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)

 

[Lightium]

Best I can say is to keep following the circuit from the transformer with the voltmeter untill you find no voltage.

 

[Ed207]

sagor1

At C1935 I get 2V (should be 8.6V measured on good monitor) and no output from pin 2 IC1936 as ON/OFF is low (this is controlled from signal board I believe, which is not getting power).
At C1934 I get 7.27V (should be 16V) and no output from pin 2 IC1933 or IC1934 . Again on/off pin 5 is low

At C1936 I get a wavering voltage between 2.8 and 7.4V (should be 117V according to a working monitor). This is the lower winding.

C1936 is giving me some funny readings and was a bit crusty on the underside. I'll probably swap this out to make sure, but that won't affect the other voltages.

Am I right in thinking that the issue may be on the live side, before the transformer?

 

[Tony Stewart]

Measure Pins on IC1901. Either that or any component it depends on is failing.
Slightly bulged lids on bulk e-caps (+) is the 1st visual check. Heat stress 2nd. Probe pin #'s on ICs and test nodes 3rd.

Mark clearly on above if you can.

 

[sagor1]

If C1936 is faulty, that could be a problem. That provides voltage for feedback to the primary switching circuit via PC1901 near the bottom. With faulty feedback, none of the secondary voltages will be working properly. Without the 117V, PC1901 will not turn on and IC1901 will not switch the primary voltage thru the transformer. Check the diode D1934 that feeds C1936 as well.
There could also be a fault with IC1901 or PC1901 itself.
If possible, compare voltages (DC voltages) on the primary side around the semi-conductors if you have another working unit. Be careful, as these may be high voltages and dangerous. Check around Q1901 and Q1902, they both look at feedback on the primary side of the transformer. Again, use the primary ground for measurements, not the secondary ground, they are isolated from each other.

 

[Ed207]

Thanks guys. I'll check when I'm back home on Wednesday. Great advice so far and much appreciated.