ADCOM GTP-400 Channel cuts out

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
Hello all. I have a friends ADCOM GTP-400 on my desk. One of the channels cuts out/static.

I traced a good signal all the way back to the ADCOM 6A Opamp (IC551) The output pin 6 will be fine, until i turn the volume knob past 1/2, then sound stops/static, and I get the full negative rail (-18VDC) on output pin 6 of the OPAMP. OPAMP Pin 2 and 3 are inputs, and Pin 3 is fine when this happens, but pin 2 will shoot all the way up a -3 volts when output goes to neg rail - pin 2 has the feedback return input...

I turn unit off and restart and all ok, until I turn up volume again...

Don't know if the -3vdc is from the output going to -18vdc (feedback to pin 2), or if the -3vdc causes the -18vdc at output.

Anyway, I think it may be the opamp itself (ADCOM 6A = LT1026?) but wanted to throw this out in case someone has a smarter opinion...
 

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Some op amps do odd things if the inputs aren't close in voltage.
I designed a class d amp triangle wave generator and I found when the inputs got close to rails the output would flip and stay at one rail.

Look at the inputs to make sure they are ok. Might be something driving the op amp causing it to latch up. If its not that then possibly op amp has died.
 

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
Whoops, it is a LT1056, not LT1026...a new one is on its way!

I have to see if I have a single Opamp - all the others are dual opamps (OPA2134, 5532, TL072, 4562, etc)

Actually I found a single TL062 and a pair of LM318P, both of which are singles?



EDIT - ok, the TL062 is also a dual (duh)...so only single I have are the LM318, and I also have a few OPA627, and OPA637, but not risking those on my friends equipment!!
 
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