Fender Princeton 650 - Schematic?

[schraedertr]

Hi,

I have a Fender Princeton 650 of recent vintage. DC on spaker leads.A loud hum is it's only sound via FX out, so I looked at first op-amp. Turns out the V+ to op amp is +3VDC and V- is -.8VDC, both with 1V P-P hum on the scope. Looking at bottom of board, it's starting to brown under 5W power supply resistors.

Pulled the output transistors and driver transistors and all test good. So something is pulling down power supply, and no ICs are obviously hot to touch.

It's time to start digging deeper, so I was wondering if anyone has a schematic.

Thanks,

Tom

[Enzo]

You can send me an email with Fender Princeton 650 schematic as the subject line and ask for it, my address is:

tmenzo at msn dot com

But your problem is DC on the output. Disconnect the speaker and any speaker loads, you don't need them drawing current. Your op amps are running on nothing. SInce both rails are low and the dropping resistors are hot - though they normally get hot - I'd be thinking an op amp is shorted rail to rail. Since the preamp ICs AND the power amp ICs run from the same rails, a shorted one anywhere will load down both. You may not be able to measure this short with a meter, all it needs is an internal diode in series with the short and you'll never find it that way.

Look closely at every IC. Any cracked or discolored? Replace those. Let the amp run a minute or so, and lay a fingertip on each IC. Any getting hot? Replace hot ones.

Seems REAL unlikely to me, but potentially both zeners have failed. Power down and check theor junction drops forwards. They are the five watt zeners, 1N5353.

Without proper power to the chips, nothing will work. The schematic won't help you determine which one it is, they are all in parallel.

[schraedertr]

I had been testing without a speaker or load, SOP for solid state initial troubleshooting.

Nothing looked bad or felt hot on the op-amps so I pulled them one at time, checking afterwords.

Well, now all the analog chips are out, replaced by sockets, and the problem persists. I pulled the V+ zener and no change to voltage. I'm suspecting a cap as there are small ones at every chip, but nothing looks amiss with them.

I'll wait for the schematic and then see if I can isolate it section by section, cutting traces if required.

Thanks,

Tom

[Enzo]

Schematic is a very large file, it is on the way.

If the ICs are not loading it, it just seems so unlikely that both rails picked the same time to collapse, but it moight be the case. Measure resistance across the zeners, that should point out a shorted cap on the supply. A shorted bypass cap at a chip is another matter, that would just be hunt and peck lifting. But they are all more or less in parallel, and since they are operating at low voltage, we are not likely to have a leakage issue. SO measureing resistance across one should tell us if one is shorted.

My experience with the little bypass caps at each chip shorting is that Karma dictates that failure will appear in a 32 channel mixer, rather than a guitar amp.

ANd I forgot the obvoius - the dropping resistors from the HV rail to the zeners. I can't look at the file while it is uploading, but if I recall, there were 270/330 ohm 5w resistors for that. Make sure they are OK as resistors, and check their solder to the board. Fender amps often have trouble with this. Mostly I see it on things like Hot Rod DeVilles though.