Hey guys, been at this for days and I'm a beginner at both SMPS and electronic repair. I'm also stuck at this point. I have an audio amplifier I am trying to fix, and I'm pretty sure the problem stems from the supply. Here is the schematic: http://imgur.com/a/e6XNY
When I press the power button it won't "turn on", i.e. no digital display, no output. Essentially appears dead. I took some voltage measurements and there is voltage at least in the primary and secondary circuits, although I think it's low for the latter. I think the real problem is the PWM controller isn't turning on, because measuring the VCC (15V) of pin 12 provides 0> voltage.
- Across the fullwave rectifier (TP30 + TP21) I measure about 135VDC.
- Across what I think are the output channels (TP26 + TP27) I measured only 27VDC, and all but 500mVDC is across one channel (TP2 + TP27).
- One other thing I noticed was the switching transformer T1 makes a constant clicking noise, though it's lightly heard unless your ear is near it. It clicks/ticks roughly 2-4 times a second.
- When I first power on the circuit, for about 1-2 minutes the Vbe/Vce of Q39-41 raises to a maximum of ~23V and drops immediately down to a minimum of ~12V, then rapidly rises back to ~23V. It cycles many times during that time period. After the voltage settles, it stays at 30V. Vbc ~450mV for each.
- No voltage across R398,R401-402. They each measure ~10ohms.
- C224/C226 get SUPER hot. I nearly burned my finger after touching it only for a fraction of a second, and I can feel the heat from about an inch away. Those two caps are just after the filter caps, C55/C56. These are a square 'box' but thinner, so not a real square. They are not a regular elecrolytic type.
- Also, I do not have a capacitor tester, but I already removed and tested the filter and secondary caps using both the resistance and voltage tests. They show high resistance and retain the voltage charged to. I tested the caps because it seems to be pretty standard thing to do, but also two of the larger caps have slightly bulged tops. But I don't know if the bulge is significant, moreso now that they tested fine so far. Here are pictures of the caps: http://imgur.com/a/WIlUk
Thank you for your help.
When I press the power button it won't "turn on", i.e. no digital display, no output. Essentially appears dead. I took some voltage measurements and there is voltage at least in the primary and secondary circuits, although I think it's low for the latter. I think the real problem is the PWM controller isn't turning on, because measuring the VCC (15V) of pin 12 provides 0> voltage.
- Across the fullwave rectifier (TP30 + TP21) I measure about 135VDC.
- Across what I think are the output channels (TP26 + TP27) I measured only 27VDC, and all but 500mVDC is across one channel (TP2 + TP27).
- One other thing I noticed was the switching transformer T1 makes a constant clicking noise, though it's lightly heard unless your ear is near it. It clicks/ticks roughly 2-4 times a second.
- When I first power on the circuit, for about 1-2 minutes the Vbe/Vce of Q39-41 raises to a maximum of ~23V and drops immediately down to a minimum of ~12V, then rapidly rises back to ~23V. It cycles many times during that time period. After the voltage settles, it stays at 30V. Vbc ~450mV for each.
- No voltage across R398,R401-402. They each measure ~10ohms.
- C224/C226 get SUPER hot. I nearly burned my finger after touching it only for a fraction of a second, and I can feel the heat from about an inch away. Those two caps are just after the filter caps, C55/C56. These are a square 'box' but thinner, so not a real square. They are not a regular elecrolytic type.
- Also, I do not have a capacitor tester, but I already removed and tested the filter and secondary caps using both the resistance and voltage tests. They show high resistance and retain the voltage charged to. I tested the caps because it seems to be pretty standard thing to do, but also two of the larger caps have slightly bulged tops. But I don't know if the bulge is significant, moreso now that they tested fine so far. Here are pictures of the caps: http://imgur.com/a/WIlUk
Thank you for your help.