Having a little trouble repairing the left channel of this amp, which camp in with shorted output FETs IRF640/9640 which I clipped all the legs off. Partially blown open source resistors (x10). I also found Q221 failed, and R255 / R228 opening up. Some of the MPSA06/56 transistors also failed and so I replaced ALL, Q229, Q230, Q209, Q210, Q224, Q225.
I installed all new 0.1ohm source resistors. Temporarily put one IRF640 and one IRF9640 into the board at Q211 and Q219.
Amp powers and draws 1.2A with bias pots CCW. With no load, Left channel at very low input has artifacts on the top half of the waveform, and the bottom half is only about half what it should be. When increasing input signal, the amp 'freaks' out and outputs garbage to my scope which sort of ends up looking like an MTX power supply drive signal.
I'm reviewing the schematic, but is there anything that I may be missing?
I installed all new 0.1ohm source resistors. Temporarily put one IRF640 and one IRF9640 into the board at Q211 and Q219.
Amp powers and draws 1.2A with bias pots CCW. With no load, Left channel at very low input has artifacts on the top half of the waveform, and the bottom half is only about half what it should be. When increasing input signal, the amp 'freaks' out and outputs garbage to my scope which sort of ends up looking like an MTX power supply drive signal.
I'm reviewing the schematic, but is there anything that I may be missing?
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Skiped lifting protection diodes for now.
I found (4) 20 ohm resistors near the MPSA06/56 had all blown open. Replacing those helped immensely.
Now under 4-ohm load, I'm still getting some artifacting on the top half of the waveform. Artifacting tends to reduce when increasing the bias.
Here is a photo, this time bias CCW (Off), no load. Scope set to 0.5vDiv
I found (4) 20 ohm resistors near the MPSA06/56 had all blown open. Replacing those helped immensely.
Now under 4-ohm load, I'm still getting some artifacting on the top half of the waveform. Artifacting tends to reduce when increasing the bias.
Here is a photo, this time bias CCW (Off), no load. Scope set to 0.5vDiv
I installed all new IRF640/9640 and I'm taking a step back, amp freaked out and starts drawing a bit of current over about 5% input signal. Under that and it still shows like above photo.
D202 & D205 I believe are for this channel's protection. Removing them helped me diagnose Q227 as being bad. Replaced. Reinstalled D202 and D205. Amp is working out to about 2/3 drive signal now.
Related but new problem. Repaired channel under no load and high input signal the amp draws too much current. Voltage across repaired channel's source resistors reaches about 0.025vDC. The fets get a bit warm.
D202 & D205 I believe are for this channel's protection. Removing them helped me diagnose Q227 as being bad. Replaced. Reinstalled D202 and D205. Amp is working out to about 2/3 drive signal now.
Related but new problem. Repaired channel under no load and high input signal the amp draws too much current. Voltage across repaired channel's source resistors reaches about 0.025vDC. The fets get a bit warm.
Found R238 and R264 blown open, but replacing them did not help the problem, which is still;
"Repaired channel under no load and high input signal the amp draws too much current. Voltage across repaired channel's source resistors reaches about 0.025vDC. The fets get a bit warm."
Still hunting for faults...
"Repaired channel under no load and high input signal the amp draws too much current. Voltage across repaired channel's source resistors reaches about 0.025vDC. The fets get a bit warm."
Still hunting for faults...
Yes all left channel source resistors are measuring significant DC across them all. Right channel isn't doing this at all.
Fixed that problem. It ended up being that I didn't make the best solder connections on some of the new SMD resistors. I guess since I had to replace a few dozen SMD components, missed a few. Seems to be working now. Will mount up and test some real loads.
Thanks
Thanks