NAD - Ampeg GT-10 Loud Hiss

[maches]

I recently picked up a 70's Ampeg GT-10 solid state amp. I bought it real cheap on CL and didn't have a chance to test it out. I planned to stick in my garage so I could test pedals that I'm working on without bringing them inside. I wasn't too worried what it sounded like. When I got home and plugged it in it had a really loud hiss to it. Since I have some experience with amps I thought I try to fix the problem myself if it's not too extensive or expensive.

The amp came with a 2 prong cable which the original owner cut and extended with another 2 prong cable. When I opened the amp I noticed some of the electrolytic caps looked like they may have leaked. I ordered an 8ft. 3 prong cable and all the electrolytic caps to recap the amp. I'm pretty sure that the 2 prong cable isn't the culprit of the loud hiss (just safer), but I wasn't sure about the caps since they are 30+ years old..

From some of the reading I've done on upgrading from 2 prong to 3 prong cables an isolation transformer is recommended if one is not present. I'm pretty sure this amp doesn't have one. Would it be a big deal to go without one? Any other thoughts on the cause of the noise?

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[celeste]

It does have an isolation transformer, The PT does double duty. It is the amps with no PT that you would need to add an iso trans to.

The electro's in a 30yo amp that has not been used need replacing, you can pretty much count on that. So you are off to a very good start on bring it up to snuff.

Hiss is usually broad band white noise, and that is usually not just a cap, but it might be, you got to replace them or do some heavy diag to find out, replacing them is much easier.

 

[maches]

Thanks for your input celeste. I don't think there is a PT. I don't see one anywhere. Do they look different in solid state amps? I see the OT, but not a PT. I should have the amp recapped and the new power cable installed by Friday. so I guess I will find out if it works then.

I emailed ampeg for a schematic, but I'm not sure if they charge for them or not. Hopefully will hear back from them soon.

 

[celeste]

Transistor amps usually do not have an OT. What you see is the PT

 

[maches]

Oh, okay. Thanks. That makes me feel more confident on replacing the cable. Should I just be able to put the white and black in their respectable places on the board and then put the green wire under one of the nuts on the PT?

 

[celeste]

Oh, okay. Thanks. That makes me feel more confident on replacing the cable. Should I just be able to put the white and black in their respectable places on the board and then put the green wire under one of the nuts on the PT?

That should do it just great.

 

[maches]

So I replaced the electrolytic capacitors and changed the power cable to a three prong and I am still getting white noise. I noticed a couple of other things that I thought I would ask about to see what you guys think. First thing that I thought was odd is one of the input jacks doesn't have a ground wire. I tested it with a multimeter and I am getting conductivity through the chassis though. There are also few diodes and resistors that have crud on the leads. Not sure if it is just oxidation or caused from something else. The back of the PCB doesn't show any signs of abuse, although the traces for one of the 2 power transistors (Q4 I think) has brown goo mixed with it. Looks like it could just be flux or some of the goop they used to secure the capacitors.

Schematic: http://www.megaupload.com/?d=LN09OJBU

Some pics
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[maches]

Another Update. After doing a little more research I decided to take the amp back apart and re-examine the PCB. I sprayed the pots with some contact cleaner and took the board out again to spray it down as well. When I took it out this time I had a better view a trouble spot that I overlooked the first time around. One of the transistors overheated and caused the plastic (as well as whatever they coat it with) to bubble up.

I tried to be as careful as I could removing the transistor, but because the plastic was covering the solder joints it was extremely difficult to get good contact. I ended up tearing a couple of traces and I still don't have the piece completely off the board. It has a heat-sink attached and it is a PITA to get loose.

The transistor is an RCA 40389 (CXF 518). Even if I can remove the transistor without causing more damage I'm not sure how I would go about getting a replacement for the unit.

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