API 550A oscillation problem

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la-2a

Well-known member
Joined
Apr 27, 2005
Messages
96
Location
Germany
Hi, I have an vintage API 550A (2x 2520) module here with an big problem. The module oscillates and makes a kind of distorted sine wave noise
like an Moog Synth without any signal on the input. The frequency changes the sound by moving the switches  and it does not matter
which band. The noise disappears by pushing the left button L|F  (Low pass ??).  No input signal going trough . I replaced  both 2520,  I recapped the unit . Now I was
going to check the Transistors . Does somebody out there has an Idea .??

The version I have has no component numbers printed on the board. Can somebody post an picture from inside. Thanks. ;)

Any suggestions highly appreciated
Thanks in advance.  ;)
 

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what was the unit doing before the recap and 2520 replacement? Same issue? I would check and make sure traces are intact, check that power is getting to the right places, then test some transistors.
 
hi, yes same issue before recaping. i have a working lunchbox no problems with  other modules .
Thanks
 
I ment power once it gets to the card. I had a API 550A original on the bench this past week that would distort audio the moment it was in circuit. End result was there was a faulty 2520 and a few capacitors needed changing. I would check your recap work with a magnifying glass.  cap orientation, etc,etc. After that start testing the transistors
 
Could be a broken wire on one of the switch banks, also a dodgy transistor can cause it. I've had a few with a similar problem, but never a common fault!
I drew up this to help me, it's the view from the component side
 

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