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

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Sundog
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@doz I've been thinking about the circuit pathology and think I might understand it.

The negative feedback path is through C71. The top of the pulses cause the VDR to conduct, so the average DC becomes more negative on the top of the VDR, thus driving the grid more negative.

The current through resistors R85 & R86 oppose this, causing a slight rise in average grid voltage. Thus the balance of values of C71 and the resistors is critical.

It would be interesting to disconnect the resistors and see the result. My hunch is that if you increase the resistor value or the capacitor value, the problem might go away.

 

Having said all that, it still might be a simple fault such as the VDR or preset pot!

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Posted : 21/01/2022 7:27 pm
Doz
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@sundog Lots of fun this morning, trying out some new freezer spray (on the cheap) and trying out your suggestion of lifting the feedback chain. 

I think I've got conclusive proof that the VDR is thermally shot. 

I'll make a post in wanted. It's a E298/ED/A265 according to the diagram. Bound to be unobtainiun, knowing my luck.

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Topic starter Posted : 05/02/2022 2:36 pm
Sundog
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@doz Freya posted this document some time ago. Whilst not identical types, I do remember the black/blue tip being in line circuits. What colour band(s) has yours?

100104=19434 mullard20vdr

 

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Posted : 05/02/2022 6:18 pm
Doz
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@sundog Beige body with a white end. There's a close up in the video that I'm just rendering.

 

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Topic starter Posted : 05/02/2022 7:22 pm
Sundog
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@doz And what is the type in the TV who's output transformer you used?

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Posted : 05/02/2022 7:39 pm
Doz
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Topic starter Posted : 05/02/2022 8:20 pm
Doz
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It's R2169 on the diagram (Philips 300 chassis), but doesn't name a type 😣 

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Topic starter Posted : 05/02/2022 8:23 pm
crustytv
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Posted by: @sundog
 
And what is the type in the TV who's output transformer you used?
 
Posted by: @doz

but doesn't name a type

Hi guys, I have the Philips 300 series manual, and in the part's section R2169, the VDR, is the same type as the one you have in your PYE, an E298.ED/A265. As you can see they are rated for 910V

20220205 213521
20220205 213503

I've all manner of NOS E298 series VDR's in stock, and plenty of these A265. PM me your address and I will get one off in the post to you. Just hope RM don't go and lose it.

20220205 202905
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Posted : 05/02/2022 8:51 pm
Doz
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@crustytv Thanks Chris!

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Topic starter Posted : 06/02/2022 2:45 pm
crustytv
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It's posted, guaranteed delivery tomorrow A.M.

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Posted : 07/02/2022 3:20 pm
Doz
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Thanks Chris. It arrived safely this morning. 

Hopefully I get back in the workshop before the end of the week.

Place your bets 😆 

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Topic starter Posted : 08/02/2022 3:26 pm
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Doz
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Yipeeee! VDR in, EHT was sky-high at switch on , which is a change. Adjustment of the width control brought things down under control to 14 and-a-bit KV , where it stayed for hours. 

Boost settled itself down to 660V , and was steady. 

IMG 20220212 191227
IMG 20220212 191217 1 2

I've now got some work to do as I have some disturbance to the line to iron out - it's probably just lead dressing around the replacement transformer, as I buggered about with it somewhat. 

Thanks to Chris @crustytv for the VDR and @sundog for your thoughts! I'll post the last video up when I've sorted the noisy verticals!


 

Admin Note : Off-topic posts split off here

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Topic starter Posted : 12/02/2022 8:56 pm
Doz
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Bingo .... 

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Topic starter Posted : 14/02/2022 10:32 pm
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Sundog
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@doz I think you were very brave even to attempt the LOPT transplant, and really glad to see it's now working well.

I still have my suspicions though. VDRs will have a manageable temperature coefficient but a heat gun is a pretty blunt instrument. I'm not suggesting you try, but I wonder if you were to do the same to the new VDR, whether there would be a step-change in EHT.

The reasoning behind this thought is that by changing any component (that has tolerance) in a feedback loop it alters pole placement, and thus the stability of the system.

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Posted : 15/02/2022 12:09 pm
Doz
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Posted by: @sundog

@doz I think you were very brave even to attempt the LOPT transplant, and really glad to see it's now working well.

I still have my suspicions though. VDRs will have a manageable temperature coefficient but a heat gun is a pretty blunt instrument. I'm not suggesting you try, but I wonder if you were to do the same to the new VDR, whether there would be a step-change in EHT.

The reasoning behind this thought is that by changing any component (that has tolerance) in a feedback loop it alters pole placement, and thus the stability of the system.

It ran for 3 hours with the back on last night..always the acid test !  I'm convinced.

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Topic starter Posted : 15/02/2022 12:25 pm
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