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?

 

[sagor1]

The DC at the transformer is pulsed by the switching device IC1901. It might be running at a high frequency, possibly at many kHz. That produces the secondary voltages. PC1901 provides feedback to the switching device. R1916 may be (hard to tell by schematic) a thermal feedback, causing Q1902 and/or Q1902 to do some form of "control".
The output voltages are regulated by IC1931, 33, 35, and 36. Check the input (pin 1) and output voltages (pin 2) on those. If input voltages to those are "bad" (12V and 6.3V have higher input V), then the transformer switching may not be working properly. Input to the 12V and 6.3V regulator should be a few volts above 12V at least.
Measuring the secondary voltages as AC may not be accurate if your meter does not respond to the many kHz frequency of the AC signal. Measure the DC voltages at the capacitors instead - C1935, C1934 rated at 16V and 25V respectively. Measured voltage should be a fair bit less than the capacitor rating. Note that the grounds may be different on either side of the transformer. Measure secondary voltages against secondary ground (ground side of C1934, 35)