Sony BVM 2010P PSU Problem

[Gizmo]

I'd like some thoughts on troubleshooting an old Sony BVM monitor PSU.

I bought it in a non-working state, when the power button is pressed the monitor powers up for a split second and then powers itself off. While powered for the brief time, the lights on the control panel at the front appear to behave correctly.

I've had a look at the service manual, and the power board is split into GA and GB sections. The GA section is the main power supply and the GB is an over-voltage protection sub-board which shuts down the PSU in the case of voltage issues.

The PSU uses two TL494CN chips for voltage control and according to the service manual the capacitor input to the chip which is used to set the oscillator frequency is grounded in case of over-voltage. I've checked, and this pin does appear to be grounded when the power to the monitor is shutdown. This is just tested with a DMM, seeing a small voltage drop to 0 as soon as the power goes.

I've recapped the board, but this hasn't made any difference to the symptoms.

Next I need to check the voltages with a scope to see if they actually are overshooting a lot, looking just with a DMM, they seem to be ok, but the reading is obviously only for a split second.

If anyone has any experience of these monitors or any general advice on PSU troubleshooting, that would be appreciated!

[Gizmo]

After more digging the LED's on the GB board seem to indicate if there's a problem in either the HIGH-B or LOW-B sections. For my monitor, the HIGH-B LED lights on powering up. Attached is the reverse engineered schematic for the HIGH-B detection section of the circuit.

It seems to be a latch formed with Q3 and Q4 with the input to Q4 base being the trigger for the latch. I'm not quite sure when that circuit would trigger the latch, I'm not very good outside of simple digital electronics! I'll do some probing on the input to Q4 and see what's there.

I've just probed the 150v rail, and it's coming out at pretty much smack on 150v. There's about a 2-3v overshoot very briefly, then settles down. The base input to Q4 raises to about 700mv with the delay from C1 before it latches and shuts down the PSU. So I'm starting to suspect the top section of this circuit to detect over-voltage is failing.

[Gizmo]

Aha, I've found the actual schematic of the GB board now.