CRT Monitor HV Issue

Xef said:

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?

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Nigel Goodwin said:

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.

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throbscottle said:

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!

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