Switching Power Supply Won't Start - Partial Schematic Inside [Repaired]

[Peeps]

I'm working on a power supply I pulled out of a dead wine cooler which drives two fans and two thermoelectric coolers at 12V. Inspecting it and testing some parts, nothing obvious is blown and power is getting through the primary side rectifier and charging the mains caps to 340V. Q6 and Q7 both turn on and stay on instead of pulsing and there is therefore no voltage on the secondary side. The model of the board is HYS2A130-H and I've attached part of a schematic I put together here. I unfortunately do not have a DSO so I cannot capture any startup waveforms. I'm not sure what this kind of switchmode topology is called either. I tried searching around but didn't find anything matching it.

[xavier60]

The 130K resistors are there to lightly bias on Q6 and Q7, enabling them to self oscillate.
This is supposed to generate some output voltage, powering up the 494 which then takes control of the half bridge, Q6 and Q7.
Check for shorted secondary diodes.

[Peeps]

If both Q6 and Q7 are on, it should blow up.

That's what I assumed would happen, but the chain of 130k resistors seems to limit the base currents enough that they just get warm.

[Peeps]

The 130K resistors are there to lightly bias on Q6 and Q7, enabling them to self oscillate.
This is supposed to generate some output voltage, powering up the 494 which then takes control of the half bridge, Q6 and Q7.
Check for shorted secondary diodes.

You saved me a lot of time. I checked the rectifier diodes and while not shorted, both legs are reading about 300 ohms reverse biased and out of circuit. I'm going to hack in a temp replacement and see what happens.

[Peeps]

Well I spoke too soon. Looking at the datasheet, these large TO-247 (original D83-004, tried replacement MBR4060PT) diodes have a reverse leakage current as high as 100mA so they read anywhere from 200 to ~4k ohms reverse biased and depending on how hot they are. Temporarily replaced the 16V 2200uF output cap to be sure but no change on that side either...

[xavier60]

Is there any output voltage?
Check D4 and D5 also.

[xavier60]

The problem can be a short somewhere or an open in the power path. Although they rarely fail, the primary coupling capacitor should be included as suspect. How are Q7 and Q6 sharing the HVDC? It's ok for uneven sharing.

[shakalnokturn]

As said by xavier60 above.
Actually C16 would be my first suspect here.

Edit:
From a similar topic...
https://www.eevblog.com/forum/repair/need-help-identifying-this-component-(pics)/?action=dlattach;attach=590233

[Peeps]

Is there any output voltage?
Check D4 and D5 also.

No output voltage. D4 and 5 test good.

The problem can be a short somewhere or an open in the power path. Although they rarely fail, the primary coupling capacitor should be included as suspect. How are Q7 and Q6 sharing the HVDC? It's ok for uneven sharing.

Most of the voltage is being dropped across Q6 with Q7 having maybe 2.5V across it.

As said by xavier60 above.
Actually C16 would be my first suspect here.



Edit:
From a similar topic...
https://www.eevblog.com/forum/repair/need-help-identifying-this-component-(pics)/?action=dlattach;attach=590233

It's marked as 1u0K 250C which I'm guessing is 1uF 250V? Seems like a low voltage for the application. Testing it reads 0.95uF with 0.5 to 1 ohm ESR which I think is a bit much for a polyester capacitor.

[xavier60]

Q7 could be partly broken down causing Q6 to produce all of the heating that you noticed.
Test for C-B leakage although it might not show on a DMM. Is there any yellow/brown glue, possibly causing corrosion and leakage?
And all resistors in the area.

[bdunham7]

Don't overlook C13 and C14.  Can you test them?

[Peeps]

Don't overlook C13 and C14.  Can you test them?

Definitely bad. ESR of one was around 30 ohms, the other about 50. Decided to check the rest of the caps and found most of them over spec with one of the 47u and the 22u also reading >15 ohms. I replaced all the caps except the mains caps which tested good and now I get 0.7V at the output and you can hear the transformer ringing very faintly, so it is trying to do something now.

Q7 could be partly broken down causing Q6 to produce all of the heating that you noticed.
Test for C-B leakage although it might not show on a DMM. Is there any yellow/brown glue, possibly causing corrosion and leakage?
And all resistors in the area.

While they didn't have goo, the flux around the leads was totally black possibly from heat? The board is looking a bit baked around them so they've had a lot of thermal stress. The first thing I did when I looked at the supply was to clean those areas but to no change. I'll remove them now and run some basic tests and see what comes up.

[Peeps]

Aha smoking gun. Q7 BE reads as s a ~1k ohm resistor. CE reads as a diode on my component tester (but not multimeter). It still recognizes the whole mess as a transistor with an hfe of 1. Q6 on the other hand seems to be okay (hfe 28). I'll replace them both and see what happens. 

[Peeps]

After replacing those transistors (and capacitors) it fired up and seems to be working great now. Big thanks to everyone that helped me narrow down the issue here!