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Forum 135

[Closed] Restoration of a possibly unique pre-war TV

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Brian Cuff
(@briancuff)
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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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Posted : 07/05/2014 10:06 am
Brian Cuff
(@briancuff)
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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 .

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

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Posted : 07/05/2014 8:16 pm
Till Eulenspiegel
(@till)
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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.

 
Posted : 07/05/2014 9:14 pm
TVJON74
(@tvjon74)
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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

Jon
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Posted : 07/05/2014 11:00 pm
Brian Cuff
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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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Posted : 07/05/2014 11:06 pm
Brian Cuff
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A bit more progress. Here are a couple of screen shots:

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!

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.

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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Posted : 12/05/2014 1:12 pm
Brian Cuff
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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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Posted : 27/10/2015 3:20 pm
Till Eulenspiegel
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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.

 
Posted : 27/10/2015 4:17 pm
Brian Cuff
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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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Posted : 27/10/2015 4:31 pm
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