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Restoration of a possibly unique pre-war TV

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Sun May 04, 2014 8:23 pm

A reasonable amount of progress today!!
Firstly I powered the timebase and RF sections from my bench PSU and got both field and line lock from an Aurora input! Needless to say, I was delighted. I tidied up the wiring with a bit of lacing and decided to bite the bullet and install the newly rewound mains transformer. Unfortunately, it wasn't just the EHT secondary that was faulty on the original but the whole thing had been well and truly toasted so a complete rewind was necessary. This is not a bad thing as it does mean that the transformer is A1 and built using modern insulation materials - important where mains EHT is concerned! I have finished for the day and will power the set up via a Variac tomorrow and if all goes well, it will be time to mount the CRT and try for first light. Just a couple more pictures!!!

Underside-without-transformer.jpg
The two rectifiers (EHT in the middle) are just about accessible for wiring after the Xfmr is fitted.


Underside-with-transformer.jpg
That thick heater wire was routed like that in the original!


underchassis-before-web.jpg
This is what it was like before the start of the project.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Sun May 04, 2014 11:17 pm

The pictures of Pye 838 TV radiogram. One thing is sure, this set is in terms of build quality certainly not in the same league as the Marconi 703. I regret selling my 703. I should have sold this one instead. Anyway, I'll keep faith with the 838 and will restore it to full working order.
The record deck is on the left and radio on the right.The curious bent spindle on the gram deck is correct. The radio unit is similar to the 802 table set. This is the receiver with a continuous tuning system. To change wavebands one simply turns the tuning control to the extreme end of the waveband, then just keep turning and another waveband is selected. In practise the system is simply a nuisance.

Till Eulenspiegel.
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838_0124.jpg
838_0125.jpg

 
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Re: Restoration of a possibly unique pre-war TV

Post by Cathovisor » Mon May 05, 2014 1:54 am

Till Eulenspiegel wrote:The record deck is on the left and radio on the right.The curious bent spindle on the gram deck is correct.

It'd be interesting to compare with the Dulwich example of the 838 what the correct deck should have been, assuming that too has retained the correct deck. Pye were quite fond of Collaro record changers before the war in their upmarket 'grams like the T18RG. In something of this vintage, it'd either have been an RC4 or maybe an RC1A if Garrard machines were fitted (the RC1 would have been discontinued by the time this set came out) or the Collaro 'bridge'. However, it may equally have just been a single player. It didn't always follow that an expensive instrument like this would have had a record changer.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Mon May 05, 2014 9:49 am

Cathovisor wrote:However, it may equally have just been a single player. It didn't always follow that an expensive instrument like this would have had a record changer.

The depth of the lid suggests that there needed to be headroom above the deck which, of course, is needed if a normal type of changer was fitted but not necessarily required for a single player.

It's a pity you sold your 703. They are such icons of the bygone age of luxury as demonstrated by the top of the range sets - Baird T14, RGD 382RG, the Scophony sets etc. I think provided that I still have the space, the 703 will be the last set I sell - hopefully, not too soon :qq1 .

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Mon May 05, 2014 10:17 am

Hi Brian,
Goodness knows what motivated me to sell the Marconi 703. I sold the set for what I paid for it.
As the set was part of the shop stock, it was bought by the business, so when I sold it the purchaser had to pay VAT on it. At least the Chancellor got something out it.

One thing is sure, I'll never find another one. Unless some battered remains are found in a basement somewhere.

Till Eulenspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Tue May 06, 2014 5:18 pm

At last results. There are still a few jobs to do to this set but least it's showing a picture. The CRT is not too bad at all but the video drive needs boosting up a bit. Remember that this set does not have a video amplifier, the demodulator drives the grid of the CRT via the contrast control. The CRT is designed to give a fully contrasted picture with only 15 volts of video.
Seems that my repair to the sensitivity potentiometer is a success. Operation is nice and smooth. By rights the set really needs a new set valves in the RF unit, it might have six stages of RF amplification but in reality the gain is not all that good. The MSP4 pentodes don't have a high gm. It's only 2.4mA/V. The final RF stage uses an MSP41, that valve is a little better @ 3.2mA/V.
It is well known that design work on these sets was started in 1935 and it is all too evident by some of the old fashioned valves and components used in these sets. For us it only makes the set even more interesting.

Till Eulenspiegel.
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702_0111.jpg
702_0132.jpg

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Tue May 06, 2014 6:22 pm

The Pye TT1 transistor TV set of 1960 comes to mind. In that set a special high sensitivity 14" Cathodeon CRT was made for the set.

till Eulenspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Tue May 06, 2014 8:29 pm

The predecessor of the Pye 915 was the 815. In common with the Marconi 702 the 815 did not have a video amplifier. The full wave vision detector which consisted of two T6D diodes supplied positive going video to the CRT grid direct. The 915 has an anode bend detector which supplies high level negative going video to the CRT cathode.

T6D diode: the valve was recoded as the EA50. http://www.radiomuseum.org/tubes/tube_ea50.html

Till Eulenspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Tue May 06, 2014 10:49 pm

Driving the CRT cathode also meant that the video was hi-level negative going which was exactly right for DC restoration on the grid of the sync separator saving an inverter valve or other method of producing negative going video.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Wed May 07, 2014 12:13 am

That's right. Negative going video for the cathode of the CRT and positive going sync tips for the sync separator. The grid-cathode "diode" clamps the sync tips to ground and negative going sync pulses appear at the anode of the sync valve. This arrangement was to become the norm right up up to the end of the valve TV era. Previous sync separators that were driven with negative going sync pulses were often tricky to adjust and there was always a certain amount video component, however small, in the output.

The 915 chassis employs an EB4 double diode. Is this the first application of the Pye positive interlace filter?

Till Eulenspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Wed May 07, 2014 9:06 am

Till Eulenspiegel wrote:The 915 chassis employs an EB4 double diode. Is this the first application of the Pye positive interlace filter?


Could be, Till. Anyway, we'll see if it works today (hopefully) as the CRT is mounted in the 12C chassis. I first powered up the sound section as this is switched by the volume control and I got CH1 sound from the Aurora booming through. Great, I thought so I switched on the vision chassis - not a peep, not even timebase noise. By this time it was about 22.00hrs so I gave up in disgust and went in doors. During a relatively sleepless night, it suddenly hit me - I had removed the HT rectifier to check the AC voltages on the holder and DIDN'T PUT IT BACK!

I've just thought that you might be thinking "poor old s#d. He's loosing it!" Nonsense, I've always been like that :aaj :aaj

Ah well, the exciting time is after a trip to the platers! Whatch this space.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Wed May 07, 2014 7:16 pm

I have done a few more tests, mechanically assembled the CRT mountings and fitted the CRT.
On switch-on, the EHT rose to 4.5kV (enough?) and a picture, of sorts, appeared on the screen. The focus is not good, the scan sizes are a bit on the small size and the brightness control doesn't function but it's a start. I think that for some reason, the HT is low which would account for the errors so it's out with the CRT and start a bit of fault finding.
One thing though, the CRT looks as though it's a goodun :thumb .

First-Light-1.jpg

First-Light-2.jpg


All in all, I'm quite pleased with the first tests.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Wed May 07, 2014 8:14 pm

Hi Brian,
There's no doubt about it, that CRT is a good one. And that's good news because I believe it is one of the very rare Mullard/Philips tubes with the side contact base. When I last tried it out the MW22-1 CRT in the Pye 838 it displayed a bright picture. The MW22-1 has a complex hexode gun assembly, has your 12" CRT a similar gun?
Today I received the 915 service manual from your goodself along with a useful copy of the original. This gave me the opportunity to examine the sync separator circuit. The sync separator valve is an EF50. What is interesting is that the frame sync is derived from the screen grid and the line pulses are developed across the anode load. I hope I've got that the right way round. The EB4 double diode functions as a separate pulse clippers for line and frame syncs. The EF6 side contact valve is retained as the line oscillator.

Till Eulenspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by TVJON74 » Wed May 07, 2014 10:00 pm

Great work Brian :aad

I have been following this thread with great interest and it's great to see a picture on this very rare set.

You must be pleased with the results so far? It looks like it will be a cracker when its all finished.

Best regards.
Jon

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Wed May 07, 2014 10:06 pm

Thanks Jon. But I know there will be a lot of hard work to get it right!!!
And thanks Jeffrey, for your kind words too.

Till, The 12C uses either a 31/3 or 31/6. The only difference between them seems to be the heater current (.65A and .6A respectively). I assume the gun is the same as the 22/1 as the base is the same and all three are drawn as tetrodes. Unfortunately, the type label is missing so I have no idea which CRT I have.
As to the sync separator, it follows the double anode pentode, the 4TSP, used by pre-war cossors with the line and field syncs taken from different anodes. The Pye method sounds much cheaper than designing and making a new valve!

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Mon May 12, 2014 12:12 pm

A bit more progress. Here are a couple of screen shots:
Second-Light-2.jpg
Not the best testcard in the world!

The non-linearity looks a bit like too much overshoot after retrace. I haven't scoped the scan waveform yet but there is something wrong at the beginning of scan! I guess that a bit of damping may help - a snubbing network perhaps!

Second-Light-1.jpg
2.5MHz just visible - not bad on a slightly out-of-focus CRT! Cogging? Sync sep. checks needed.

I'm quite pleased with the resolution seeing as I had to re-wind all the coils to get down to 45MHz!

I'm not very happy about the short-term streaking (HF loss) but it seems to me that it must be in the video stage (anode bend detector) as the frequency gratings look reasonable! More 'scoping in order.
Pye-12C-Vision-Detector-circuit.jpg
The last IF stage and the anode bend vision detector/video amp.

The detector/video amp valve has an un-bypassed resistor in the cathode so I could add a small puffage across it to give a bit of tip-up at the HF end. I've just noticed the tuned circuit in the detector anode. I guess this is a rejector for the carrier and should be tuned to 45MHz. I'll have to check that! The other HF peaking coils are non-adjustable so a bit of thought is required here!

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Tue Oct 27, 2015 3:20 pm

Does anyone have experience of the method of varying gain (contrast) by varying the screen grid potential? This is the method used in lots of WWII kit and the Pye 12C which gave birth to the Pye 45MHz radar IF strip.
In the 12C, varying the potential does vary the gain but in a seemingly haphazard way with dips at both ends of the control. It's very hard to calculate what the range of potential variation should be becaus, of course, the screens are taking varying current as the potential varies. The circuit, in it's simplest form, is just a pot across the HT supply controlling the first 3 screen grids as can be seen in the circuit in the previous post.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Tue Oct 27, 2015 4:17 pm

Hi Brian,
there is a comprehensive write up about the EF50 in the Radiomuseum website:

http://www.radiomuseum.org/tubes/tube_ef50.html

We know that controlling the gain of the RF pentode amplifiers by varying cathode bias can cause crushing of the vision waveform. The sync pulse amplitude increases but the active video part is crushed.

Perhaps by varying the screen grid volts of the EF50 prevents this characteristic?

Till Eulenspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Tue Oct 27, 2015 4:31 pm

According to the Edwards paper, varying the screen volts to vary the gain does not affect the input capacitance. Varying the g1 bias affects the input impedance so I guess the capacity varies as well. I find it off that course and fine sensitivity controls are required - does that mean the control is fairly fierce?

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