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Old 24th Feb 2021, 4:56 pm   #1
matspar
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Default Baird T164 portable no EHT.

Afternoon all,

It looks like I've had to admit defeat on this one so its time to call in the big guns for some advice.

My Baird Portable that I restored last year was working very nicely until the width potentiometer VR4 decided to have a little burn up and the EHT disappeared. With this replaced EHT was still not forthcoming so at the time I came to the conclusion that the LOPT was at fault.

A few months later another LOPT was sourced and this along with the original were tested on a bench Lopt tester and found to be ok, although I have not yet performed a ring test.

With this in mind I am now suspecting the line output valve (V14) to be at fault, which is the self oscillatory 20P1. The problem I have is that the valve refuses to oscillate, so no line drive or EHT.

Voltage readings are showing 128v on pin 4 when the circuit quotes 76v. The anode voltage will only rise to around 150v.

I have tried 5 different 20P1s, and today tried a brand new old stock item with exactly the same results. The Anode voltage will rise to HT potential with the top cap disconnected.

I'm satisfied wth all the other components in the line stage. All capacitors have been replaced and all resistances within spec. The line hold pot is intact, HT is present, visconol taken out of circuit. I have been through it countless times and cannot find anything incorrect.

I've ran out of ideas now. What am I missing on this one chaps? is the 20P1 one of those fussy valves? am I looking in the wrong place?

Any thoughts more than welcome!
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Old 24th Feb 2021, 6:19 pm   #2
ronbryan
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Default Re: Baird t164 portable no eht

If the currently installed transformer has never oscillated in your set, have you tried reversing the phase of the feedback winding to see if it will then oscillate.

I have had a re-wound transformer on a Townsman (same chassis as the portable) where the windings were reversed compared to the Baird original. It was the Series II circuit however, which has a slightly different line output stage to the version you have posted.

Ron

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Old 24th Feb 2021, 8:08 pm   #3
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Default Re: Baird t164 portable no eht

Hi Matt,
That's an interesting circuit - I've not come across that arrangement before!

Presumably, the valve in combination with the transformer forms a combined oscillator and output stage? Just using the one valve? That seems a bit too cunning for its own good!

Where do the line sync pulses get in? I guess almost anything will cause the oscillations to be damped out which makes fault finding a bit hard! You've obviously done everything that's, well, obvious. My only suggestion would be continuity of windings & interconnecting wires including the line coils.

It's bound to be something silly if you are confident of the other components. Like you, my first thought would be LOPT, but if it tests OK on a tester, that's less likely. It would be interesting to 'ring' it and see what the trace looks like. That would at least show if it was excessively damped.

Mind you, I've had issues with LOPT's that 'ring' fine, but suffer insulation breakdown as soon as the line stage starts to work. This isn't always obvious either if the breakdown is next to the core.

I'll be interested to see what the cure is!

All the best
Nick
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Old 24th Feb 2021, 8:23 pm   #4
stevehertz
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Default Re: Baird t164 portable no eht

Sorry I'm not contributing to the fix, but just to say that I had a Baird Everyman with the same line generation circuitry, and the resultant picture was almost ludicrously unlinear. The test card circle looked like an egg!
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Old 24th Feb 2021, 8:33 pm   #5
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Default Re: Baird t164 portable no eht

There are two versions of the line output transformer, they are not interchangeable without circuit mods. Found this out in the past, might be same scenario.
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Old 24th Feb 2021, 8:54 pm   #6
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Default Re: Baird T164 portable no EHT.

Hi Matt.

Interesting...... don’t be shy checking those new capacitors you’ve fitted. I’m one of many, I suspect that turn a blind eye to new components....

‘It can’t be that as it’s new.....’. Wrong!!

What caused the focus control to burn up?

Let’s hope it’s something simple��


Cheers.

SimonT.
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Old 25th Feb 2021, 11:55 am   #7
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Default Re: Baird T164 portable no EHT.

Hi Matt,
I with thermionic on this one.
As you know my Garric currently has a later Baird transformer fitted at present. It looks a little different to yours as it does not have the paxolin panel on the side, but simple solder tags the suspend the EY51 in the air.
I'm attempting to recreate the original transformer (similar concept to the 164 version, but without wavewinding and no EHT section), but I've not got it quite right yet. However I had not proplem even getting that one to oscillate.

When I do get it right, you can have the 164 style transfomer that fitted to the set at present.

Cheers
Andy
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Old 25th Feb 2021, 12:04 pm   #8
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Default Re: Baird T164 portable no EHT.

Thanks Gents I will be having another session later so I'll see if I can work any magic.

Stephen - the Lopt fitted when I first aquired the set was a later replacement type which was poorly fitted and from what I remember was wired slightly differently. The one in place now is the correct type and is wired exactly as shown in the circuit.

Nick - I will go on the hunt for some line sync pulses and see if they are absent or getting lost somewhere. I'm not convinced this is Lopt trouble (yet!) and I'm sure it probably is something silly that I haven't yet stumbled upon. The circuit is very simple really.

Ron - are you able to pinpoint the feedback winding on the circuit?

And finally Simon/Andy I have lifted out several of the new caps and checked them just in case, but haven't yet found any offenders. I've read that the wirewound pots on these aren't fantastic and can fail spontaneously. It didn't actually go o/c but it would seem that something caused it to pass excess current.

Thanks again! if anyone has any further thoughts do let me know. All very much appreciated.
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Old 25th Feb 2021, 12:47 pm   #9
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Default Re: Baird T164 portable no EHT.

Looking at my notes, there were numerous component changes between the lopt versions, the reason to mention it initially was because the exact same failure to oscillate issue on a set someone had tried to replace the lopt but didn't realise there were component mods involved. Once the changes were implemented in this case it was fine.
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Old 25th Feb 2021, 1:40 pm   #10
ronbryan
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Default Re: Baird T164 portable no EHT.

Quote:
Ron - are you able to pinpoint the feedback winding on the circuit?
The feedback winding on the Series II lopt is connected to the left-hand and right-hand tags shown on the attached photo of the faulty unit. The feedback winding must have shorted to the anode winding and burnt out.

As I said in post #2, the replacement (re-wound) transformer had the feedback winding phase reversed, so I had to swap the wires around to make it oscillate.

Reversing the phase of the feedback was a suggestion for you to try if you have nothing to lose, other than search for another lopt, but as stated, my experience was with a Series II model, with the scan coils fed from the anode winding, not from the control grid (pin 5) circuit.

The second photo shows how the original transformer that failed was wired and the associated components on the tag-board. It is not relevant to the Series I circuit, if that is what you have.

Ron
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Old 25th Feb 2021, 2:15 pm   #11
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Default Re: Baird T164 portable no EHT.

Ron thats most helpful thanks for that!
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Old 25th Feb 2021, 2:47 pm   #12
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Default Re: Baird T164 portable no EHT.

This was the lopt that was previously fitted when the Baird was working.

This wasn't the original however as it was badly fitted and when I first got the set I could see the 'outline' of the original through the dirt on the chassis.

I think I'm right in saying that the circuit diagram I have is correct for the type currently fitted. My set doesn't have a line linearity control which I know the later ones had.
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Old 25th Feb 2021, 6:24 pm   #13
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Default Re: Baird T164 portable no EHT.

Try as I might, I cannot get the replacement lopt to produce any EHT and I can't identify what version it is, nor do I have anything detailing the mods (or it it's actually any good for that matter!)
I refitted the original one and this has produced a pitiful 2kv of EHT and a very dim raster, so there has been some slight progress at least. The HT appears to be only around 220v which strikes me as a bit low, but appears to agree with the circuit.
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Old 25th Feb 2021, 6:43 pm   #14
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Default Re: Baird T164 portable no EHT.

Just looked, the later version circuit is near the bottom of the genuine manual on Jon's website.
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Old 25th Feb 2021, 9:14 pm   #15
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Default Re: Baird T164 portable no EHT.

Hi Matt,
Have you tried unsoldering the anode of the EY51 and sticking an EHT diode in the air, to see if the heater winding is breaking down?

Cheers
Andy
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Old 25th Feb 2021, 9:55 pm   #16
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Default Re: Baird T164 portable no EHT.

I haven't done so yet Andy, I will look for a suitable diode tomorrow and try.

I did replace the EY51 as it looked old and tired and I had a NOS one to hand.
With the line stage now trying to function I went through all my other 20P1 valves (6 in total) to see if any would provide an improvement.
One would not produce any EHT whatsoever, 4 will generate around 2.5kv and one brought it up to 3.5kv.

I'm currently passing a bit of DC through the overwind to see if this has any effect.
I replaced the bad width pot with a 270R (closest big wirewound value I had) whereas the original was 500R. Not sure if this difference in value isn't helping matters so I will source one of the correct value.
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