JVC TM-1500PS no power

[ostlar]

Hi, I am working on a dead JVC TM-1500PS video monitor.
It is in great condition, other than it does not power on.

I need some help diagnosing it because I'm a little lost.

Upon power on I get green power LED and a relay click, a constant whining noise coming from the PSU, another relay click, and then silence and no power LED.
The relay clicking is RY501.

It does not want to try to power on again until the capacitors have discharged and power is lost from the PSU.

This what I've done so far:

- Checked for cracked solder joints
- Checked all the capacitors, resistors and diodes in the PSU
- Checked for shorts in the B1 circuit, flyback and deflection yoke
- Checked capacitors in horizontal and vertical deflection
- Checked for shorts in HOT
- Checked diodes and resistors in the B1 circuit
- Checked fusible resistors
- Disconnected the degauss coil

The PSU is on a separate board, so I am able to isolate it and test it.

I have measured the B1 voltages upon power on.
I get 84V while it is whining, then it jumps up to 129V after the relay clicks and whining stops.
The B1 upon operation should be 110V according to the service manual.
After the whining stops and RY501 relay clicks, I then have stable 129V on B1 and C521, while nothing else happens.

I disconnected the cable from the PSU to the main board and hooked up a 230V 100W light bulb (39Ω) to the B1 circuit to load test the PSU.

It draws 0,28A at 110V for a total of 30,8W.
This is less than half the power the monitor uses at normal operation (68W according to the specs) but the PSU handles it well.
There is some faint whining from it.

I added another 100W lightbulb in parallel for a total of 19Ω measured with the cables.
That results in 0,56A at 108V, or 60,5W.
The PSU makes a whining noise a bit more higher pitched than it did when I try to power on the CRT, but it seems to handle the load and somewhat keep the voltages up.

This is where I am kind of lost. There is no indication of a short on the B1 rail, but the power supply is acting like there is.

I'm kind of stuck as of now, been on and off on this project for a couple of weeks. Having some new eyes look at this usually helps.

[vol.2]

Originally Posted by ostlar

Hi, I am working on a dead JVC TM-1500PS video monitor.
It is in great condition, other than it does not power on.

Another important piece of information is whether or not the high voltage is kicking on or not.

Use a frequency app to see if you can detect a 15kHz spike when the set is switched on. Turn on the app, then watch what happens when you switch on power to the set. It might turn on briefly and then switch off when it goes into protection. Or it might just be on and fine. The app I use on my android phone is called Spectroid, but there are lots of them out there for free.

It the high voltage isn't turning on, then I would suspect that the B+ is correct at 129V because it's not being loaded. Your test with the bulb shows it regulates when it's loaded.

So check to see if you're getting 15.1kHz and then you'll know if the HV is working. That in itself won't tell you what to fix, but it will get you closer to a cause

The fact that you get 110V when you load it is also suggestive, but you also might have a weak power regulator IC (IC901), which could show issues only under load

Not sure if I can be of any more help, but hopefully that gives you some paths to check out

[ostlar]

Originally Posted by vol.2

Another important piece of information is whether or not the high voltage is kicking on or not.

Use a frequency app to see if you can detect a 15kHz spike when the set is switched on. Turn on the app, then watch what happens when you switch on power to the set. It might turn on briefly and then switch off when it goes into protection. Or it might just be on and fine. The app I use on my android phone is called Spectroid, but there are lots of them out there for free.

That's actually genius, thanks for the tip.

I do get a peak at 15633Hz which is almost bang on the HV frequency. I couldn't hear it over the PSU whine, and frankly my hearing has gotten a bit worse over the years even though I am only 29.

It stops as soon as the monitor goes into protection.

So that lets us know that the HOT is doing its job. Now my suspicion of the PSU being faulty is slightly increased.

Could a diode or resistor fail while being loaded, but otherwise measure okay unloaded? Or it might be the IC as you say.

The IC costs $10, so it is very tempting to order and replace it.

[vol.2]

Originally Posted by ostlar

Could a diode or resistor fail while being loaded, but otherwise measure okay unloaded? Or it might be the IC as you say.

A diode or transistor can fail under load, but test okay. The junctions can degrade and stop working to spec. A resistor is almost impossible to test perfectly and fail under load. There's some really edge cases where the body of the resistor has a small crack that heats up and separates when under load, but it almost always goes complete open, and if it was cracked, it would fall apart in your hands when you took it out of circuit.

One trick to isolating a bad semiconductor is to spray it with cold spray while it's on. Sometimes the quick change in temperature corrects the problem momentarily. Doesn't always work

The IC costs $10, so it is very tempting to order and replace it.

There's voltages shown on the schematics for the IC. I think it would be a good idea to check them out, although I'm not 100% sure that they are all good to probe. Sometimes those areas can be tricky, but the service literature usually tells you not to probe them if so. Just in case, I might use a multimeter I didn't care a whole lot about for the first test.

Or hopefully someone else here can chime in and give the go ahead.