OK, where did I go wrong on this Ampeg BA115HP?

[glebert]

Got this amp on a trade, was working well, so sold it off without opening it up. I would say that I always thought it was a little quiet sounding for 200 watts but not at all quiet. Week or so later the customer tells me it is sounding weak and distorted. I pick up the amp and it definitely sounds different, but still "working." Sounds good through the headphone out, and putting a signal into the CD input sounds distorted, so I know it has to be power amp related. I measure the voltage across the 0.33 ohm resistors and am getting pretty much zero for everything. I measured the resistance to the wiper of the bias adjustment to make sure it hasn't gone open, but it is within range but seems to be cranked all the way high or low (don't know which). I bump the bias adjustment just a tiny bit (like the smallest change I could) and I hear the power transformer hum change a little. I measure the voltage across one of the resistors and am getting 400mV+ (should be 15-20mV). Before I can reach the power switch it pops the fuse. Long story longer, I pull the FETs and one of each side is shorted out.

So where did I go wrong on this, or was this just a failure that was going to happen soon anyway? Ideally I would have been monitoring the voltage across a .33 ohm as I adjusted the bias but the way that they are mounted makes it hard.

I've only had two of this model amp (well, one was a non-HP) and they both have done this same thing to me, where they were good when I sold them and then failed shortly after.

Oh, and I don't claim to be a great tech or know everything, but if you are the kind of person who makes yourself feel good by making others feel bad, I would just say move along.

TIA,
Greg

adding schem:
ampegba115hpschem.pdf

[g1]

If the bias trimmer was all the way to an end, and just barely moving it caused such a jump, I would think there may be a fault elsewhere in the bias circuit.
Or the trimmer was defective/dirty and the meter did not detect it when measuring. When you measured the resistance 'to the wiper', did you measure to both other lugs?

[glebert]

When you measured the resistance 'to the wiper', did you measure to both other lugs?

Yes, it was 500 to one lug and 0 to the other.

[The Dude]

I'd strongly suspect a dead spot in the trimmer.

[glebert]

Still working on fixing this amp, and noticed something a little odd. I'm getting 58-59V rails instead of the 50V listed in the schematic. I measured the mains at 122VAC. Doesn't that seem a little high? Also, the IRFP power FETs are only 100V parts, which seems like a bad idea with rails approaching 60V. The schematic doesn't make much sense as far as which wires are supposed to be connected to where on the primary side of the transformer. The transformer has all the wires shown in the export diagrams, but the amp is labeled as a 120V unit and I doubt someone replaced the transformer. Anyone have a diagram or connection list?

edit: to clarify, the rail measurements are with the power FETs removed.

[glebert]

Still working on fixing this amp, and noticed something a little odd. I'm getting 58-59V rails instead of the 50V listed in the schematic. I measured the mains at 122VAC. Doesn't that seem a little high? Also, the IRFP power FETs are only 100V parts, which seems like a bad idea with rails approaching 60V. The schematic doesn't make much sense as far as which wires are supposed to be connected to where on the primary side of the transformer. The transformer has all the wires shown in the export diagrams, but the amp is labeled as a 120V unit and I doubt someone replaced the transformer. Anyone have a diagram or connection list?

edit: to clarify, the rail measurements are with the power FETs removed.

Anybody?

[Helmholtz]

The somewhat increased rail voltages probably just mean that there is little current draw from the power supply.
There's nothing in the amp that could actively increase rail voltages.

100V Mosfets should be ok.

[g1]

The only mis-wiring of the primary that I can see raising supply rails would be if it were wired for 100V AC line.
Although the 120V wiring seems a bit vague, you should be able to verify that it is not wired for 100V. Also, the 115HPT schem. shows white and black of primary used for 120V operation.

Agree with Helmholtz that rails are probably just high because outputs are not running.

[glebert]

Thanks for the replies. I was mostly worried about 100V mains wiring, as g1 mentioned, or something that is neither of them but still wrong. The white and black to the transformer are wired to the mains (as are the purple and brown), and also found a video of a guy fixing one and his wiring looks the same. I guess I can put that to bed then, thanks.

Regarding power FETs, would it be acceptable to use similar FETs with a higher voltage rating? I already have IRFP240/9240 on hand.