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Hey guys, got a GT1200H that's self oscillating on the "Solo" channel. Plugging into the "insert" jack gives me a good clean sound on all channels, so that narrows it down to the preamp. In my limited experiences, when an amp starts to self oscillate it's a cap problem. I'm thinking it's probably the big filter caps (a pair of 4700uF suckers, one for each +/-40V). Unfortunately, I don't have any caps laying around of similar value so I can't just clip on to test. Before I replace them, is there anyway to narrow down for sure?
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I dug into it a bit, and if you look at the schematic, the oscillations are being introduced somewhere between TP3 and TP4. Coming out of IC2:B the oscillations are about 5V peak to peak. The inverting input of the opamp is clean. Would a bad opamp cause this? -
Crate GT1200 Schematic
Here is my take on it.
The Solo signal is produced in the IC2:A part of the circuit.
Yeah a bad opamp could do all sorts of wierd stuff to the signal.
At IC2:A pin 1, what does the signal look like in Solo mode?
Monitor the + side of C15 (2.2 uf) & toggle between Overdrive & Solo.
What happens to the signal?
With all of the jfets that are being used for switching logic, a leaky part could screw up the signal good.
I would not jump to IC2:A pin 4 right away.Comment
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Well, I fixed it without knowing I did. When I started poking around that opamp I noticed Vcc- was closer to -30V. I had nothing across the Schottky that's controlling the -16V rail, and continuity check showed that that the anode leg had been lifted. I soldered it back in, but was still getting the 5V ptp oscillation at T4. For shits, and to see if it made a difference, I plugged it into a cab, and it sounds clean, no more issues. Apparently that 5V signal is just...there? I'm a bit frustrated that I let that take me down the wrong path, but the amp is working, so I suppose I should be satisfied.Comment
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