The last of CRT here

[tvtech]

Hello all

This thread directed specifically at Nigel )

Samsung CRT........54CM........
PSU using STRW 6753
SMD Jungle and Micro in one.
Tac sw used to power TV on or OFF

Problem : Line stage running, all voltages 100%
Picture only appears if I turn G2 up a bit.
And than down.

Picture than 100%.

Its not a Samsung "cap thing"............checked them all......................

........................

tvtech

 

[Nigel Goodwin]

I would suggest two (related) possibilities (from distant memories of CRT sets!), probably it's down to the auto-greyscale circuits. These only have a limited range, and once the CRT loses too much emission then it's out of their range - hence no picture. By increasing the G2 this brings it back within range, but then it's too bright - but you can then turn it back down as it's already within the range of the auto circuit.

So either CRT (most likely), or a fault in the auto-greyscale circuit - most of which is in the chip, but if you're lucky it could be some high value resistors associated with the cathode drives and feedback for the auto circuit.

Most of my experience with such faults, mostly on Sony sets, was that it was almost always the CRT.

This is the 'problem' with the auto greyscale circuits, it makes the picture still look good, even though the CRT is worn out - without the auto circuits it's obvious the CRT is failing, as the greyscale drastically changes.

I would suggest you twiddle the G2 until it's working, displaying greyscale bars if you have them, and then measure the voltages on the three cathodes - you'll probably find that two are OK, and one is obviously low - the low one is the gun that's low emission, or has a fault in the drive/auto for that gun.

 

[tvtech]

Bingo

Thanks Nigel, am gonna try your advice today and report back.

Thank you,
TV

 

[tvtech]

You nailed it !!!

Two guns were down on the tube.....third one barely passed.
Rejuvenated it and bingo, picture comes on straight away.

Rejuvenation probably wont last but customer knows its time for another TV.

Cheers Nigel. Youve been a big help as always with your spot on advise

 

[schmitt trigger]

I am always held in awe by Nigel’s paleonto-electronics knowledge.

 

[unclejed613]

Rejuvenation probably wont last but customer knows its time for another TV.

how long the emission of those guns lasts will depend on who made the CRT. up to about 1990 most american and japanese manufactured tubes had enough active cathode material that the tubes could be rejuvenated several times. when China began making tubes, they began skimping on the cathode material, and tubes were good for only one or two rejuvenation attempts. this was true for computer monitors as well as TVs. since TVs were considered a "throw-away" item, it was TV CRTs that had the thinnest oxide layer on the cathodes. i repaired computer monitors for about 10 years, and as time passed and more monitors were built with chinese CRTs, the lifetimes of the tubes got shorter and shorter. for me, CRT replacements were rather uncommon at first (about 1 in 20), but became more common around the year 2000 (1 in 5). then came LCD monitors, and it was no longer cost-effective to repair CRT monitors, because the prices of CRT monitors dropped 50% in about 6 months after the prices of LCD monitors stabilized at $300. just before this, CRT monitors averaged about $300 new, but had dropped to $150 or less after LCDs were introduced. since the average repair bill for a CRT monitor was about $150, and the price of new CRT monitors dropped to $150, repairing CRT monitors was no longer viable.