Need help repairing Sony CRT

[MaarioS]

I have a Sony CRT KV-29FX20K I try to repair. It worked perfectly fine before, however it started to change the picture into green hue for a long time. Sometimes it changed colors but changing it to green was probably the most common one. After that the picture started shutting off completely and the LED blinked 5 times. Sometimes the picture reappeared and disappeared and the LED blinks 5 times whenever the picture is off. Now it almost never works. I need to wait 20-30 minutes and the picture sometimes appears but disappears after a few seconds. The LED still blinks 5 times

I tried replacing the RGB transistors as suggested in other threads but unfortunately it didn't fix the issue. Symptoms are identical. Well, at least I didn't make it worse. I read somewhere this could be a power issue and this TV probably needs a full power supply recap. However, when I unscrewed the TV, there is no sign of leakage or bulging anywhere. There is nothing suspiciously obvious inside, just kinda dirty

What could be the issue here? How likely is the recap going to solve the issue?

 

[Nigel Goodwin]

I have a Sony CRT KV-29FX20K I try to repair. It worked perfectly fine before, however it started to change the picture into green hue for a long time. Sometimes it changed colors but changing it to green was probably the most common one. After that the picture started shutting off completely and the LED blinked 5 times. Sometimes the picture reappeared and disappeared and the LED blinks 5 times whenever the picture is off. Now it almost never works. I need to wait 20-30 minutes and the picture sometimes appears but disappears after a few seconds. The LED still blinks 5 times

I tried replacing the RGB transistors as suggested in other threads but unfortunately it didn't fix the issue. Symptoms are identical. Well, at least I didn't make it worse. I read somewhere this could be a power issue and this TV probably needs a full power supply recap. However, when I unscrewed the TV, there is no sign of leakage or bulging anywhere. There is nothing suspiciously obvious inside, just kinda dirty

Well I don't know why anyone would have suggested that?

The answer is simple and obvious, the CRT is low emission. It's a very, very, common fault and error code, and back in the CRT days I saw countless examples of it.

5 flashes (as far as I can remember) is the error IKB, which is a failure of the automatic greyscale circuit (inside the video decoder chip). The reason is that the CRT has lost so much emission (on one or more of the guns) that it's now outside the range of available auto-adjustment. This is also obvious from the fact the TV had a green tinge for a considerable period, before it finally died.

You could try slightly increasing the A1 control (first anode voltage), depending on the model it might be on the LOPTX, or a separate (largish) pot on the CRT base. Increasing the A1 voltage slightly 'may' manage to move it back within the auto-adjust range, But even if it works, it's likely to only give a couple of months extra use - and as you've already found, the picture isn't very good anyway.

BTW, as part of my previous career, I did Sony TV Warranty repairs for 40+ years

But any half competent TV service engineer could have told you the CRT is duff.

 

[MaarioS]

Aha so long story short, this is a sign the CRT is starting to die? Well, I will try to give it last ditch effort. I have nothing to lose. The error code seems to be correct

FIVE Blinks---AKB circuit (AKB), the timer/standby indicator blinks for about 30 seconds then goes to the self-diagnosis function. Something is wrong with the video. Check video out, Q705, 732, 761 and other components on the C board, also check Q218, 219, 220 on the A board. Also unit be in IK blanking, try turning up screen slightly.

Here's the service manual

H Board Removal; S1 Board Removal - Sony Trinitron KV-25FX20A Service Manual [Page 13]

Sony Trinitron KV-25FX20A Manual Online: h board removal, S1 Board Removal. 2-5. S1 Board Removal Release The Clip Indicated To Release, Push The Claws In The Direction Of The Arrow As Indicated. Eht Lead 20Mm 20Mm 13...

www.manualslib.com

This sounds simple but I have never done this sort of regulation once in my life. Now which pot am I supposed to turn in which direction?

 

[Nigel Goodwin]

It's RV701 labelled 'SCRN' on the CRT base - turn it a little one way, and see what happens - if no better, turn it back to where it was, and try a little the other way.

Essentially the auto-greyscale has a limited range - by increasing the A1 voltage it affects all three guns, and while you're increasing the failed gun from been too low you're also increasing the other two guns towards been too high.

This is why only minor adjustments are required.

 

[MaarioS]

Doesn't look like I have any luck. I turned the pot up to 45 degrees in either direction and it still blinks 5 times. How far can I go? Any thoughts before I give this thing final verdict?

 

[Nigel Goodwin]

Doesn't look like I have any luck. I turned the pot up to 45 degrees in either direction and it still blinks 5 times. How far can I go? Any thoughts before I give this thing final verdict?

You only need to turn it a fraction - but you can't turn it once the set has started flashing - you have to turn it slightly with the set switched OFF, then turn the set ON and see what the result was.

The final verdict is blindingly obvious, the CRT is knackered, and has been for a long time.

 

[MaarioS]

The final verdict is blindingly obvious, the CRT is knackered, and has been for a long time.

This is the final verdict

My new old TV is Samsung CW-29Z408PQ

 

[MaarioS]

Also I forgot to mention (how could I?)- thank you for your help!

 

[Nigel Goodwin]

Also I forgot to mention (how could I?)- thank you for your help!

It's no problem, I did countless repairs on those era of sets, as a major Sony Service Dealer - which is why I'm all too aware of how short they last after adjusting them, really it's too late as the CRT has already failed, but very careful tweaking can often eek out a couple of months.

Unfortunately (or not?), because of the auto-grey scale adjustments built-in the chip, sets of that era don't gradually 'fade away', as it self adjusts until it runs out of range, then 'poof' it's gone