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Old 8th Feb 2012, 6:02 pm   #1
matspar
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Default Pye VT2 EHT problems

Hi all,

I've been working on a bakelite Pye VT2 for the last few days, it is a bit of a wreck but is coming together slowly. It has been hidden away in a lockup garage since 1963.

So far I've done the usual cleaning, de-rusting, and recapped the chassis, and today powered it up to see how it fared. It didn't disgrace itself entirely but there is an issue with the EHT which needs a proper look into.

I'm getting a nice strong line whistle but no raster. All the valves and tube heater are lighting up nicely and it looks like there's plenty of line drive, but the EHT is too low.

I can draw a very tiny spark (about 3mm) from the anode of the EY51 EHT rectifier, and there is a very faint glow from the heater, but not as bright as it should be.

I can't get a spark from the top caps of the line output or boost diode valves, a PL81 and PY81 but the voltage readings on the valve bases aren't far off the circuit.

I'm getting about 168v HT and the A1 voltage is around 190v which seems a bit low. I'm suspecting the Lopt may be at fault, unless there's something I may have missed. The EY51 heater winding is intact and reading just under 1 ohm.

All the valves are known good ones, and apart from the new capacitors I've checked all the resistors in the area and they all seem ok.

If anyone has any thoughts or ideas, or even a spare Lopt which I could buy, I'd be more than grateful!
I've wanted one of these sets for years and am really hoping to get this one minted and mended. I've attached some photo's to give you an idea. The chassis now looks a lot better than it did!!

P.s. Ignore the bodged in EY86 recifier which is shown in pic 3 (this has been replaced with the correct EY51)

Best regards

Matt Spanner

Portsmouth
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Last edited by matspar; 8th Feb 2012 at 6:11 pm.
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Old 8th Feb 2012, 6:19 pm   #2
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Default Re: Pye VT2 EHT problems

The H.T. quoted is 174v so your 168 is about spot on. I doubt if the LOPT is faulty as the EY51 is attempting to heat with very little AC on it's anode.
If you are sure you have replaced the caps in the line output stage, check they are fitted on the correct tags. C52 the boost cap and C51 cathode bypass decoupler to the PL81 and the H.T. decoupler, C67 12uf [10uf]. The PL81 may be low emission but is such a simple circuit that not much can go wrong. The LOPT is possible but I have a big doubt about that. The line oscillator transformer may have very low Q or partially O/C. Worth checking the resistance values. It has been known for them to continue working of a fashion when virtually O/C. Not a bad start to be honest. I'm sure you will get there. Regards, John.
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Old 8th Feb 2012, 6:57 pm   #3
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Default Re: Pye VT2 EHT problems

Many thanks John, I will double check these caps over the weekend. The electrolytics are still the originals, forgot to say its just the wax ones which I've replaced so far. I'll also check the osc transformer and report back.

I don't appear to have C67 on my circuit (only goes up to C54) I'm using a trader sheet 1132/T52 Glad to hear it may not be the Lopt!

Best regards
Matt
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Old 8th Feb 2012, 7:27 pm   #4
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Default Re: Pye VT2 EHT problems

You could try a silicon EHT rectifier to prove whether you have problems in that area.
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Old 8th Feb 2012, 7:32 pm   #5
Kevin19
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Default Re: Pye VT2 EHT problems

Hi Matt

I did one of these Pye VT2 sets a while ago and being new to TV repair restoration John W above guided me so fantastically and the set now works very well you can down load the proper Pye VT2 manual here link below well worth having.

http://www.thevalvepage.com/tvmanu/pye/body_pye.htm

You may like to read all my mistakes! Re when I did my Pye VT2 set I could not get a raster! But with the help of this great forum and John W got the set sorted it one of my favourite sets now and works very well!

Hope you get yours going here is my posts re mine below.

http://www.vintage-radio.net/forum/s...ad.php?t=52010


Best wishes

Kev.
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Old 9th Feb 2012, 9:59 am   #6
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Default Re: Pye VT2 EHT problems

Yes you are quite right Matt. The circuit in the Newnes books is a microdot! C49 is the 12uf. C51 .1uf cathode decoupler PL81 and C53 .001uf across the width coil. Not a lot to go wrong. The scan coils should be OK as they are not constructed on the same lines as the V4 series but 60 bad storage years have passed, so they cannot be ruled out completely. On the good side they are very reliable. They can be disconnected for a test and if the line output stage perks up considerably they may be worth closer inspection. The LOPT may be very damp and a simple test is to run it for say 15 mins and see if the overwind is getting warm. If it is, the saturation problem may be the cause of weak EHT and a long drying out may improve matters. Sometimes just switching it on for a period each day, increasing the time by 10 mins or so does the trick.
The PY81/800 or more likely the PL81 may be a bit low forcing the LOPT to struggle.
[Glad to here you VT2 is till working Kevin! ] Regards, John.
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Old 10th Feb 2012, 3:15 pm   #7
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Default Re: Pye VT2 EHT problems

Hi John, Jeffrey, and Kev,

A big thankyou for your advice and assistance, very very much appreciated.

I've spent a few hours on the VT2 today and done the following. C49 (12uf) replaced (along with C35 & 44 - contained in same can)
Oscillator transformer checked out of circuit and appears to be in good health.
Width coil and line hold coil also read ok. I had the set on continuously for about 30 mins and from what I can tell the lopt didn't appear to get warm.

I've temporarily sustituted the PL81, PY81, and EY51 from known working sets just in case any of the new ones I've fitted were duff, but unfortunately I've not yet had any improvements.

I also couldn't see any heater glow from the EY51 today, although the line whistle still sounds nice and strong. It was very bright in my conservatory however and may have just been to hard to see, so I might wait until it gets a bit dark and run the set up again.

I've not taken the scan coils out of circuit yet, or tried a silicon diode but I will explore both of these during my next session. Apart from that I'm at a bit of a loss as to what to try next.

My gut feeling is the Lopt (although I don't really want to admit it), possibly the saturation problem. The resistance readings are again very close to the service info but I suppose a single shorted turn wouldn't make a lot of difference.
Is there anything else worth trying or anything you can think of that I may have overlooked?!!

If anyone has a Lopt available to try/buy I'd be very happy. Alternatively do you think I may be able to adapt a lopt from another similar sized set? I have good working lopts available for Ferguson 991T, Pye V4, Ultra V814.

Best regards
Matt
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Old 10th Feb 2012, 3:30 pm   #8
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Default Re: Pye VT2 EHT problems

Are you sure that the PL and PY top cap leads are on the correct way round

Failing that if you remove those and simply fit the top caps from another lopt without any other of its leads connected and you then can draw a spark from the top caps,
it is a fair bet that your LOPT is duff.
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Old 11th Feb 2012, 3:14 pm   #9
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Default Re: Pye VT2 EHT problems

Hi,

Yes the top cap leads are correct, did get me thinking for a moment however.
I've disconnected the scan coils today and still no EHT improvements.

I've removed the lopt and what I propose to do now is leave it somewhere quite warm for a few weeks in the hope that the saturation problem is causing the low EHT. Its baking up quite nicely in the conservatory as I write this (finally a bit of sunshine in Portsmouth today!)

I'll also advertise for a replacement in the hope that someone may have a good one hidden away. Failing all that I may think about removing the EHT overwind and experimenting with a tripler, but this will be last resort stuff as I'm not too sure how to go about this.

Thanks again for all your time and help, its been a couple of years since I've attempted a set from this era and as I'm only 33 years old I still have plenty to learn!

All the best
Matt
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Old 11th Feb 2012, 4:19 pm   #10
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Default Re: Pye VT2 EHT problems

Hi Matt,

If you're worried about your lopt a "ring test" might be an idea if you have an oscilloscope handy...http://www.vintage-radio.net/forum/s...0&postcount=27

Dom
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Old 11th Feb 2012, 10:35 pm   #11
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Default Re: Pye VT2 EHT problems

I have a 'Scan test' that will check it out reasonably reliably. As Peter has suggested, if you have any known good LOPT from a valve receiver, you only need to substitute the boost diode and line output top caps and if you can then draw a good spark from the overwind output of the donor transformer, there is a good chance the original is below specification. [Damp] The line output stage is very simple with a narrow angle tube. Almost any lopt of vaguely similar spec will give you a picture. You need to compare the circuit of both transformers to get the connection right. I've fitted a Ferguson 968T in a Pye LV30 and an Ekco TS88 in an Ultra W720 with correct linearity and EHT. You can't do any harm to the VT2 if you get anything wrong. Good luck with it. Regards, John.
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Old 12th Feb 2012, 1:59 pm   #12
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Default Re: Pye VT2 EHT problems

Hi Dom,
Unfortunately I've not used a scope before, but many thanks for the suggestion. Something I must learn about one day..

John,
I have tried a couple of other lopt's today by swapping over the pair of top cap connectors. I refitted the suspect one before trying this (am I right in thinking I should leave the original wired in place underneath?)

Both donor lopts behaved in exactly the same way as the original - no sparking from the top caps but a very small one present from the EHT anode connection. One was better than the other although I do know they both worked a few years ago and have been dry stored ever since. I didn't have a rectifier valve fitted when trying this.

Don't know what you think but this has cast a bit of a doubt about my duff lopt diagnosis, I may have a go at working out the connections and completely fitting the slightly better one (a 'coffin' type from a Ferguson 991T) and try again.
I feel a bit stumped with this set now, not much left to try or replace!

The can type smoothing capacitor is still the original one and looks to be in fair condition, no bulging etc. Do you think it may be worth replacing this just to draw a line under it?

Also would it help to carefully removing the pitch from the overwind on the original lopt? I've been reading the very detailed posts about the dampness acting like a shorted turn. Was wondering if this may be worth a go if I don't get any improvements from leaving it in the warm.

I won't get another chance to work on the set now until next weekend so everyone can breathe a sigh of relief!!! Thanks again & speak soon!

Matt
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Old 12th Feb 2012, 2:41 pm   #13
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Default Re: Pye VT2 EHT problems

Must admit Matt I feel the same way about this. I'm not completely convinced this is a LOPT fault. I would not melt the original pitch but if you remove the transformer and give it blasts from a hot air gun, just enough to give the pitch a glaze as such and then let it cool, repeating several times, it should improve matters if the transformer is damp without causing any actual damage. You are more than welcome to cart it up the A3 to my place if you want to have a play with it in my workshop. Failing this it will have to be a warm spot such as on the top of the central heating boiler [trusting it's not an AGA!] Your H.T. is correct so I don't think it will be the caps. You probably have but check the screen volts on the PL81 pin 8. Should be approx. 112V. Clutching at straws here but if the screen resistor 8.2k has risen in value it will give identical symptoms to yours but don't get too excited. John.
PS AS jeffrey suggests, if you have any spark on the anode of the EY51 it might be worth using a solid stick rectifier with the EY51 disconnected. You can also heat the EY51 from a 6v battery[heater winding disconnected] to see if you can get anything on the screen [roomlights out] providing the ion trap has not been disturbed. It may be difficult to reset it with such low EHT. The tube should give a glimmer at 500v.
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Old 12th Feb 2012, 8:31 pm   #14
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Default Re: Pye VT2 EHT problems

Thanks again John, I shall give all of this a go next weekend and see how I get on. I've been through all the resistors and there's nothing drastically high, the only incorrect thing I found in this area was C50 (47pf) which was a 15pf cap in my set. It was an original component so must have worked ok. I swapped it for a 47pf and it doesn't make any difference.

If I don't make any progress I may well take you up on your kind offer of a visit as I'm only down the road in Portsmouth, maybe you could cast an expert eye over it!

Best regards
Matt
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Old 13th Feb 2012, 11:08 pm   #15
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Default Re: Pye VT2 EHT problems

Here is that infernal Lopt! I've been zapping it with a hot air gun this evening, and it is currently living in a small foil container placed on top of a nice hot pipe on the central heating boiler. I'm going to heat it gently with the gun every evening this week and then give it another go in the VT2 again next weekend - it'll be interesting to see if there's any improvements and I'll report back on my progress!
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Old 14th Feb 2012, 8:14 am   #16
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Default Re: Pye VT2 EHT problems

Sir Winston Churchill would have been proud! By the look of the rust it has been subjected to a lot of damp. You can only give it a try. I've seen a lot worse that work.
I think it is a very similar if not the same transformer that is employed in the Pye LV30/51series. John.
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Old 19th Feb 2012, 8:34 am   #17
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Default Re: Pye VT2 EHT problems

Time for an update! if anyone's still reading!

I re-fitted the original iffy lopt yesterday, crossed my fingers and hoped, but unfortunately there was no improvements, was worth a try I suppose.

I also experimented with a lopt from a 17" Dynatron TV38 (1959)
A very common type (I've had Ekco and Ferranti sets in the past which used the same chassis)

I chose this one as it looks fairly simple and only had 3 connections, 2x scan coils and what I think is the A1 connection. I didn't have a circuit diagram to hand but I know I have one somewhere.

With this lopt fitted there was considerable improvement, I now have a very tiny spark on the top caps of the PL and PY81, and getting on for about a half inch spark on the rectifier anode. Nothing on the screen however. The A1 voltage has shot up to about 400v. Too high but I can probably reduce this. I haven't used the A1 connection from this lopt, I've only connected the scan coils.

I think the EHT its still low with this one installed, I imagine the HT was a bit higher in the original set. At least it proves a point about the original lopt however.

My plan of action now is to either - remove the overwind from the original lopt and go down the Tripler route. I know the basic theory on how this is done but my only concern is if I can generate enough EHT to light the tube up.

The pye needs 7.5Kv - do you think I can get anything like this from the anode of the PL81 using a tripler? I'm not convinced personally, but I've not attempted this before so all this is unchartered waters for me.

Alternatively could I raise the EHT using the Dynatron lopt which is showing some signs of life? there is a large mica capacitor between one side of the scan coils and the boost diode anode - think I've read somewhere that you can play around with the value of this to vary the EHT?

Any thoughts on either of these would be very welcome! I'm not going to give up on this set!

Matt

Last edited by matspar; 19th Feb 2012 at 8:41 am.
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Old 19th Feb 2012, 10:15 am   #18
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Default Re: Pye VT2 EHT problems

Good work Matt. Interesting, if you have 1/2" of EHT on the rectifier anode and 400v on the first anode of the CRT the screen must light up. The EHT rectifier should be lit but remember the Ekco transformer uses a U25 with a 2v heater as apposed to the 6.3V EY51 in the VT2 You could add a few turns to the heater winding if you want to retain the EY51.
If it lights, you will have some EHT. The tube employs an ion trap and this might have become dislodged if you have cleaned the interior. If it is stuck fast, leave it alone for now. Check the grid and cathode voltages on the CRT base. A tripler will probably give way to high EHT. A doubler as used with the small screen Thorn 1500 chassis would be a better bet. Looking forward to the results. Regards, John.
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Old 19th Feb 2012, 1:32 pm   #19
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Default Re: Pye VT2 EHT problems

Thanks John,

I think I was being a bit optimistic about a half inch spark, just tried again and its probably about a quarter of an inch. The U25 is struggling to light up as well. I'm getting nowhere fast with this set really...

I tried shorting the crt cathode to chassis just to see if I could get any illumination but again no joy. There's just nowhere near enough EHT being generated. The ion trap is undisturbed, I've loosened it slightly just to break the seal from all the built up grime, but I've resisted the temptation to move it.

If I took a tripler from the anode of the line output valve, after first removing the overwind, how much EHT do you think would be available?
I tried briefly yesterday on the original lopt connecting a tripler to the anode of the EY51, but it just damped the lopt down even more.

Best regards
Matt
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Old 19th Feb 2012, 3:26 pm   #20
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Default Re: Pye VT2 EHT problems

With the line output stage working correctly I would guess and it is a guess, with a tripler around 12kv. You could certainly try it. It will not harm the MW31-74 even if it is a bit high. Nothing to loose as such. Have you tried a spark test at the DC end? Touch screwdriver to chassis. Should get a reasonable crack. J.
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