SRM1801 SMPS bad

[rf7]

This unit had shorted outputs on the power amp PCB, which took out the Power Supply also. MOSFET's Q1 & Q2 were shorted in the Power supply along with the mains fuse. I've replaced those, along with Diodes D9 & 10, and transistors Q3 & 4, just because they were in front of Q1 & 2. Now, the PSU turns on, but the rails are very low. The Inputs to the MOSFET's look good (15Vpk - pk at hi freq.) , but there is no evidence of switching on the output of the MOSFET's. I'm seeing 325V on the hi voltage line, but it's not getting switched. What would cause that?

[Enzo]

A schematic might help....

If this is like some others, there are two switching supplies on the board, the main one powers the outputs, but the other one covers the low voltages needed by the control circuits. If that one is not right, the bigger one will not be either.

[rf7]

Here's the schematic: SRM1801 PSU ASSEMBLY 0027030-XX SCHEMATIC.pdf

I'll have to check the LV rail voltages.

[rf7]

The LV rails will not be up, because they depend on the initial Mosfets Q1 & Q2 switching. VDC-H is just sitting at 325Vdc and is not switching at all, so nothing will be working. My guess is that the filter caps might be bad, but I don't have an ESR meter. Have to get one.

[nickb]

If you have 325V DC on VDH-H then that is good. This is a half-bridge converter so you should see the switching AC 160Vpp on C10 not on VCH-H. Not switching? What is the level on pin 10 of U1?

On page 2 is a flyback converter - you should have switching on TP41. Nothing? Check TP63 for VIN supply voltage.

Do you have an isolating transformer?

[rf7]

If you have 325V DC on VDH-H then that is good. This is a half-bridge converter so you should see the switching AC 160Vpp on C10 not on VCH-H. Not switching?

It is not switching, the output is only .5Vpp. The inputs to the MOSFETs are 12Vpp, and the MOSFETs are new.

What is the level on pin 10 of U1?

It is sitting at ground.

On page 2 is a flyback converter - you should have switching on TP41. Nothing?

It seems to be switching, there is a high frequency 140Vpp sine wave output with a decreasing envelope that repeats. But the low voltage supplies there on page 2 have no output DC voltages.

Check TP63 for VIN supply voltage.

That is at 4.8Vdc.

Do you have an isolating transformer?

Yes, I do.

[nickb]

If pin 10 of U1 is low then that converter is should be in shutdown - if it's even got any power. The LV converter generates the VCC supply for the HV one so I would look there first since there are no LV outputs. Measure VCC on TP42. I expect it will be near zero.

The waveform you describe sounds like it might be doing cycle-by-cycle current limiting. Have you checked all the output diodes D23-D27. I'm assuming you are testing this with no load connected.

I don't understand why you are seeing the gates of Q3 and Q4 being driven if the driver is in shutdown Could it be noise pickup from the flyback converter?