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

[heffree]

So I know this may not be the best place for this, and if someone could suggest another forum that could help I'm open to ideas.
Anyway,
I just got a Lee Jackson SP-1000 that I picked up off Craigslist
Got it here, plugged it in, turned it on, and it just buzzes very loudly.
I opened it up and all the wiring looks okay but I'm no electrical engineer so who knows...

Just wondering if it might be a relatively easy fix before I call up the guy that sold it to me.
I'm somewhat DIY inclined so if I had to resolder something I think I could handle it.

Any ideas would be appreciated, thanks guys!

 

[JohnRoberts]

Check that PS reservoir caps are healthy and in place... DC PS rails good without lots of ripple or missing voltage..

JR

 

[okgb]

maybe describe the  " buzz " a little more ,  are the input jacks isolated from ground ?
maybe a connection is loose and either became grounded or un grounded .
both channels same problem or is this mono ?

 

[glorymorris]

An audible buzz (as opposed to a hum) will almost invariably be 120hz leakage caused by aging reservoir caps. Over time, as the electrolyte dries up, the ESR of the caps becomes too high and they no longer filter the rectified DC in the supply.

 

[heffree]

@okgb
the buzz occurs in both channels even with nothing plugged into the inputs or even with the volumes all the way down.

@JohnRoberts @glorymorris

i'm attaching a photo here, are the reservoir caps the 4 6800uf ones?

 

[JohnRoberts]

Nice photo.. caps look OK, but.....

Buzz in both channels supports power supply as common problem.

Could also be low voltage rail, if the amp uses one.

Don't see anything obvious from a picture, but that is often the case.

JR

 

[heffree]

@JohnRoberts
I was just posting the photo in hopes that you could tell which caps are the reservoir caps.
Would they be visibly deformed if they were no good anymore?

 

[glorymorris]

Most of the time, they're not physically deformed when they go bad. That definitely does happen sometimes. the (4) black 6800uF caps are a good place to start, though I'd recommend replacing all electrolytics in the until if you have bad filter caps.

 

[Sylvain D.]

Maybe a long shot, but make sure it's AC voltage is set to (117) for North America as opposed to (220) for Europe...

I've been bitten by that one before

 

[heffree]

Alright, thanks for the suggestions guys. I'll check the voltage switch first, if there is one.
Replacing the caps would be a good project for me, I've never really done any sort of repair on stuff like this. Any tips aside from making sure the caps are discharged?

 

[glorymorris]

Just leave it turned off overnight before you start. This being a solid-state power amp, no dangerous DC voltages are likely to be present.

 

[pucho812]

Be careful when you desolder your parts, caps or otherwise.  Depending on how the PCB is made, Could easily destroy or pull the traces.

 

[mich]

i think you better know more of the problem before starting to replace parts...
maybe the output transistors are shorted(or one of them)and that's what gives you the buzz - especially if its a loud buzz.
is it passing signal?
do you have a DMM ?
you can look around for shorts or open circuits and obviously check voltages  in the power supply

 

[glorymorris]

i think you better know more of the problem before starting to replace parts...
maybe the output transistors are shorted(or one of them)and that's what gives you the buzz - especially if its a loud buzz.
is it passing signal?
do you have a DMM ?
you can look around for shorts or open circuits and obviously check voltages  in the power supply

A buzz isn't a mystery. It's the fundamental (in this case US 60hz hum) with heavy harmonics, ie 120hz. It's always caused by a power supply failure-- notably filtering caps. I've encountered this problem hundreds of times.

 

[pucho812]

i think you better know more of the problem before starting to replace parts...
maybe the output transistors are shorted(or one of them)and that's what gives you the buzz - especially if its a loud buzz.
is it passing signal?
do you have a DMM ?
you can look around for shorts or open circuits and obviously check voltages  in the power supply

A buzz isn't a mystery. It's the fundamental (in this case US 60hz hum) with heavy harmonics, ie 120hz. It's always caused by a power supply failure-- notably filtering caps. I've encountered this problem hundreds of times.

plus it is in both channels which common to both channels is the PSU.

 

[Harpo]

Is the buzz humming at 120Hz or at 60Hz?
If the latter, check diodes in both bridge rectifiers as well.

 

[mich]

sure it sounds like a PSU failure - but im just saying it could also be the output transistors failing.
and im saying it because 2 days ago i worked on a solid state Fender M80 (65W per channel) that had exactly this issue -
one of the output transistors was shorted ,and the amp was buzzing Loudly at any setting.
it took out some other components around it(driver transistors,diodes) - but the caps were fine....

if you are working blindly - just changing components to see when it comes back to life - i guess desoldering a cap is easier than a transistor

 

[s2udio]

Whats about DC offset on the outputs ?
Have you measured it ?
Sounds like depleted ouput devices to me .........Check all diodes and semis.

 

[glorymorris]

sure it sounds like a PSU failure - but im just saying it could also be the output transistors failing.
and im saying it because 2 days ago i worked on a solid state Fender M80 (65W per channel) that had exactly this issue -
one of the output transistors was shorted ,and the amp was buzzing Loudly at any setting.
it took out some other components around it(driver transistors,diodes) - but the caps were fine....

if you are working blindly - just changing components to see when it comes back to life - i guess desoldering a cap is easier than a transistor

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?

 

[JohnRoberts]

It is difficult to troubleshoot from a distance with vague symptoms.

Hum/buzz in both channels suggests common vector... like PS voltage. Is product getting correct design mains voltage?

If yes... Does it mostly pass signal but has some buzz, or is sound distorted by buzz, or is there only buzz?

How much buzz is normal for a XYZ... ?

JR