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B&W TV Pye-ish 92 Rambler - 169 chassis.

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Doz
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Remember my Pye 169 with the Philips Lopt transplant? https://andydoz.blogspot.com/2016/03/pye-92-17-transportable-television-set.html   

Well, over the Christmas period I thought I'd take another look, as it looked to have developed a faulty lopt again.

After a bit of diagnosis, indeed the loptx looks to have failed again. 

 

After a period of about 20 minutes of running, the EHT disappears (not slowly fades, but instantly). The set keeps running, with no overheating of the PL504 or PY800. 

I had obtained a Thorn jelly-pot with a view to substitution, but a look at the diagram shows me this is going to be a lot of work ... I thought I'd try one thing first... 

What if the rectifier in my replacement philips lopt is failing short? (it's an aftermarket affair with internal solid state rectifier) 

I'll do what we used to do back in the old days with black and white portables, snip the anode lead and insert a stick rectifier. 

I soldered two TV13 rectifiers in series, as this set has about 16KV, and snipped the anode lead and dangled everything in place! 

Switched on .... it's fizzing slightly and there's a slight whiff of ozone, but it's only a lash up .... 

OK, after 20 minutes, presumably just as the internal rectifier gives up the ghost, there's suddenly a lot more fizzing and corona discharge. I had the EHT probe to hand and a quick measurement shows my EHT has risen to 20KV... along with a change in width and brightness. I think we'll switch off at this point. 

My question to you wonderful ladies and gentlemen is ....

1) Is my internal diode in such poor condition it's only managing to give me 16KV before failing short, and allowing the two TV13's to do their job once it's finally gone short?

2) Have I got some other fault present which is manifesting after 20 minutes which is causing my internal diode to fail, as it's suddenly being asked to do 20KV and breaks down? 

3) Does anyone have a TV20 they'd like to sell me?

 

Cheers, and happy new year,

Doz.

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Topic starter Posted : 03/01/2022 11:00 am
Nuvistor
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@doz 

Is there a tuning capacitor that’s going O/C after 20 mins or so? Not a lot of help but a thought.

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Posted : 03/01/2022 11:50 am
Doz
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My thoughts exactly. 

The Pye doesn't have a conventional lay out of the line stage. The harmonic cap is usually in parallel with the scan coils. In this case, there isn't one, but I'm thinking it's C73, although the value seems too high at 180nF  ... they were all replaced when I did the conversion, but that's not to say they're OK, as they weren't exactly in the first flushes of youth then!

Screenshot from 2022 01 03 15 29 38
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Topic starter Posted : 03/01/2022 4:35 pm
Doz
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That should read C74 - 180nF ... it's actually 150nF in this set (17") - and I've subbed it out - the fault is still present ...

C73? 22nF ? 

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Topic starter Posted : 03/01/2022 7:54 pm
Katie Bush
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Posted by: @doz

That should read C74 - 180nF ...

That makes more sense, even to my dense grey matter!

To be honest, I thought that was the one you meant - It just seems logical when you view the schematic.

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Posted : 03/01/2022 9:41 pm
Cathovisor
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@doz I'd have said the 180nF is the S-correction cap, and I'd be inclined to look at C72/72A/73. There's always a danger that an NOS component can fail, of course.

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Posted : 04/01/2022 3:54 pm
Doz
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For those of you who suffer with insomnia, here's a video I made of "Part 1" where I discover the high EHT.

Capacitors are on order !

 

 

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Topic starter Posted : 08/01/2022 2:37 pm
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Something of an avalanche no?

Is the construction of the replacement transformer "diode split"?

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Posted : 08/01/2022 9:31 pm
Doz
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Posted by: @sundog

Something of an avalanche no?

Is the construction of the replacement transformer "diode split"?

Somewhat ... I think I've sussed it. 

Unable to see if the replacement transformer is diode split or not. I presumed not, but I may be wrong. The Philips 210 original certainly wasn't, but as I say, this was an aftermarket replacement, which did away with the valve rectifier.

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Topic starter Posted : 11/01/2022 11:47 am
Doz
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OK, replacement of C73 - 22nF and C72 - 47nF have made no difference to the fault. I was even worried about the make up of the capacitors, to see if that was an issue, so I made some others up out of 3KV ceramic discs... made no odds... 

R80 (1M0), R81 (1M2), R85 & 86 (4M7) measure fine. I'm currently monitoring the voltage between the width VDR and the width pot to see if that changes when the fault manifests itself.

When the fault is evident the PL's Grid goes very slightly +ve at the top of the waveform, and the -ve peak is reduced. 

and , because this set hates me... it's been running now for about 35 mins without so much as a wobble on the EHT. Most it's ever run for is about 25 mins.

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Topic starter Posted : 15/01/2022 5:57 pm
Doz
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Anyone any ideas? I'm fresh out!

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Topic starter Posted : 18/01/2022 8:21 pm
Sundog
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@doz OK here's my guess, which if correct, is not good news!

You've said the windings get quite hot, this could be because of some compatibility issue.

As the windings get hot, so does the core, who's effect on the inductance will change. As the inductance reduces, so does the impedance, and more current flows, heating the core further.

So maybe it's simple thermal runaway?

Here's a paper on the subject that google showed up: http://przyrbwn.icm.edu.pl/APP/PDF/133/app133z4p65.pdf

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Posted : 18/01/2022 8:58 pm
Doz
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Posted by: @sundog

@doz OK here's my guess, which if correct, is not good news!

You've said the windings get quite hot, this could be because of some compatibility issue.

As the windings get hot, so does the core, who's effect on the inductance will change. As the inductance reduces, so does the impedance, and more current flows, heating the core further.

So maybe it's simple thermal runaway?

Here's a paper on the subject that google showed up: http://przyrbwn.icm.edu.pl/APP/PDF/133/app133z4p65.pdf

Perhaps hot was a bit too extreme, warm yes... That's an idea except, the EHT suddenly goes sky-high. It's not a gradual thing.

The other interesting fact is the width doesn't change.  

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Topic starter Posted : 19/01/2022 10:31 am
Sundog
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@doz Fair enough.

If the EHT alone was increasing, the width would appear to reduce. So both EHT and line drive are increasing proportionately.

What happens to the boost voltage when the change occurs?

Can you point me towards a full schematic?

 

 

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Posted : 19/01/2022 8:53 pm
Nuvistor
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@sundog 

The data library has both the L Larry-Johns TV Servicing and R&TV information, both have circuits and lots of other info.

 

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Posted : 19/01/2022 10:45 pm
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Doz
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Posted by: @sundog

@doz Fair enough.

If the EHT alone was increasing, the width would appear to reduce. So both EHT and line drive are increasing proportionately.

What happens to the boost voltage when the change occurs?

Can you point me towards a full schematic?

 

 

As Nuvista states, there's a manual in the library. I'll measure the boost this evening. 

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Topic starter Posted : 20/01/2022 10:22 am
Doz
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Posted by: @doz
Posted by: @sundog

@doz Fair enough.

If the EHT alone was increasing, the width would appear to reduce. So both EHT and line drive are increasing proportionately.

What happens to the boost voltage when the change occurs?

Can you point me towards a full schematic?

 

 

As Nuvista states, there's a manual in the library. I'll measure the boost this evening. 

You can see the line drive waveform change at 27 mins 11 seconds in on my video

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Topic starter Posted : 20/01/2022 3:30 pm
Sundog
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@doz I'm not even sure whether measuring the boost voltage will indicate anything much. It's bound to go up in fault mode isn't it? But if it stayed the same or went down, that would be curious.

Looking at the line drive circuitry I see they found the need for width stabilisation using a VDR. 

Thinking aloud here: Negative feedback works well as long as the phase shift is within limits. Once beyond, it becomes positive feedback.

You mention that the grid goes a little positive, but the purpose of that feedback through R85,R86, C71 is to drive the grid more negative as a control.

Sorry for nothing positive, just a few thoughts.

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Posted : 20/01/2022 5:08 pm
Doz
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@sundog 

 

Any thoughts are helpful at the moment. I will measure the boost anyway, as nothing is really making much in the way of sense anyway! I did measure the voltage at the end of the VDR that feeds the width control, and that stays steady at about 36V

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Topic starter Posted : 20/01/2022 5:22 pm
Doz
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Clearly something is VERY thermal! I'm going to have to sell off a kidney and go and buy some freezer spray ...

 

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Topic starter Posted : 21/01/2022 10:25 am
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