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Dynaco SCA-80 loud buzz

69.206.244.26
Posted on October 8, 2009 at 21:34:36
saeger
 I have a Dynaco SCA-80 that buzzes loudly through both channels, irrespective of volume or other controls. I posted this question before but stated that it was only one channel. I brought the amp out of storage today, and I found that my recollection was faulty. I tested the voltage test points and got the following results:

PC-18
Actual Normal
1. 0dcv 0dcv
2. 0dcv 0dcv
3. 0dcv <0.5dcv
4. 50dcv 74dcv
5. 18dcv 36.5dcv
6. 18dcv 37.5dcv
7. 16dcv 36dcv
8. 16dcv 36dcv
9. 0dcv 0dcv
10.50dcv 71dcv
11.0dcv 0dcv
12.0dcv 0dcv

PC-19
Actual Normal
1. 50dcv 71dcv
2. 50dcv 74dcv
3. 16dcv 36dcv
4. 16dcv 36dcv
5. 18dcv 37.5dcv
6. 18dcv 37.5dcv
7. 0dcv 0dcv
8-9.54acv 54acv

Thank for any help anyone could offer.

RE: Dynaco SCA-80 loud buzz, posted on October 9, 2009 at 15:12:11
Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004
 There is a STRONG possibility that you have shorted diodes on PC-19! Do NOT operate the unit until this problem is corrected or you might destroy the Power Transformer. The other likely culprit is shorted filter capacitors. Or BOTH. My "long-distance guess" is that the filter caps shorted, taking out the diode(s).

I see very few SCA-80's that aren't suffering from bad electrolytic capacitors these days. The larger the capacitor, the more likely it is toast. While I don't deal with them all *that* often anymore, I would consider it almost MANDATORY to replace all the large electrolytics - especially on a unit that has been "in a closet" for any length of time.

Hope this helps.

Bill

RE: Dynaco SCA-80 loud buzz, posted on October 10, 2009 at 19:16:40
saeger
 Thank you for pointing me in the right direction. Is the large green capacitor near PC-19 with the yellow wire that is grounded and the three red wires one going to PC-19 and the other two going to the left and right PC-18 boards the likely culprit? Is there anyway to test whether or not the capacitor is shorted? Should I replace the diodes first and then the only capacitor if necessary. Can I test a diode with an electrical tester? Thanks again. I knew the problem must have been before PC-18 because it is on both channels, but I never wold have thought to replace the capacitors. Thank you again. Also are the diodes simple Radio Shack stuff or do I have to special order them?

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