breadtimesam
New Member
Hello, I have a question about an sx-780 I want to fix up.
A little background on me, I've done some tube radios, both AM and one AM/FM. I'm familiar with basic soldering/desoldering, recapping, and limited troubleshooting. Use (in series) an isolation transformer, variac, and dim bulb tester. However with this receiverI have been leaving out the variac.
I picked this up at an estate sale so I was given absolutely no information about it. I looked over everything to make sure there were no glaring problems. I didn't see any burnt resistors or shoddy connections. I plan on doing a full recap (was going to leave the filter caps original, but would replace if needed) and am considering replacing failure prone semiconductors. Someone like AH-Fix-It pretty much shotguns caps, diodes, and transistors, whereas someone like xraytonyb replaces some problem components but more less does a repair and recap. I'm also thinking I'll buy the darlington replacement kit and build that no matter what. Given the readings below, they may be fried anyway.
When I put power to it, I did not get it out of protect. I put it on AM and FM, and the lights and meter indicate these are working. My signal generator and scope are in storage at the moment, so I haven't scoped the output of the board.
When I took both darlington packs out the relay came out of protect.
One reason I took out the packs is I am getting some very weird voltages on the darlington pins traces with the packs removed. With packs removed, here are the readings I am getting on traces where the pins go:
R channel
1 -34VDC, -70VAC
2 -27VDC, -60VAC
3 and 8 no voltage
9 34VDC, 70VAC
0 34VDC, 60VAC
L channel
1 -20VDC (pin was wavering by about 10V), -70VAC
2 -27VDC, -60VAC
3 and 8 no voltage
9 34VDC, 70VAC
0 34VDC, 60VAC
So that AC looks real bad. Would you suggest looking at the filter caps or the bridge rectifier? I have the Peak cap and semiconductor testers, so I can remove and test if you think it's worth it. I've seen some videos where a simple recap and control cleaning (I have both F5 and D5) solves all sorts of wonky issues, but I'm thinking there is a much bigger issue here. I've gone over the schematic but I don't see how AC voltage would be getting into the audio signal, and also don't know how such high DC would be getting to the audio either.
My ideal would be to find the actual issue and fix that rather than swap a bunch of parts, have it work again, and not know exactly why.
UPDATE: I ended up desoldering the filter caps and large rectifier diodes. Everything tested great, so I'm not sure. I also ohmed out the filter caps cathods to chassis before desoldering and it was a short as expected. If the filter caps are good and have good connections and the rectifier diodes are good, where could the AC be coming from?
A little background on me, I've done some tube radios, both AM and one AM/FM. I'm familiar with basic soldering/desoldering, recapping, and limited troubleshooting. Use (in series) an isolation transformer, variac, and dim bulb tester. However with this receiverI have been leaving out the variac.
I picked this up at an estate sale so I was given absolutely no information about it. I looked over everything to make sure there were no glaring problems. I didn't see any burnt resistors or shoddy connections. I plan on doing a full recap (was going to leave the filter caps original, but would replace if needed) and am considering replacing failure prone semiconductors. Someone like AH-Fix-It pretty much shotguns caps, diodes, and transistors, whereas someone like xraytonyb replaces some problem components but more less does a repair and recap. I'm also thinking I'll buy the darlington replacement kit and build that no matter what. Given the readings below, they may be fried anyway.
When I put power to it, I did not get it out of protect. I put it on AM and FM, and the lights and meter indicate these are working. My signal generator and scope are in storage at the moment, so I haven't scoped the output of the board.
When I took both darlington packs out the relay came out of protect.
One reason I took out the packs is I am getting some very weird voltages on the darlington pins traces with the packs removed. With packs removed, here are the readings I am getting on traces where the pins go:
R channel
1 -34VDC, -70VAC
2 -27VDC, -60VAC
3 and 8 no voltage
9 34VDC, 70VAC
0 34VDC, 60VAC
L channel
1 -20VDC (pin was wavering by about 10V), -70VAC
2 -27VDC, -60VAC
3 and 8 no voltage
9 34VDC, 70VAC
0 34VDC, 60VAC
So that AC looks real bad. Would you suggest looking at the filter caps or the bridge rectifier? I have the Peak cap and semiconductor testers, so I can remove and test if you think it's worth it. I've seen some videos where a simple recap and control cleaning (I have both F5 and D5) solves all sorts of wonky issues, but I'm thinking there is a much bigger issue here. I've gone over the schematic but I don't see how AC voltage would be getting into the audio signal, and also don't know how such high DC would be getting to the audio either.
My ideal would be to find the actual issue and fix that rather than swap a bunch of parts, have it work again, and not know exactly why.
UPDATE: I ended up desoldering the filter caps and large rectifier diodes. Everything tested great, so I'm not sure. I also ohmed out the filter caps cathods to chassis before desoldering and it was a short as expected. If the filter caps are good and have good connections and the rectifier diodes are good, where could the AC be coming from?
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