la-2a

API 550A oscillation problem
« on: September 17, 2011, 10:15:53 PM »
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.  ;)
« Last Edit: September 17, 2011, 10:33:35 PM by la-2a »


pucho812

Re: API 550A oscillation problem
« Reply #1 on: September 18, 2011, 02:03:47 AM »
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.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

la-2a

Re: API 550A oscillation problem
« Reply #2 on: September 18, 2011, 06:52:27 AM »
hi, yes same issue before recaping. i have a working lunchbox no problems with  other modules .
Thanks

pucho812

Re: API 550A oscillation problem
« Reply #3 on: September 18, 2011, 10:31:18 AM »
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
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

Walrus

Re: API 550A oscillation problem
« Reply #4 on: September 19, 2011, 05:59:10 AM »
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
Kevin.


 

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