B&W TV Philips 21TG100U Line timbase fault.
This 21" Philips has many of the circuit design elements from the previous 90 degree CRT models including the Stella ST1007.
Bad luck with this set, without any doubt the line output transformer has failed. A 50 volt P - P line drive waveform is present on the grid of the PL81 line output valve but there are no sparks on the top caps of the PL81 and PY81, same goes for the EY86 anode connector.
Oh dear, but which winding, is it repairable? An impressive set, well designed for servicing.
Some LOPT's are a real pain in the overwind.
I feel for you.
Hi Andrew, upstairs there is a Philips 19TG108U which also has a faulty line output transformer. I don't think it is the EHT winding that is faulty.
The 19TG108U was introduced early 1961 and has a similar chassis as fitted in the 21TG100U. CRT is the AW47-90.
Hi Derren, one solution is to have the main winding rewound and dispense with the EHT overwind and use a tripler. That's what Rank-Bush-Murphy did with the replacement transformers for the sets fitted with the A774 chassis.
We've got nothing to loose so I've hooked up the bench power supply unit to the line output transformer. 30volts is being supplied to the EHT rectifier anode connector and the top cap connector of the PL81.
The bench power supply is also a Philips, it's a type SBC519. It's quite an old instrument, was bought sometime in the eighties from the Philips Service depot in Newcastle.
It is apparent to me now that the 21 inch CRT is in a larger cabinet. I assumed them to be the same as the 17 inch version because the escutcheon on the 17 inch seems overly large, with the CRT looking almost lost in the middle. Here below is a Stella 17 inch offering....
Thanks for having a look at this, as you know it happened on my watch so I feel party responsible. Whilst I did not employ your warm up method, the set was powered up carefully over a two week period on a Variac and showed every sign of being in good shape, eventually giving a superb bright picture and line and frame form.(with the appropriate repairs) On the last day of that period however when full mains was applied for the first time, the failure occurred after about two hours' use. Sadly we will never know if any other method would have made it more reliable. What I did think was particularly interesting about this set was the trouble a couple of TV engineers had gone to to make a note of the repairs done to the set on a couple of repair tickets which I have retained here. I thought it would be interesting to replicate it here;
31/08/64 - PL81 Line Osc, ECL80 Line Drive
30/12/64 - 3 resistors frame, 1 x ECL80 1/2 frame osc.
12/05/66 - Focus resistors, Turret dismantle and clean.
18/05/71 - New scan coils fitted (!) PCL82 frame, clean v/c track
7/07/72 - C115/R113 , PCL83 sound stage.
A nice record to have and something you don't often see! Clearly a set someone cared a great deal for at one time and worked well into the 1970s.
Hi Derren, it's not good news. The transformer was allowed twelve hours of warm up time but after reconnecting the valve top caps the results are just the same, no EHT and the PL81 is glowing red hot.
A replacement line output transformer is out of the question, availability of this part ceased by the end of the seventies.
In an earlier post I mentioned that I still have the 19TG108U a set which is almost electrically identical to the 21TG100U. Here's an idea. If the jellypot line output transformer is still in the set why not transfer the parts into your 21" set? I'll go upstairs tomorrow and take a look at the set and report back my findings
From the UKVRRR, the topic about the jellypot conversion: https://www.vintage-radio.net/forum/showthread.php?t=67147
Yes of course, if you are willing to do that. Maybe the width issue will be less of a problem in a 21 inch set? Just a thought, otherwise I am sure you might well think of a way around it. By the sound of it these Philips lopts do not have a great reliability record, a similar thing happened to my 17TG106U, but only the primary failed as I recall, as I had it rewound by someone who does not do overwinds. I no longer have that set however. After what has happened any solution is worth considering, it's a very impressive sight when that 21 inch tube is lit up, and I can well understand why someone tried to keep it going for so long.
The picture of the 19TG108U shows I gave up on the jellypot line transformer substitution project, at least for this set. However, I still have a few ex-BRC transformers and I'm willing to have go at fitting one in your 21TG100U. We've got nothing to loose.
Can't say I've ever seen a 19 inch set quite like that? I thought they were usually later dual standard types. Worth a try but depends which set you would rather save (first) I suppose. Why did you give up on it do you think, because of the width issue?
Hi Derren, I can't remember why the project was abandoned and it possible the modified line timebase box is somewhere in the shop. If it is it'll be upstairs.
About the 19TG108U. The "square corners" 19" CRT was introduced early in 1961 and Philips like many other manufactures quickly cobbled up sets which used the chassis from previous 17 and 21 inch models.
Other 19" sets that come to mind are the Ultra V1980 and Murphy V519.
very service ale philips, i guess it´s made in GB? hmm a PL81 for a 21" 110 degress screen, that = short lived PL81
a PL81 for a 21" 110 degress screen, that = short lived PL81
I have to admit I thought it must be stretching the limits for a PL81 a bit.
Hi Alex, One would of thought the PL36 was the logical choice for the line output valve in 110degree CRT receivers but in fact many 405 line sets equipped with the slim 110* tube used the PL81. And it seems they got away with it! Reliability of the little PL81 was good but maybe not as good as the PL36. The PL81 in Derren's set was fitted in the mid sixties.
The 21TG100U is now on the service bench again. The EHT winding has been removed from the line output transformer and now the whole transformer assembly is ready to be refitted on the chassis. I'll report back my finding later today. If it works an EHT multiplier unit will be employed for the final anode voltage.
Well, that didn't work, the PL81 is glowing red hot. Have found another TG100U line output assembly. I'll try that one in the set and report back later.
And that one didn't work either.
Tiny spark at the anode of the EY86 which lasts for only 10 seconds or so, proving at least the line timebase circuit oscillates.
But soon after the PL81 begins to glow red hot again.
Late production Rank-Bush-Murphy line output transformers which were made as service replacements for the A774 series employed a tripler for the EHT instead of the usual overwind.