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Old 15th May 2010, 6:23 pm   #41
dominicbeesley
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Default Re: Oh no - another V4!

Hello all,

I did a bit more and got some improvement but not much better on the width - I've ordered a PL81,PY81 and pair of PY82's in the hope that they will help - if not they'll be good for spares....

For now I've bypassed the PY82's with a big diode and 47ohm resistor and this has brought HT to bang on spec - I worked out that high AC on anodes and low HT on Cathodes must mean low current - I'm slow but I get there in the end. Then spent an age looking at PY82 characteristics to work out the correct series resistor...was a shock when it came out bang on spec. Just goes to show I really should just sell my valve tester!

Unfortunately that didn't improve width, EHT or boost - still comes out at 7kV on my measuring probe (380V for boost) so hopefully a new pair of valves in the LOP tin will help.

Then I decided I'd look in the tuner to find the most unhappy looking caps I've ever witnessed (see picture). They're still close on capacitance but quite leaky so I guess its something has reacted with (or eaten) the coating. Replacing these and resoldering a lot of dogy joints in the tuner has improved sensitivity on sound by about 40dB but picture is still odd.

Anyway now got a fairly good picture but the APC is playing up quite a bit so will attack that next whilst waiting for the new valves. There's also quite bad hum-bars but not sure if that isn't some sort of H-K leak in the line valves.

I'm a bit disappointed that I might not get to use my wobbulator - can just make out the 3 meg gratings when the APC decides to behave so it might not need twiddling! Pictures don't quite do it justice due to long exposure times and shaky hands!

What I am a bit worried about is that the focus assembly is quite off centre on the tube neck - and quite loose. Plus I can't get the picture to centre - John warned me off disturbing the CRT in case there's the cellophane coils so not sure what to do!

Dom
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Old 15th May 2010, 7:44 pm   #42
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Default Re: Oh no - another V4!

Replace the electrolytics in the power supply area - C35, C36, C37. Then C76, C88, C89.

Try also swapping the ECC82 (V22) and PL82 (V23) as well.

Cheers,

Steve P.
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Old 15th May 2010, 9:04 pm   #43
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Default Re: Oh no - another V4!

Hi.
As Steve says you have a hum problem, hence the caps he has suggested, are you using an aurora? If so can you try it with a signal from a freeview box etc, does it stop the hum? I have this on my sets (not them all) perfect with a program but bendy on the testcard, smoothing is perfect and checked out on the scope.

Regarding the centreing try loosening the four nuts on the focus assembly that bolts onto alloy tube cradle and reposition till the space around the tube is roughly equal, retighten the nuts whle holding the focus magnets so the assembly doesnt move tighten the nuts in an X sequence just a bit at a time. This should help the centreing.
I know John has said about the scan coils, perhaps I have been unlucky but I have had the later ones fail too, I believe though any Pye with a 70 degree tube will have scan coils that will do.
good Luck
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Old 16th May 2010, 1:05 am   #44
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Default Re: Oh no - another V4!

Thanks Steve, Trevor,

I went round the main smoothing electrolytics and checked them for leakage - they're all ok then I piggy backed some modern caps to see if they were dried up and 'scoped the lines to see if they changed much - is that a valid check? -It's what I do on radios...I'll try and do a proper swap at some point but for now will piggy back some caps on them again tomorrow and replace the others, also I've not replaced all the valve chain caps yet...

In the meantime I found some new looking PY801s which looking at the spec's seem to be a suitable sub for a PY81 so I gave it a go and got another half an inch of width! Plus boost is now upto over 400V, it's a bit late to drag out the valve multimeter for a proper check.

I then remembered I also had an old PL81 (taken from my TV24) and gave that a go, the width went back down again but much of the hum went - I probably didn't describe it well last time - there's not hum bars as such (bright bars) but there was quite harsh "wobbliness" to the edge of the picture - this is now reduced to a slight wateriness! So would that be poor h-k insulation in the PL81 causing it to alter the bias point of the valve?

Also tried the aurora on a few different channels and some come in much better than others - I think the channel change switch is a bit mucky as it is a bit hit and miss and usually needs a few goes back and forth before it takes properly. Wish I'd cleaned it when I had the can open!

The picture is still quite grainy - hopefully sorting out the APC and Contrast control will cure this...However it now looks pretty respectable - can make out all but the 3MHz grating all the time and the 3MHz some of the time on some channels - is that's as good as I'll get - would these have been that good when new?

Trevor, as to the wobbly test cards, even with a bit of extra smoothing etc I never got totally rid of the wobbly test card on my TV24 - like you say some sources are ok others not - off air UHF TV and test cards are wobbly. Some freeview boxes do seem to be mains-locked though. I had one that seemed to be - unfortunately it stopped working when they did the HD upgrades a while back!

I think the centreing might be ok - once I'd got the boost voltage up with the PY801 I can now get the picture centred ok. Would that have been some odd distortion in the flyback making it go off true?

I still have the focus wound right up - is that something I can fettle or is it likely just duff magents in the collar? Also the picture goes blank with the focus at the other end - made me panic a bit when I did that by accident!

Dom
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Old 16th May 2010, 3:28 pm   #45
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Default Re: Oh no - another V4!

Hello Dom,
If you only have 7kv the focus magnet will be at an extreme setting. When you manage to get the EHT up a bit you will find it focuses mid range. Looks very good so far. Check the screen feed resistor to pin 8 PL81 and the anode load resistor to the line oscillator for high resistance. Grand work. John.
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Old 17th May 2010, 1:34 am   #46
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Default Re: Oh no - another V4!

Thanks John,

I think the EHT is probably a lot closer now - not had a prod at it yet but will tomorrow.

I had a look at any >20% out resistors in the line bistable and lop stage but only got a small improvement...hopefully a new PL81 will bring up full width and EHT.

I attacked the contrast circuit and now it sort of works but not as well as I'd have hoped! It only works over a small dynamic range so I think there's still something not quite right - I'm getting about -5V or so of control voltage but that only when the tuner is overloaded so something isn't getting through properly - more study needed of the circuit I think!

However the hum problem seems to have almost disappeared - so possibly there was some hum on the grid of the 1st IF valve?

I have to say this has kept me nice and busy and I've not even started on the cosmetics yet!

Dom
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Old 18th May 2010, 5:23 pm   #47
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Default Re: Oh no - another V4!

Hello all,

I have to admit to being a bit stumped now. I've replaced PL81, PY81, PY82s and tried various ECL80s in the line-bistable, replaced out of tolerance resistors in line stage. All valve voltages given in Trader sheet are within 5% and waveforms given in Pye manual are either spot on or a bit over - except for [17] (line feedback?) which is 300V instead of 400V.

Only thing I can think of now is LOPT - what's the best (safest) way of removing these? Is it worth giving it a few hours in the oven?

Any advice would be gratefully received.

Dom
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Old 18th May 2010, 5:46 pm   #48
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Default Re: Oh no - another V4!

Hello again Dom,
It might be the LOPT core a bit saturated by a damp overwind but it is such a good picture that I feel it might get better with extended use. I don't fancy the idea of removing a 60 year old LOPT from the chassis unless it is absolutely necessary. The insulation may well be brittle but fine if not disturbed. Just my thoughts.
I recently unearthed my original V4 manuals and was surprised at the early release dates for the V4. The full manual is dated January 1953 so these sets must have been released very late in 1952 with the usual Pye advance service data to cover these early releases. The abridged data is dated April 1953.
I also found this data sheet dated 1-11-53 covering a few notes that dealers must have asked Pye to clarify.The original manuals are a work of art! Regards, John.
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Old 18th May 2010, 5:54 pm   #49
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Default Re: Oh no - another V4!

Might be worth a good spray with WD40 then leave for a while and repeat.

David
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Old 18th May 2010, 5:58 pm   #50
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Default Re: Oh no - another V4!

For Forum members who do not have the original service data I have scanned the service notes. These were probably faults discovered during the testing stages of the receiver after assembly so may not relate to actual faults in the field but I feel you will agree that any information is a bonus. J.
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Old 18th May 2010, 6:38 pm   #51
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Default Re: Oh no - another V4!

Get the TV and put it in a DRY room right up against the radiator. Get the LOPT closest to the radiator if you can.

Leave the radiator on and the set there for a couple of weeks. Then try it.

Cheers,

Steve P.
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Old 18th May 2010, 10:22 pm   #52
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Default Re: Oh no - another V4!

Just for my curiosity...you are saying the EHT and boost are both still low?
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Old 19th May 2010, 12:23 pm   #53
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Default Re: Oh no - another V4!

Thanks lads,

I've not got a hope in hell of getting this one past the boss to put it over a radiator - not until I've refinished it anyway. Instead I'll set a fan-heater on it in the workshop. After the refurb the workshop is nice and dry...

As to EHT I've not measured it after the set has been running but boost was up to 430V at switch on yesterday, not sure what it fell back to later.

Thanks John for the service sheets!

No here's a poser - I'm still puzzled by the provenance of this set. The tuner looks to be a pukka VT4 factory fitted item. However the blue plate on the LOP can says V4. Now another contradiction - the Flywheel modification note attached by John above is NOT fitted so the chassis must be from before November 1953! But it looks like it never had V1 or the Band I tuner fitted! Weird - could it have been a pre-production set for IBA transmission tests or a Pye test set?

Dom
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Old 19th May 2010, 12:45 pm   #54
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Default Re: Oh no - another V4!

Hi Dom
What's the serial number?
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Old 19th May 2010, 1:15 pm   #55
dominicbeesley
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Default Re: Oh no - another V4!

Hello Trevor,

Had a really good look but can't find any serial number on the actual chassis.

Back Cover Ref: 690 460
BBC Licence: B6 27405
Cabinet Label Date: 2 Jun 1954

Had a good look at the screw holes round the band I tuner area and V1 valve hole and there looks to be no trace of screws having been used (no self-tap threads like the other used screw holes).

Unfortunately non of the electrolytics look to have dates on them

Dom
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Old 19th May 2010, 1:33 pm   #56
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Default Re: Oh no - another V4!

There usually is a number on the chassis but I can't remember where, also I can't check with mine as I'm off the road with a slipped disc, perhaps OTT or someone else could check on their's and let you know.
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Old 19th May 2010, 1:37 pm   #57
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Default Re: Oh no - another V4!

Hi guys,

The serial number is stamped on the chassis at the rear under the tuning knob, its where the coax enters the chassis to the left of it.

mine is "115465" and my case has a stamp of 2nd September 1953

Chris

P.s.
Sorry to hear of your back trouble Trevor, I know how painful that can be, they used to say rest but doc's seem to say move about it helps..... aha ha that means you won't be able to boot my butt

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Old 19th May 2010, 3:02 pm   #58
murphyv310
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Default Re: Oh no - another V4!

Quote:
Originally Posted by oldticktock View Post
P.s.
Sorry to hear of your back trouble Trevor, I know how painful that can be, they used to say rest but doc's seem to say move about it helps..... aha ha that means you won't be able to boot my butt
Nice one Chris!
I was thinking about you last week when I saw a BMW 5 Series with the reg plate B4 OTT and it made me think about some of the threads before OTT became on the scene, surely your V4 thread has had the highest views etc on the TV category.
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Old 19th May 2010, 3:25 pm   #59
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Default Re: Oh no - another V4!

Aha! Found it under a load of rust! 274973 - so it looks like a later chassis BUT with a V4 LOP plate and without the flywheel mod...

Cheers

Dom
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Old 19th May 2010, 3:33 pm   #60
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Default Re: Oh no - another V4!

Hi.
I have done a few V4-VT4's and have never seen the mod for the flywheel but I think I will do the mods on mine at sometime. Something in my head tells me that chassis number is close to mine.
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