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Old 30th Jul 2021, 5:39 pm   #1
radiograham
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Default Pye VT4.

Hi,decided to have a look at this pye vt4 i have had for a while,first of all there was a valve ef80 missing,this was replaced and valve heaters lit.I gave it 100volt ac via auto transformer first snipping the suppresor caps out.After an hour the voltage was increased to 200 and i could hear sign's of life through the speaker,the main smoothers were just warm,volts increased to 240,my daughter's 18 year old ears were borrowed at this point to listen for line whistle,she could'nt hear any so i switched off at this point and checked the lopt.The eht overwind was ok plus the anode winding,both valve's tested ok.after a few more check's i'm pretty sure the line linearity coil is open circuit,i have never come across this before was this a common fault? i was thinking is there a way to get it working without this just to get first light before going any further? also what is the correct way to change the ey51,this has those circular metal rings at the ends, do these come off? Graham.
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Old 31st Jul 2021, 5:52 pm   #2
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Default Re: Pye vt4.

Yes i admit i got sidetracked by checking the lopt first,i then checked for line drive at the grid of the line output valve,the ecl80 line osc is not working,valve tests ok and i have tried a substitute,just found the line hold pot is intermittent and am fitting another,the line linearity coil is definitely o/c so this will have to addressed later. Graham.
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Old 31st Jul 2021, 6:49 pm   #3
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Default Re: Pye vt4.

One of the most complicated receivers of the period and it remained so for a few years.
An O/C line linearity coil is almost unheard of with the Pye V4. Did you clean the chassis before starting your work? If so you may have broken a fine wire from the coil. Worth having a very careful examination of the winding.

Start with the HT line. Make sure this is something near the published data. The VT4 are prone to line output transformer and scan coil failure. If you can scope the drive waveform to pin two of the PL81 it will confirm if the line oscillator is working. Replace the boost cap, line coupling cap and test. The scan coils can be checked by disconnection. It's the screened wire but I cannot remember the colour of the other one. Connections are under the scan coil mounting on a tag strip. Don't tackle anything else until you get the line timebase working. You will only mess yourself up! Good luck with it. The chassis has a very high component count. John.
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Old 31st Jul 2021, 7:22 pm   #4
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Default Re: Pye vt4.

This may help. J.
https://worldradiohistory.com/UK/Pra...on-1955-08.pdf
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Old 1st Aug 2021, 12:29 am   #5
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Default Re: Pye vt4.

Hi,the vt4 is indeed not one for the beginner,i also have a v4 which i got going a few years ago this was a marathon as you say it has a very high component count.I haven't cleaned the chassis much apart from dusting so don't think i have broken any wire's.One thing i have noticed the main ht seems high well above normal,i am going to concentrate on the line tb and get this working first,not the easiest of chassis to work on,i will also change the 2 cap's you mention,the boost cap and the line coupler. Graham.
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Old 1st Aug 2021, 4:37 pm   #6
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Default Re: Pye vt4.

The H.T. might be reading a bit high due to lack of current being drawn from the line output stage due to a fault. Mains voltage these days appears to be higher than it was and worth a check for comparison. I'm 248v here and never less than 240. Make sure that the mains dropper has not been messed about with and take a visual look at that lin coil. J..
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Old 8th Apr 2022, 11:11 pm   #7
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Default Re: Pye vt4.

Hi all,revisiting this tv after a gap since last august,on rechecking the linearity coil this is definitely o/c and i wanted to check if what i am doing is ok?when the coil is in circuit one end is from the cathode of boost diode the other end goes to the primary of lopt and there is a tap to the secondary from the coil.In wanting to get first light before removing the coil to find out where the o/c is i have fed the primary and secondary direct from the boost diode cathode,should this work?i have replaced most of the caps around the line tb and am replacing the 12uf electrolytics at the moment two of them in line tb one in frame tb.Also checked resistors in line tb some of them are a bit high but i dont think enough to stop it working.At the moment there is virtually no eht,there is a slight spark from ey51 anode,i have checked the line deflection coils for continuity and resistance both ok also they are not shorting to anything.The two 12uf d/couplers were both duff,didnt try reforming them replaced with 10uf/450volt types,no line whistle heard so i had a listen to my other V4 and have found that i cannot hear 10kcs any more,about 5 years since i restored that one,well i am 66 after all. Graham.
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Old 9th Apr 2022, 9:33 am   #8
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Default Re: Pye VT4.

Good luck with the VT4. I did one a few years back. I made one from 3!

Two V4s and one VT4. I chose a V4 as that was the first TV I tried to get working when I was around 14 years old. That one had EHT failure, probably was a duff boost cap from my memory of the symptoms. I never did quite get it working in the end though.

The trickiest job was replacing the Hunts heater de-couplers in the RF/IF section. Not easy to get at!

I replaced all the wax capacitors and any significantly out of spec resistors.

I was lucky that I had one good functioning CRT and one working LOPT.

I removed all the pitch off the LOPT and ran about 30mA through the overwind for a few days to dry out any moisture. Then varnish over the winding.

The EHT is still a little low (they always seem to struggle a bit in that department from what I have read and experienced).

That is probably why the focus control is at one end of its travel to get good focus.

Initially on the chassis I chose there was no line drive.

I subsequently found a wax covered mica capacitor was short circuit and stopping oscillation.

It is almost unheard of these types of capacitor going short!

If you need any parts then let me know. I have a complete VT4 chassis and tuner - with dead LOPT.

If it is the LOPT then I have dismantled cleaned up one which would make for a much easier rewind if you go down that route.

I also have a box of bits, maybe a line linearity coil if you need one.

PM me if you need any parts.

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Old 9th Apr 2022, 8:43 pm   #9
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Default Re: Pye VT4.

Hi Andy, thanks for the offer of the parts etc. I may ask for some especially the linearity coil, I have a V4 which I restored a few years ago remember the caps you mention, the VT4 is a bit easier having the turret tuner, not as many screens etc to remove. The set should work without the linearity coil with HT from boost diode to primary and secondary of LOPT just to get first light do you agree?

Graham.
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Old 12th Apr 2022, 10:46 pm   #10
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Default Re: Pye VT4.

Update,went through all the resistors in line/tb which were all high in value,replaced with 0.6 watt metal film types,also checked 100pf fixed mica and 100pf preset line hold trimmer ,the 3 12uf cap's have been replaced plus all the cap's in tb which were orrible unt's have been changed.The tb now seem's to be working but not very strongly there is a waveform at the line output valve grid but still no eht with a small spark from the ey51 anode.both the pl81 py81 and the ecl80 test 100 %
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Old 13th Apr 2022, 9:25 am   #11
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Default Re: Pye VT4.

Did you disconnect the line scan coils? Also worth while checking the actual screened cable to the coils as this was known to go S/C frame to line especially if the insulating sheet material was cellophane and not polythene. When changing the tube the scan coils would usually fail within the week. The customer had to be warned of this.

If the spark at the anode of the PL81 is about the same size as the anode of the EY51 I would say the LOPT is faulty.

When I was first in the trade around 1962 the PYE V4/VT4 was at the end of life but there was still a number around. Thinking about it the VT4 was only eight years old in 1962. What fantastic technical changes in eight years! You could still still get a new transformer from Pye spares dept. Shiny new Perspex case!
The width coil can be checked by disconnection. It is shunted across part of the LOPT winding and will severely damp the LOPT if the width coil has s/c turns, a common fault back in it's youth. John.
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Old 13th Apr 2022, 9:43 am   #12
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Default Re: Pye VT4.

Souds like its a fairly long ongoing project, well done for perseverence.

Like Heater/cathode short, I remember repairing these in 1962, amazing to think how good the spares backup was for something which was 'old' in TV terms.
I also have a VT4 which I have been meaning to get round to rebuiding, & I was hoping to get it done by the Queens' platinum Jubilee, to display with some suitable recordings of the time. (O.K. I know that the VT4 came out later, but I watched the coronation on a V4).

From the thread so far I think I might not get mine finished in time, although I have checked the CRT and it is excellent emission-wise.
Good luck with your rebuild, looking forward to hearing more.

David.
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Old 13th Apr 2022, 10:30 am   #13
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Default Re: Pye VT4.

Quote:
Originally Posted by Heatercathodeshort View Post

When I was first in the trade around 1962 the PYE V4/VT4 was at the end of life but there was still a number around. Thinking about it the VT4 was only eight years old in 1962. What fantastic technical changes in eight years! You could still still get a new transformer from Pye spares dept. Shiny new Perspex case!
The width coil can be checked by disconnection. It is shunted across part of the LOPT winding and will severely damp the LOPT if the width coil has s/c turns, a common fault back in it's youth. John.
I had a VT4 around that time. I seem to recall that it was partly comprised of bits from other units and I did some tweaking to improve the video bandwidth. The result was the nearest I had ever seen when compared to an on-line studio monitor. A lovely set. Reminds me of the JFK era.
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Old 13th Apr 2022, 12:28 pm   #14
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Default Re: Pye VT4.

Quote:
Originally Posted by Heatercathodeshort View Post
If the spark at the anode of the PL81 is about the same size as the anode of the EY51 I would say the LOPT is faulty.

When I was first in the trade around 1962 the PYE V4/VT4 was at the end of life but there was still a number around.
I was thinking exactly the same thing regarding the line transformer.

Somewhere around that time when I was a child in the early 60s, I used to go with my parents to visit a family that they knew who lived in a large house and had an adopted German son. I think he must have been in his early 20s and had a workshop in the grounds, converted from some old outbuildings or pig sheds. My memory is that he collected these Pye VT4 TVs and had several of them. At the time I'd never seen one and knew nothing about them, or TV in general for that matter, but remember him explaining their advanced circuitry and the reason he liked them and why he had more than one example in his shed. He knocked up a little super regenerative single valve receiver for me to receive VHF TV sound on. He also converted a massive octal valve car (probably more like a bus) radio to mains for me - I'm rather sorry to say that both of these items have been lost in moves over the years. It was these childhood visits that sparked the interest in me to eventually train as a TV engineer, although in the end I didn't continue it into a life long career as most on here have done.
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Old 13th Apr 2022, 11:03 pm   #15
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Default Re: Pye VT4.

Hi thank's for the suggestions,i haven't disconnected the scan coils or the screened lead which is the one between the chassis and the def coils tagstrip i think,i will do that later on.also mentioned is to disconnect the width coil to check it,if this was faulty would that be enough to stop the output stage?also as regards the linearity coil this is one winding which goes between boost diode cathode and top of pri and a tapping to secondary of transformer,it is the first part between boost diode cathode and the tap which is o/c,hope this make's sense?i have checked the lopt windings resistances which are all ok though i know this doesn't check for shorted turn's.i checked the neg voltage at line output valve grid this is only 15 volts should be a lot more about 45 if the lopt is faulty would it drag this down? voltage's on line osc are ok and valve is 100% also tried another.the main ht was a bit low at 180 volt so put a 1n4007 across the py82s which increased it to 210.i must admit to being a bit stumped now and thinking it must be the lopt,i would check with another if i had one but sadly i haven't,it would have to be a rewind which would need thinking about whether it was worth it. Graham.
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Old 15th Apr 2022, 2:50 pm   #16
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Default Re: Pye VT4.

My first success with mending tellies was with the VT4 while I was still at school in about 1967. I used to buy them from a place called I think Padgett's Radio Store, do them up and sell them to my mates!
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Old 19th Apr 2022, 12:38 am   #17
radiograham
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Default Re: Pye VT4.

Hi,after a talk with Mike Barker about a possible rewind of the lopt i mentioned i had a working V4 and he suggested i use the scan coils and tube in the good set as a testbed to see if there was a problem with the notorious scan coils that the v4 is well known for,this i did swapping the tube base the eht and a couple of temporary leads for connection to the line coils in the good set,i disconnected them from the good set and sad to say it didnt alter anything so i think it must be the lopt that is faulty.The only substitute i have is one i got a few years ago from another forum member which is for a PYE V14c,this looks in good condition and unused.i have looked at both circuits and they are very similar so hope to use this transformer as a replacement.thoughts anyone? Graham.
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Old 19th Apr 2022, 3:14 pm   #18
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Default Re: Pye VT4.

Padgett's Radio stores "The TV graveyard of the North" as they called themselves in their ads.

Where's my Tardis?!
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Old 23rd Apr 2022, 1:18 am   #19
radiograham
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Default Re: Pye VT4.

Hi,at the moment i am looking at using the lopt i have which is for a pye v14c.I have tried this by connecting up the line output valve and boost diode to their connections and now have about 12kv eht,turn's out i can hear 10kc after all! so finger's crossed this lopt will be suitable.The only problem i have is the connections are different from the diagram in the pye manual,this shows 5 conn's 5 is line valve anode,6 is boost diode,4 is one end of this winding.then there is 1+2 which a winding on it's own.On this lopt there are only 4 tag's,the line valve no5 and boost diode 6 are the same and 4 is, the difference seems to be that 1+2 are now part of the same winding.Looking at pics of a v14c this looks the same 4 tags not 6,i cant find any mods in the pye manual to explain the difference. Graham.
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Old 23rd Apr 2022, 9:18 pm   #20
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Default Re: Pye VT4.

I have seen two chassis views of a v14c one shows the lopt with 6 tags on the paxolin board of the lopt and another that shows the one like mine with 4 tags,so i think they must have changed the conns to the lopt at some time but i cant find anything in the service manual about it. Graham.
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