Need Help Repairing SMPS (in audio amplifier)

Thread Starter

Charles Gallup

Joined Nov 8, 2016
9
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.
 

AlbertHall

Joined Jun 4, 2014
8,381
- 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.
If they were getting that hot I wouldn't expect them to last very long. Are you sure it is the capacitors that are hot and not something near them?
Don't check the temperatures with your finger while it's switched on unless you like electrocution.
 

AlbertHall

Joined Jun 4, 2014
8,381
Then you should replace them pronto, but I don't think they are anything to do with the problem.
They are connected across the mains input and are part of the interference supression. If you do replace them they should be X-class capacitors and rated 275V AC and they not any type of electrolytic - they are film capacitors. Being X-class means they are suitable for connection across the mains.
 

Thread Starter

Charles Gallup

Joined Nov 8, 2016
9
Can I just remove them for now to 'ease' the circuit and continue to try and find the issue? I do not have replacements yet. Why wouldn't the PWM IC be getting any voltage?
 

AlbertHall

Joined Jun 4, 2014
8,381
Yes it should work fine without those caps.
The transformer 'clicking' suggests to me that what is happening is it fires up and then detects some problem (perhaps too much current) and shuts down again to prevent further damage. SMPS often exhibit this behaviour. It can be very difficult to find the actual fault.
Check around for shorted components. The diodes on the secondary of the transformer and the amp output transistors would be good places to start.
 

Alec_t

Joined Sep 17, 2013
10,384
If you're getting only 135V across the main bridge rectifier I'd suspect the bridge has failed. With power off, a DMM test of the bridge might reveal a faulty diode; or it could be that a diode is breaking down when high voltage is applied.
 

Thread Starter

Charles Gallup

Joined Nov 8, 2016
9
So I removed the rectifier, D24 from the circuit and I got "0L" from every measurement across the leads. If this the core of the issue?
 

Dodgydave

Joined Jun 22, 2012
8,411
Put the dvm on "Diode test" with beep, test between an Ac input and positive out,negative out, do it on both Ac inputs.

If the bridge is ok it should only beep one way across each diode.
 

JoeJester

Joined Apr 26, 2005
4,077
If you did, and it was OL in both directions, you have a bad bridge. Did you take the readings with the bridge unsoldered from the circuit?
 

Thread Starter

Charles Gallup

Joined Nov 8, 2016
9
I took it out of the circuit completely. I also have some caps I'm worried about as mentioned in my op. I've since bought a capacitance meter and it's reading 3.6mF and it's rated at 3.9mF. That's about 7.7% off. I don't know what tolerance I should be expecting.
 

hoop

Joined Nov 17, 2016
38
looks like a crown audio amp
i think albert is correct the amp is going into protection
and has separate circuitry for that
 
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