013 flip/flop Q1. Also check the voltage at CD4013B Pin 1.
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