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Old 17th Aug 2019, 3:49 pm   #1
EF80TVVALVE
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Default Pye LV20

Hi all,

I've been working on this LV20. I like these PYE sets, they always seem to work very well although the layout of the set doesn't make it exactly plain sailing to recap as they are hiding everywhere, but the results are always well worth the restoration, this is actually the third set I've completed of the B18/20 series of sets.

I've got the set going, it has had a full recap as I'm no fan of keeping any of the old types. I think even the 70 year old electrolytics probably aren't up to the job and even if they test fine now, it would be a shame for them to fail soon after restoration possibly leading to even more work so I just swapped the lot, although I kept the original MICA caps in place as these always seem to be fine.

I've had a few dreaded problems with the sliders. Firstly vision sensitivity would only work in the most extreme position, causing the set to be overloaded with signal. I bridged in a 10k and turned the slider right down just for now as that mocks up a low position setting that will give me a fairly decent input level.

The other is the brigtness, I have found a spot that gives me an okay(ish) picture, but anything over half way seems to baloon the picture and it goes extremely bright and washed out, or the picture almost disappears. A bright line about 2 inch begins rolling down the screen scanning down the test card. I'm wondering if there is any way to replace these sliders without losing the look.

Overall quite pleased with this one, it obviously had a long service life as the screen isn't the greatest and there is some ion burn but really it's working excellent for its age.

Some initial photos of it. Overall around 40 caps replaced, no resistors as of yet but a few will probably come out as I get fettling it to a good standard.
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Old 17th Aug 2019, 4:27 pm   #2
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Default Re: Pye lv20

Many years ago an old TV engineer told me how to rub a graphite pencil along the tracks of worn pots. Ok, they're sliders that you have there, but no reason why the principle shouldn't work just as well.
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Old 19th Aug 2019, 8:41 am   #3
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Default Re: Pye LV20

Quote:
Originally Posted by EF80TVVALVE View Post
I've been working on this LV20. I like these PYE sets, they always seem to work very well although the layout of the set doesn't make it exactly plain sailing to recap as they are hiding everywhere, but the results are always well worth the restoration, this is actually the third set I've completed of the B18/20 series of sets.
I too have restored a few of these sets and have been lucky with the wirewound pots, although I have one in the roundtuit pile that will need replacements fitted.

There seems to be two types of these, cylindrical or flat types. I have successfully rewound similar pots on an HMV 1807, but it is a fiddly job!
It is a real pain to remove/replace the sliders on the Pye chassis, as the connecting wires are so short and in the deepest part of the chassis

Your set appears to have a good picture, I would not worry too much about the ion burn, it was a common problem back when these sets were in use.
I find it does become less noticeable as the CRT wakes up with use.

Mark
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Old 19th Aug 2019, 2:14 pm   #4
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Default Re: Pye LV20

Ah right, graphite won't work on wirewound pots, so forget that suggestion.
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Old 20th Aug 2019, 10:37 am   #5
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Default Re: Pye LV20

Well keeping the set on for a few hours seems to have woke up the CRT a bit more, the picture isn't too bad at all now. EHT does seem a bit lower than it should be but the EY51 looks roasted so I'll fit a replacement and see how that improves things.

Mark, I was thinking of attempting to rewind these but it will be fiddly, they are the cylindrical Welwyn types so that does give me some advantage. I do recall a forum member replacing these by cracking open a ceramic wirewound and fitting that but this was a long time ago and I can't really recall the thread.

A good tip Steve, shame it won't give me the results on these wirewounds but a good tip worth remembering none the less.

Here's a photo I took of it back in the cabinet, it's performing quite well now. A few small jobs to do but overall I'm very happy with this little set.
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Old 20th Aug 2019, 10:41 am   #6
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Default Re: Pye LV20

Excellent picture!
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Old 20th Aug 2019, 9:10 pm   #7
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Default Re: Pye LV20

Quote:
Mark, I was thinking of attempting to rewind these but it will be fiddly, they are the cylindrical Welwyn types so that does give me some advantage. I do recall a forum member replacing these by cracking open a ceramic wirewound and fitting that but this was a long time ago and I can't really recall the thread.
This was how I rewound my HMV pot, it certainly was a fiddly job though!
I used epoxy resin to hold the windings in place.

Glad to see the improvement in the CRT, good old Mullard's always seem to wake up and give a nice bright picture.
The ion burn also gets less noticeable too.

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Old 21st Aug 2019, 8:24 pm   #8
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Default Re: Pye LV20

I've had the set running for a few hours this evening. I've fitted a new EY51 which has improved things a little more. After around 2 hours the LOPT was fairly warm/hot to the touch and the focus began to go off a bit. Just wondering if this is a sign of some moisture in the LOPT? It did look like some of the black pitch has gone a bit drippy when I opened to replace the EY51 but it looks to have happened a long time ago.

Any tips on driving out the moisture if this is the case? I don't want to strain it too much and end up with it failing.
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Old 21st Aug 2019, 10:43 pm   #9
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Default Re: Pye LV20

Passing a current through the overwind may drive out the moisture in a couple of days.

My LV51 loses focus after an hour or so, but it appears to be quite common problem with these LOPT's

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Old 22nd Aug 2019, 2:50 pm   #10
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Default Re: Pye LV20

That looks a real nice job you have done there. The B18T series is capable of giving an excellent picture but unfortunately 70 years on the LOPT, in a lot of cases has lost it's 'Zing' giving reduced EHT. Your example looks a cracker. At least they have an outstanding Mullard 9" tube many of which were sourced from the Netherlands. Good luck with it! John.
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Old 22nd Aug 2019, 3:28 pm   #11
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Default Re: Pye LV20

Surely just using the tele for a short amount of time each day will warm the LOPT in just the same way as passing current through the overwind and have the same drying effect? Warmth chases out damp.
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Old 22nd Aug 2019, 4:18 pm   #12
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Default Re: Pye LV20

Quote:
Originally Posted by stevehertz View Post
...Surely just using the tele for a short amount of time each day will warm the LOPT in just the same way as passing current through the overwind and have the same drying effect?...
This was certainly the case in a GEC early 60s set that I restored a few years back.

Very nice picture, by the way
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Old 22nd Aug 2019, 5:08 pm   #13
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Default Re: Pye LV20

Thanks for all of the replies, I'm very pleased with this one. It has worked a treat and I've not had many problems at all with it, other than the sliders everything is working well.

I think I will just run it for a short time every day or so, I'll remove the side from the can just to help vent the LOPT a little better. It doesn't seem to be too strained so I think it will be fine to dry it out in situ.

The EHT seems to be at a good level still, despite not having actually measured it I'm not having any symptoms of low EHT although it definitely drops after a long run due to the dampness of the TX.

I've noticed some rusting on the sound output, I've changed all of the associated caps so it may be the output valve which I'll replace with another and see if that cures it, if not I'll try and trace its origins.
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Old 22nd Aug 2019, 6:42 pm   #14
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Default Re: Pye LV20

Quote:
Originally Posted by stevehertz View Post
Surely just using the tele for a short amount of time each day will warm the LOPT in just the same way as passing current through the overwind and have the same drying effect? Warmth chases out damp.
I agree Steve but for some unknown reason it does not work in stubborn cases.
If very damp it can lead to breakdown of the overwind before it has dried out.
I did a lot of research with a Ferguson 988T many years ago and posted the notes.
It appears that it needs to be warmed from the inside out so to speak. I think dampness can be trapped between the tar/pitch/ wax and the winding itself etc and can be difficult to remove with outside heating. It is very deep set having taken decades to seep into the windings and insulation material.
Drying out a narrow neck bottle comes to mind. Just my thoughts. John.
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Old 23rd Aug 2019, 8:27 am   #15
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Default Re: Pye LV20

Quote:
Originally Posted by Heatercathodeshort View Post
Quote:
Originally Posted by stevehertz View Post
Surely just using the tele for a short amount of time each day will warm the LOPT in just the same way as passing current through the overwind and have the same drying effect? Warmth chases out damp.
I agree Steve but for some unknown reason it does not work in stubborn cases.
If very damp it can lead to breakdown of the overwind before it has dried out.
Hi all,

I have no proof on this but my idea is that with the set in service you have between 13-15kv across the overwind which means there are several hundred volts between adjacent turns deep within the LOPT, the fact too that the insulation around these windings is damp contributes towards a likely breakdown between them.

I believe that to use a DC current from a bench PSU to drive out the damp from the insulation and windings is the way forward because you only need 20-30v to pass enough DC current through the overwind to generate a moderate amount of heat to dry it out, this also means that voltages between adjacent windings are negligible therefore greatly minimising any risk of developing a shorted turn.

Cheers
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Old 24th Aug 2019, 1:33 pm   #16
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Default Re: Pye LV20

Hi all,

Yes I suppose it is more safe to remove the lopt to dry it out. I think really it is down to the lopt construction and actually how bad it is. I'm not seeing any concerning problems relating to it so I don't think it is too bad so I think with this one I'll leave it in the set. I've seen some restoration threads where the lopts have really suffered resulting in picture problems and poor performance but this seems nowhere near as bad as those. I'll just run it lightly for now until it has dried out.

I'm thinking of changing the sliders for wirewound pots, just wondered if anyone else has done this? Spacing in the set is going to make it a little difficult but it should be possible to do. Anyone know what wattage should be suitable? I'm going to try and measure the power across the resistors but I may not be able to devote any time to it over the bank holiday weekend.

Also noticing some 'fuzzyness' on the picture, almost as if the line scan is slightly oscillating side to side as it scans across the screen. I've seen this on some other sets I have previously owned so it may just be the PSU intefering with the aurora. I'll probably buy a better one first before trying to fault find something that may not be the set.
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Old 24th Aug 2019, 8:14 pm   #17
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Default Re: Pye LV20

Quote:
Yes I suppose it is more safe to remove the lopt to dry it out. I think really it is down to the lopt construction and actually how bad it is. I'm not seeing any concerning problems relating to it so I don't think it is too bad so I think with this one I'll leave it in the set. I've seen some restoration threads where the lopts have really suffered resulting in picture problems and poor performance but this seems nowhere near as bad as those. I'll just run it lightly for now until it has dried out.
I think it is a wise decision not to remove the LOPT, it is a real br to remove.
Connect up your bench PSU with it in situ, it will need drying out, I had no luck with running my set daily in the hope it would get better with use.

Quote:
Also noticing some 'fuzzyness' on the picture, almost as if the line scan is slightly oscillating side to side as it scans across the screen. I've seen this on some other sets I have previously owned so it may just be the PSU intefering with the aurora. I'll probably buy a better one first before trying to fault find something that may not be the set.
Try running the Aurora on a 9v battery, this should show if the problem is the PSU or not.
I have had similar problems on this chassis that was caused by dodgy EF50's, substitution cleared the fault.
Also make sure the valve pins & the sockets are clean, the sockets on these sets are not the best quality.

Mark
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Old 26th Aug 2019, 10:24 am   #18
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Default Re: Pye LV20

Good idea regarding the aurora and 9v battery, I'll give it a go. I'm just looking at potentiometers to replace the sliders at the moment, is there any reason to NOT fit carbon types in place of the sliders? I was going to replace with wirewounds but the most that I can find are those 10 turn precision types which will be a bit of an overkill really. Found some nice single turn carbon types rated at 2w which I may go for as they look just right.
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Old 27th Aug 2019, 9:20 am   #19
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Default Re: Pye LV20

Unfortunately neither the Pye or trader sheets give any wattage ratings for the pots.

One of my unrestored sets has been modified with rotary pots on a metal plate fixed to the rear of the cabinet to replace four of the sliders, these are largish wirewound pots that are physically too big to fit in the space where the sliders are fitted.

I would just tack on the replacement pots and test before going to the trouble of mounting them permanently.

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Old 28th Aug 2019, 5:57 pm   #20
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Default Re: Pye LV20

Hi,

Pye slider pot repair:

The easiest way I've found to repair these pots is with Electrolube SCP (Silver Conductive Paint). It's adhesive as well as being conductive and sets rapidly.

The ceramic tubular ones unclip easily after the tags are disconnected and the slab type unscrew. Clean gently and identify the break with an ohmmeter (and magnifier!). After a good shake apply a small (about 1mm) blob beside the break. When set trim off any loose ends. The paint is a bit pricey but you only need tiny amounts. It's widely available - eg RS 101-5621.

For when the pot winding is beyond repair I'm investigating the use of a ceramic resistor. I've recently bought 2 10k 10w resistors (for 99p) from dmcomponents2011 on eBay. The outer ceramic sleeve is easily broken off to reveal the resistive element inside of just the right length. The problem now is how to physically mount it between the clips - a ptfe or perspex bar is my current idea. A work in progress...

Ian
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