HELP! Power amp buzzes when powered on (solid state)

[mich]

In the case of something like that, the power supply probably still needed addressing. If a short causes a buzz, more than likely the reservoir caps are nearly done for and the increased load on the power supply (due to the failed transistor) is increasing the ripple from an already out-of-spec level. In my shop, if there is a buzz, my policy is to immediately recap the power supply (and usually the entire unit) because 99% of the time, this will solve the problem and 100% of the time, it's part of the problem. If the unit is more than 15 years old, a recap should be done as preventative maintenance anyway.

I have never seen anything in the output stage cause a buzz in the half decade I've been repairing and restoring recording equipment full time.

(as a side question for Mich) What sort of mains power do you use in Israel?

- Thats a very good practice.
i just fixed the output and kept the same 4700uF electros....
your approach is obviously better(thinking of the future)...

we get 220V AC from the wall here.

 

[heffree]

I will try replacing the larger caps first. They aren't soldered directly to the board (both leads are soldered to resistors that are soldered to the board) so it should be pretty quick and easy. Hopefully I can make a trip soon to a store in Salt Lake that actually stocks these caps and work on it this weekend. I will post back with results when I get a chance.
Thanks for all the help/suggestions guys!

 

[glorymorris]

I will try replacing the larger caps first. They aren't soldered directly to the board (both leads are soldered to resistors that are soldered to the board) so it should be pretty quick and easy. Hopefully I can make a trip soon to a store in Salt Lake that actually stocks these caps and work on it this weekend. I will post back with results when I get a chance.
Thanks for all the help/suggestions guys!

You can order them from mouser or digikey.

 

[heffree]

Well, I finally had time to go pick up some caps last weekend and switch them out.
The amp works great now!
Lots of bite and punch to it.

Anyway, just wanted to say thanks for the help!

 

[heffree]

Well, I feel a bit sheepish having to come back here and post this but..
I went to power up my amp today and I'm getting the same buzz like before.
I got the caps from a local place and they may have been surplus. Is it possible I got some bad caps? What else would cause the same noise to return again so quickly?

 

[Speedskater]

Have you tried taking the amp to a friend's house?

 

[heffree]

I sent an email to whoever runs the Lee Jackson website and they told me the caps should be at least 63v, not 50v. Would that cause the issues I'm having?

 

[pucho812]

Yes... When ever replacing a cap you should use the same value. If you use caps rated at a lower voltage then before you risk destroying the cap. If you knew that you were not going to pass 50V then your 50V cap would be fine but since your not sure, best to put back in the same value.

 

[s2udio]

So the amp must have already been modded then as the original pic shows 6800uf 50v caps ?

???

Have you actually measured the voltage across the cap....if they were running over V then they would get warm ! .

As with all tech issues...............Lack of info leads to all sorts of goobldygook theories

 

[heffree]

So the amp must have already been modded then as the original pic shows 6800uf 50v caps ?

???

That's what I've figured. Perhaps the guy before me replaced them with the wrong ones and it stopped working so he decided to just sell it.
I'll be putting the correct caps in this week sometime.
The guy that emailed me actually recommended using caps with the highest voltage rating that will still fit in the space available.

 

[pucho812]

Or he replaced them with what was available and was hoping it would work without issues. 

 

[CJ]

if the caps failed due to over voltage, you would probably know it,

look for a broken or loose ground somewhere, check the vol controls, are the controls soldered to a pc board?  what about the input jacks?

a lot of abuse occurs at the 1/4 inch inputs, maybe a resolder on the input and front panel controls where they enter the pc board, it is cheap to do and will be a good preventive maintenance thing, just like the recap.