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jcm900 dual reverb: most op-amps blown. how come?


 :
10/31/2003 6:10 PM
sebastian jcm900 dual reverb: most op-amps blown. how come?
I am repairing my marshall 4501 dual reverb amp right now and I've found most of the op-amps blown, including the reverb driver's tl071, fx loop driver's tl072 and most of the switching op-amps (m5201, hard to find a source for them...in europe). I soldered in ic sockets and threw a few tl071/072's in. Most features are now working again (I'm still waiting for my m5201's). But how could that happen?  
Where should I look at next?  
 
Thanks in advance for any advice.
 
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11/1/2003 12:44 AM
Enzo

When a bunch of things fail, you need to look for what they have in common. ANd in this case it is probably power supply. Thes chips run on +/-15V - or was it 12? - and if one side went nuts, it acn overvoltage them. Make sure those supplies are solid, nothing intermittant in the regulation.
 
1/12/2004 2:04 AM
Pete

My bro in law did something like that to the same amp by plugging the output of a SF bandmaster into the line out jack or FX loop, don't remember which. He set a pickup truck on fire the same week. Go figure.  
 
I have also seen the DC filament supply burn up and turn off the first couple preamp tubes on those amps. Not related to your problem, just FYI.
 

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