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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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22nd Jul 2006, 4:42 pm | #1 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Pye V4 Restoration
So far, I have got to the stage of a raster modulated with test card C via Aurora. The raster is full size and linear. The focus control does not quite go far enough for good focus. Is this tired ferrite focus ring magnets? If so, what next? The raster is very dull. Over volting the CRT heater to 8 volts improves brightness a little, but still not enough to be really viewable. The CRT is a 14 inch rectangular Mullard. Since I have a good line whistle and the EY51 is glowing reasonbly and since I cannot measure EHT, I plan to check the cathode current of the CRT at maximum available brightness hoping this will give me a clue as to CRT emission. Any ideas as to what current to expect? Also, I find that the usual sources of valves are not good for TV tubes, because of storage space problems. Where might I look for a replacement or for re-gunning?
Sorry for all the questions. Nigel |
22nd Jul 2006, 6:29 pm | #2 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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Re: Pye V4 Restoration
The bad focus, dim picture and a raster easily filling the screen points to low EHT rather than a bad tube.
I don't know this set. I'm presuming it's too new to have an ion trap magnet that could need adjusting. What happens when you turn the brightness up ? If the picture 'balloons', the eht regulation is poor - first replace the EY51. If the picture goes flat and tends toward negative it could be the tube. How hot does the LOPT run (check this after switching off!) ? If it's hot you could have shorted turns. You should expect more than 100 microamps emission from a serviceable tube, unless the EHT is low. So clarify the EHT situation first. If the tube emission tests low, leave it on for a few hours and see if it improves. Steve |
22nd Jul 2006, 6:57 pm | #3 |
Retired Dormant Member
Join Date: Oct 2003
Location: Halesworth, Suffolk, UK.
Posts: 413
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Re: Pye V4 Restoration
As the back is off of my V4 I have just glanced inside.
It does have an ion trap, so do as Steve says and try a wiggle, if you have not already done so, its a Mullard MW36/24 tube so it might be ok after a bit TVden Last edited by Mike Phelan; 26th Oct 2007 at 8:22 am. Reason: Caps & punctuation added. |
23rd Jul 2006, 5:22 pm | #4 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Re: Pye V4 Restoration
Hi to Dennis and Steve,
After running the set for a few minutes, the EHT overwind on the LOPT is almost too hot to touch, so it looks as if that is at least part of the problem. It is probably easier to get an LOPT rewound than to find a CR tube, so I will go for that first. By the way, yes, I do know about ion trap magnets and that is not the source of the problem this time. Thanks for the help and advice. Nigel |
25th Jul 2006, 7:31 am | #5 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Pye V4 Restoration
Hello Nigel,
Drat that LOPT! I hope you can get that rewound but it may well be easier to get a MW36-24/44 tube or equivalent.. Another point with the V4. Try not to disturb, in any way the scan coils. The original units were insulated between the line and frame coils with thin cellophane film. After a period of time this became brittle [like a sweet paper in the fire] and breakdown occured between the windings. This was not repairable. The replacements had polythene insulation. Any Pye scan coils suitable for a 70 degree tube can be used but you may have to trace the connections. Replacements for the V4 LOPT were made well into the 1960's. Regards John. |
25th Jul 2006, 8:36 am | #6 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Re: Pye V4 Restoration
Dear John,
Thanks for the advice, particularly about the scan coils. I do need to do a complete strip and re-finish on the cabinet, but had decided to leave this until the set was working. I will now see if I can do the cabinet work with the tube and scan coils in situ. As to the LOPT, I feel as though the only way to be sure the present one is faulty is to rewind or replace it. The EHT winding certainly runs hot. Given that spares were made into the 60s, do I take it that the V4 LOPTs were fairly unreliable? Cheers Nigel |
25th Jul 2006, 10:01 am | #7 |
Heptode
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 988
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Re: Pye V4 Restoration
Hi Nigel,
I have only just noticed this thread. I have spare Mullard 14" tubes and a complete V4 chassis in my parents loft. I'll send you a PM. Cheers Andy |
25th Jul 2006, 8:06 pm | #8 |
Retired Dormant Member
Join Date: Jul 2006
Location: Bournemouth - near the Sea :o)
Posts: 5
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Re: Pye V4 Restoration
Nigel,
I have restored a couple of V4 / VT4's and LOPT trouble has seemed common in both restorations so I am not surprised that yours is faulty. I scrapped a V4 recently but I believe the LOPT is good, I also have the CRT too. If you need either then you can collect from Bournemouth ( I can post the LOPT) Dave N |
25th Jul 2006, 10:00 pm | #9 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Pye V4 Restoration
Hello Nigel,
The V4 LOPT was not unreliable as such and a lot were changed when other faults were the cause of the problem. [a bit like valves] The reason for failure is simply damp that has been ingested into the tar and wax insulation, the only insulators that were available at the time. Daily switch on dried this out but these sets have probably not been switched on for 40 years and a lot of moisture has been absorbed into the windings. The interleaving tissues have also broken down and the V4 has a lot of taps and sub windings as such. Its just an age thing...Yes its a bit like the bits that fall off over 55 year old ex service engineers.... Regards John. |
27th Jul 2006, 8:52 am | #10 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Re: Pye V4 Restoration
Hi to all.
To Dave and Andy - I have sent PMs. Thanks for the offers of spares and parts. It is great news that there are some CRTs about. As a progress report, I am getting a brighter picture the more I run the set. It seems that the tube is recovering in some way and is on its way to being bright enough to watch in dim light. I am also getting less suspicious about the LOPT. I was at least partly wrong about it getting hot as this occurred when I was running the chassis on its side with the Line O/P and booster diode under the LOPT. I was reluctant to test EHT by drawing a spark as I thought this unkind to the EY51. However, I had a go and can draw a spark of more than a quarter of an inch. Can anybody advise on what spark length corresponds to 14 KV?! Cheers Nigel |
27th Jul 2006, 9:52 am | #11 |
Dekatron
Join Date: Nov 2003
Location: Near Leeds, West Yorkshire, UK
Posts: 4,609
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Re: Pye V4 Restoration
Nigel
That really depends on how sharp the electrodes are, the humidity in the atmosphere, and many other imponderables. We used to have a "Radar Kilovolter" - a calibrated spark gap, for measuring EHT, but it was really quite useless and just a gimmick. If you are repairing a few TVs it may be worth making an EHT meter, but I have never found one to be necessary. The raster will be too large if EHT is low. Good to hear that it has brightened up a bit - I have found that unused TVs sometimes do this when brought into use; think it is the CRT that causes it, but not sure how. If the overwind did have shorted turns, I think it would kill the LOP stage totally. The best thing about the V4 was the ease of cleaning the screen!
__________________
Mike. |
27th Jul 2006, 4:46 pm | #12 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Re: Pye V4 Restoration
Dear Mike,
I am gradually clearing a number of 'simple' faults. I now have control of contrast, where previously the contrast control did nothing. One wretched moldseal was hidden where I had not spotted it. It is the one which sends the black porch sampling timing pulse to the contrast and AGC valves, so it wrecked that part of the circuit. On initial switch on and warm-up, the raster is cramped at top and bottom, more on the left than the right. It does not respond to frame linearity adjustment. Height adjustment just pushes more of the raster into the cramped area. (I have replaced the frame oscillator and O/P valves.) After a few minutes running, the picture fills the screen nicely. The width control is more or less in the centre of its range. I am beginning to trust the LOPT, but in view of Heatercathodeshort's witches warning, I am wondering if I have an incipient scan coil problem. As regards brightness, which is not yet up to par, one of the next logical steps would be to try a different CRT, but I don't know if I dare remove the scan coils. I am not totally freaked by the prospect of fitting scan coils from a later 70 degree Pye set if necessary, but it would be good for my V4 to be original where possible. I've already changed about 50 capacitors! Ain't this fun. It's a hell of a sight bigger challenge than a five valve radio! Anyway, thanks to all for the encouragement so far. I sure ain't ready to give up by a long chalk. Regards Nigel |
27th Jul 2006, 10:24 pm | #13 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Pye V4 Restoration
Hello again Nigel,
That LOPT may well have 'recovered' from the damp problem. I have known this on several occasions. The damp takes months or years to enter the windings but is difficult to expel. It may be that the warm weather and the recent switch on has dried out the windings, particularly the overwind. If the EY51 appears to be lighting at the correct temperature and if your not familiar with what this should look like, try lighting a spare from a 6v source. If it appears similar to the one in your V4 the LOPT is probably ok. If you still have a dim/poor picture, try leaning the set backwards a little and gently tap the tube neck or base while the set is switched on a few times. If the tube heater brightens up you have a partial short in the heater assembly a very common fault with Mullard 14 and 17" tetrode tubes. Hopes this helps. Regards John. |
28th Jul 2006, 11:00 am | #14 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Re: Pye V4 Restoration
Hi to all.
The tube heater seems bright and tapping the tube neck does nothing, so I think we are OK there. However, I have been foxed for some time by the A2 voltage. Both the Pye service data and the Trader sheet give a voltage of 452 as measured with an Avo Model 7 on the CRT base, with brightness at minimum. However, the feed from the boosted HT rail is via a 10Meg resistor, so the voltage would have to be way above 452 if that reading were to be given by an Avo7 at 2000 ohms per volt. My Avo 8 gives about 200 volts. BUT, when I measure the boosted HT across the boost capacitor, I get about 390 volts. So,it looks as if the boosted HT is low anyway. Both the line O/p and booster diode valves are new. I have found the screen feed resistor and cathode bias resistors for the O/P valve are about 15% high and will change these. Any other ideas or are we back to a suspect LOPT? Cheers Nigel |
31st Jul 2006, 8:26 am | #15 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Re: Pye V4 Restoration
Time for a progress report!
Bearing in mind the probable need to swap CRTs, if only for comparative testing, I took the risk and decided to remove the deflector coils and take the tube out of the cabinet. Good News I think. The coils slid off without any problem and I find that they have polythene insulation, rather than cellophane 'sweet paper' . This has given me the chance of cleaning the tube, which was filthy, with black greasy dust all over the sides and close to the EHT connection. There was some dirt on the faceplate and the filter, which will not have helped the picture. I have stripped the old finish off the cabinet (after scanning the Pye Logo, so I can reproduce it in future) as it had deep scratches. When I have cleaned up the metalwork, I will probably do a further electrical test before re-spraying the cabinet. Nigel |
2nd Oct 2006, 9:57 am | #16 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Re: Pye V4 Restoration
Hi to all,
Having done everything I can think of and run the set enough for the tube to brighten up a bit, I am left with (probably) two faults. After the initial 90 second warm-up, the picture is cramped and folded over for part of the top. See first picture attached. The fold over does not run the whole width of the picture, and is not affected by frame height or linearity adjustment. It is as though the beam is being repelled by the top of the tube. After another five minutes or so, the picture has unwrapped itself as seen in the second attachment. At this stage, the picture is just about viewable, even though the CRT is clearly tired - sparkling and going negative on highlights if brightnesss or contrast are advanced. After half an hour or so, the picture has become narrow, taller than normal, dimmer and de-focussed. The picture does not get bigger as brightness is increased, though the tube may be unable to put much load on the EHT supply. Any clues please? As regards the narrow tall dim picture, I guess that this is a sign of a reduction in line output coupled with reducing EHT. Is this a known LOPT fault or do I just check voltages everywhere to see if I can find an overheating component? Regards Nigel |
2nd Oct 2006, 8:05 pm | #17 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,420
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Re: Pye V4 Restoration
.
Indeed the V4/VT4 scan coils were poor but the line transformers were ok. I never replaced one that was actually duff in the sixties and even on some that were still working in the 70's. i would try to remove the over wind and using an EHT tripler fed from the PY81 anode via a 180pf 10kv pulse cap, this will provide a reasonable scource of EHT and prove that your CRT is ok, you could try an emission test on a B&K crt tester, anything above red would indicate the CRT is ok. Going back to the transformer if you can obtain a good one a thourough soaking in WD40 done regularly will work wonders. trevor |
2nd Oct 2006, 8:38 pm | #18 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,420
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Re: Pye V4 Restoration
I forgot to say, the funny folding effect I see on your photos is actually static on the screen. Try rubbing the perspex screen vigoursly with a dry towel once the screen is normal and watching the result!!!!.
this indicates the EHT is ok but a soft tube. The folding on the left looks like a lack of line drive but also a possible width or linearity coil with shorted turns. Trevor |
3rd Oct 2006, 7:59 am | #19 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Pye V4 Restoration
Hello Nigel,
Very well done with that V4 restoration. Its a great set but not the easiest to bring back to life. Trevor is spot on with the static distortion. It almost appears as if the picture is spreading like water across a sheet of glass. Its aggravated by a slightly weak tube but its not surprising after 53 years! Regards John. |
3rd Oct 2006, 9:06 am | #20 |
Pentode
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
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Re: Pye V4 Restoration
Hi,
Thanks for the comment on static. I once re-built a vintage car ammeter using perspex as a substitute for the original glass. Just lightly rubbing the perspex put the needle over to full scale. The problem was fixed by using an anti static furniture polish, but I can't remember which one! I'll try that on the Pye. Nigel |