CRT Monitor HV Issue

[Xef]

Hi, I am trying to fix a CRT which seems to loose HV (tube turns off) after 30 sec-2 mins of displaying a picture.

The monitor service manual: https://drive.google.com/open?id=1RkczrKyDeaRsWuqN9Bnlj1r-qpG2U855

I have just measured the HOT (NPN Transistor) with the diode test on my DMM as:
B to E = 397
B to C = 341
E to B = 1
C to B = 1
C to E = 1
E to C = 1

Most seem to say that it should read between 450/500 to 900 (https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07 ) Should I assume my HOT is dead?

Also how can I test a damper diode? Mine is FMQ-G5GS from this datasheet: https://datasheetspdf.com/pdf-file/228160/Sankenelectric/FMQ-G5GS/1
It seems to be rated for 1700V so I don't know if I should test it like regular diode with DMM?

Hope I can get some advice. Thanks!

Pictures of the board:

https://imgur.com/wrBSQO7

https://imgur.com/V0aVJ3P

 

[Nigel Goodwin]

Hi, I am trying to fix a CRT which seems to loose HV (tube turns off) after 30 sec-2 mins of displaying a picture.

The monitor service manual: https://drive.google.com/open?id=1RkczrKyDeaRsWuqN9Bnlj1r-qpG2U855

I have just measured the HOT (NPN Transistor) with the diode test on my DMM as:
B to E = 397
B to C = 341
E to B = 1
C to B = 1
C to E = 1
E to C = 1

Most seem to say that it should read between 450/500 to 900 (https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07 ) Should I assume my HOT is dead?

Also how can I test a damper diode? Mine is FMQ-G5GS from this datasheet: https://datasheetspdf.com/pdf-file/228160/Sankenelectric/FMQ-G5GS/1
It seems to be rated for 1700V so I don't know if I should test it like regular diode with DMM?

Neither are likely to be faulty, as the fault is intermittent - and you can't really check LOPT transistors on a multimeter anyway, other than they will read S/C if faulty (if they are going to fail it WILL be S/C).

As always in these types of threads, you DON'T repair things by randomly trying to test individual components, and in the case of an intermittent fault you've certainly got zero chance of finding the problem in that way.

You need to 'fault find', for a start you need to find out what is actually happening - most likely problem is that safety circuits are been triggered, you need to find out if that is the case, and if so which one is been triggered. Usual options would be 'over-current' or 'over-voltage', and a prime cause in a CRT set would be the 'over-voltage on the EHT side', known as 'xray protection' in Japanese manuals.

If you look on page 13-3 Q5044 and Q5045 are listed as HV protector, and connect to the HT Ctl chip.

 

[Xef]

Neither are likely to be faulty, as the fault is intermittent - and you can't really check LOPT transistors on a multimeter anyway, other than they will read S/C if faulty (if they are going to fail it WILL be S/C).

As always in these types of threads, you DON'T repair things by randomly trying to test individual components, and in the case of an intermittent fault you've certainly got zero chance of finding the problem in that way.

You need to 'fault find', for a start you need to find out what is actually happening - most likely problem is that safety circuits are been triggered, you need to find out if that is the case, and if so which one is been triggered. Usual options would be 'over-current' or 'over-voltage', and a prime cause in a CRT set would be the 'over-voltage on the EHT side', known as 'xray protection' in Japanese manuals.

If you look on page 13-3 Q5044 and Q5045 are listed as HV protector, and connect to the HT Ctl chip.

OK, understood that makes sense.

Although I've only been taking the boards out to inspect for bad solder joints to see if that's the issue and thought that I'd extract and measure the HOT whilst I was at it.

The last board to inspect is the HV board for bad solder joints after that I will reassemble it and try to test the voltages on IC2001 (HV Ctrl), + Q5044, Q5045. Previously when it was assembled I only tested the main voltages on the Power board (DA 1,3 + DC, 1,3) which measured fine. This is because accessing the HV board to measure the voltages is really tricky and inaccessible - I will have to use alligator clips to reach the components.

For reference HV board is the brown board with the flyback connected:

https://imgur.com/7gIVbSD

 

[throbscottle]

In addition to what Nigel says, there is also the likelihood it is a thermal fault, with a component that develops a fault when it's warmed up (most likely a semiconductor), or a soldered or other type of connection which changes from low to high resistance as it warms up (which tbh is unlikely, but easily remedied by re-work).

Just because the picture is going off doesn't necessarily mean the tube supplies are failing. It could equally indicate the video signal is failing or that the low voltage supply to the tube supplies is failing, or something else entirely.

The service manual you linked to looks pretty comprehensive, you should have the information there to check some voltages and narrow down the fault after it occurs. If you can get a solid indication of what area is being affected using this method, folks here may be able to help narrow it down further.

If you don't have the necessary fault-finding skills, your friends here are likely to be a source of hot or cold air, such as a hair dryer, and a source of cold, such as freezer spray. With these you can manipulate the temperature of individual areas and components until you find where the fault is being triggered. Don't use freezer spray until you are 90% sure which component is actually faulty, it's expensive stuff!

 

[Xef]

In addition to what Nigel says, there is also the likelihood it is a thermal fault, with a component that develops a fault when it's warmed up (most likely a semiconductor), or a soldered or other type of connection which changes from low to high resistance as it warms up (which tbh is unlikely, but easily remedied by re-work).

Just because the picture is going off doesn't necessarily mean the tube supplies are failing. It could equally indicate the video signal is failing or that the low voltage supply to the tube supplies is failing, or something else entirely.

The service manual you linked to looks pretty comprehensive, you should have the information there to check some voltages and narrow down the fault after it occurs. If you can get a solid indication of what area is being affected using this method, folks here may be able to help narrow it down further.

If you don't have the necessary fault-finding skills, your friends here are likely to be a source of hot or cold air, such as a hair dryer, and a source of cold, such as freezer spray. With these you can manipulate the temperature of individual areas and components until you find where the fault is being triggered. Don't use freezer spray until you are 90% sure which component is actually faulty, it's expensive stuff!

That makes sense that it would be a component which develops a fault when warmed up since the CRT stays on for 30sec-1min and doesn't immediately fail.

Apologies, I should have specified that it isn't just loosing a picture but the CRT is in fact turning off and makes the crackling sound like when it normally would when you turn off the power. The speakers are still working, the power light is still green and the CRT filament is lit (I think the filament is powered by low voltage). There is no high voltage to the CRT since it powers off - CRT off, no raster, no picture.

I was hoping it would just be a bad solder joint but it seems not. As you say, it's probably a faulty component. I will check the voltages on the HV lines such as Nigel mentioned from page 13-3 and see if they match with the service manual. Hopefully that will help narrow things down. I've noticed that if I turn the monitor on its side, I can access the solder side of the HV board which will make it far easier to probe the voltages now.

 

[throbscottle]

Good luck! Be careful probing those high voltages - don't exceed your meter's rating and more importantly don't shock yourself!

It's good practice to clip the black wire of the meter to 0v (which may be the metal chassis) use the red probe with only one hand and keep the other hand in your pocket.