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Old 6th Mar 2023, 3:02 pm   #21
EF80TVVALVE
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Default Re: Baird T163

Further to yesterday's post I managed to get a further look into the T163. The LOP KT36 seems to lose the negative voltage on G1 which is causing the valve to overrun thus reducing the HT. Its behaviour is odd as it starts off just over -10v, the line stages begin to work and a glow can be seen from the EY51 then after a minute or so the voltage quickly reduces to almost 0v, the HT line drops and the EY51 heater extinguished.

I've attached the relative part of the schematic to this thread, after discovering this yesterday I didn't get much further as it was getting quite late. I'm trying to think of the possible cause which could make the G1 voltage dissapear from the KT36 after a short amount of time but it's leaving me stumped thus far. No leaky caps in the area which was my first go-to as I've had similar in the past, all the circuit seems to conform to the schematic too.

For reference both V8 and V15 are KT36, V14 = EB41, V13 = Z77
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Old 13th Mar 2023, 4:30 pm   #22
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Default Re: Baird T163

Well after some discussion I removed the lopt and gave it a ring test which it failed quite miserably at so presumably there is a short in the primary. It looks as though now I'll be looking for a replacement lopt or a rewind of this one. Probably the reason why the set was retired in the first place as it seems these weren't the most reliable transformers with replacement being offered by other companies at the time.
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Old 13th Mar 2023, 4:54 pm   #23
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Default Re: Baird T163

It's a strange thing for it to get going enough for the EY51 heater to strike up before failing if it was already faulty. I suspect the windings are full of damp if you didn't dry them out before power up. If you didn't (and you should) it may be too late and the damage has already been done.
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Old 13th Mar 2023, 5:03 pm   #24
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Default Re: Baird T163

Hi Techman, yes it is quite odd RE the EY51 glowing/failing though it does this with every power up, i.e. it's not glowed slightly just once and then failed to since. The circuit is self oscillating which personally I haven't had a great deal of experience of especially with LOPT failure.
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Old 13th Mar 2023, 5:13 pm   #25
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Default Re: Baird T163

Yes, I noticed from the circuit that it was self oscillating and I suspect that this is how you manage to get a repeat of the brief glowing of the EY51 filament from each power up even with a faulty transformer. Shutting the door behind the horse probably, but I would get DC power running through that transformer to give it a good drying and then try another ring test...and cross everything and hope!
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Old 13th Mar 2023, 5:49 pm   #26
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Default Re: Baird T163

Already done but alas no improvement at all. I'll hopefully get something sorted or failing that a re-wind. Still a gamble somewhat as the CRT is still unknown condition, I just hope it pays off and rewards me with good emission... I'll be keeping my fingers crossed.
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Old 13th Mar 2023, 6:08 pm   #27
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Default Re: Baird T163

Why not just test the tube?

Just a couple (or three) power supplies for a quick emission test. Heater power, HT, a bit of negative grid bias, some monitoring meters - you can add some EHT to get a bit of light on the screen, but it's not essential...that's what I do.
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Old 14th Mar 2023, 4:19 pm   #28
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Default Re: Baird T163

I'm quite far down the line now that even if the tube was flat I'd have to persue a modification to get it working, maybe with a mullard instead! It's just another hurdle I could do without but hopefully *if* the lopt failed back in the 50s and caused the set to be retired then the tube may be okay. Another collector has kindly offered up an untested Baird LOPT FOC so once its here I'll give it a ring test and dry it out and hopefully have a bit more luck.
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Old 24th Jun 2023, 4:56 pm   #29
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Default Re: Baird T163

Hi all,

Only a little update with this Baird tele. Mike Barker was happy to rewind the LOPT so it's currently with him which has given me some time to put the set to one side and work on many of my other projects. I have decided to do a little more this afternoon in attempting to make up a replacement mask for the set. I've done this by layering up lots of cardboard and gluing together so that it is around the same depth as the original mask. Today I've started to shape it somewhat, it looks rough but it is exactly how I was expecting it to look at this stage. Next I'll be strengthening up the card by applying some varnish or similar then I can lightly sand it back a little more close to the desired shape. From that point I'll use either some paper mache or filler (or both) to smooth the surface and then a final sand and shape before paint.

At some point I'll have to use the CRT face in order to shape the rear of the mask so that it fits snugly but being a Gec flat faced type I'm hoping this won't be too much trouble compared to say a Mullard or Mazda. I'm not the most crafty person but so far I think it looks like it's taking shape, I'm sure it'll look better after the next couple of stages.
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