Diagnosing a JVC DT-V100 CRT Monitor - more an electronics question...

[Ed207]

Evening!

I'm trying to diagnose a CRT monitor that appears to have no power. Don't worry, I'm used to working on live voltage systems and all precautions are being taken.

Strap in....this could be a long ride as I learn!

Symptoms - No power light or control on front panel. No HV as HV is turned on by a control circuit from the signal board, but the main power circuits still need to generate the voltages to run the signal and processing board, so I believe I'm correct in thinking I don't need the signal board connected to the main board to measure and test.

I've got the schematics (see attachment). My first question is more an electronics one.

In Sheet 8 on the attachment, it shows the incoming mains and line filter board. The part I'm interested in to start with is connector CN1003 where it shows 140V (I think this is based on Japanese 100V as I'm getting 330V DC on a UK model monitor). Now this is DC as it's coming off the bridge rectifier, but then we go to the main PCB (Sheet 4 on schematics), where it comes into the board and goes to the first transformer (T1901).

Questions:
It's coming into the board as DC and appears to be arriving at the transformer T1901 as DC. I'd presume it would need to be AC for a transformer to work, otherwise it would get very hot and burn out. Am I missing something (quite obviously yes)?

The voltages coming out of the transformer are AC as expected, although one or two do not appear to be correct (7V, 2V and a wavering 4-9V). I think the 7V is prob correct, given it's going to a 6.3V or 5V rectifier.

Any advice on where to look next?

 

[Nigel Goodwin]

So quick update. Tested all the secondary rectifiers, and discovered the clicking stopped when I disconnected D1934 from the circuit, so the fault must lie on that line. Opto isolator checked out fine, and was starting to work on testing each stage when my thermal camera arrived. I don't mind a bit of cheating if you don't!

R1963 was glowing nice and hot when turned on. Checked it and it's fine. but that led me to Q1955 MOSFET, which is shorted across S/D. New ones on the way. Will update tomorrow once they arrive.

Wow, that's an extremely risky move, as the output from that rectifier is used to set the voltage output from the PSU - so the other rails are likely to go dangerously high.

 

[Ed207]

Wow, that's an extremely risky move, as the output from that rectifier is used to set the voltage output from the PSU - so the other rails are likely to go dangerously high.

Good to know for future Nigel. I'm learning a lot from you guys and it's much appreciated.