Bush TV22 restoration online

[Panrock]

As promised, my latest restoration of a Bush TV22 will be accompanied by a blow-by-blow, fully illustrated account online, at the customer's request. He even hopes to make a guest appearance at some point during the project!

You can follow the story HERE.

The TV22 has become such a popular set to restore that I hope others will find this story interesting. As the restoration proceeds, if you have any questions about why I did this or that, or feel something needs clarifying - I hope you'll feel free to ask about it here.

Steve

[Kat Manton]

Hi,

I'm following this with interest, a fascinating and informative article, Steve. Thanks for taking the extra time to photograph and document the restoration.

I've already learned something new; there's usually the odd can of Waxoyl lurking around somewhere here but I've never considered using it in potentiometers!

Cheers, Kat

[Panrock]

Well this one has the dreaded TV22 'frame lurgy' whereby the frame expands, particularly the top frame, by an inch or so over the first twenty minutes or so of warm-up then stabilises.

I had this on another TV22 - the one that went to the Dorking pub - and never really cured it. At that time I had replaced everything in the frame timebase except the b/o transformer and the scan coils.

Although admittedly a scope has serious limitations in revealing subtle changes like this, the voltage waveform level across the scan coils doesn't seem to change as the height/linearity changes.

The HT, line timebase etc. is not affected.

Come on you real experts out there (you know who you are) what can be causing this?

Steve

[Duke_Nukem]

Whilst waiting for the experts, does the picture brightness change much in the first 20 mins ? Could it be the LOP stage (less drive -> less EHT, so width might be compensating itself whereas frame stage wont have such luck?

TTFN,
Jon

[Panrock]

Thanks Jon. Nope, the brightness holds dead steady though the focus does take a few minutes to settle.

The b/o and f/opt transformers remain cool running.

Linearity changes too: the top of the frame expands more than the bottom. I'm wondering if it something to do with conditions on (re: Trader Sheet 1003) V15a's grid, which is positively biased and this could be rather critical. Subtle changes in the mutual conductance of this valve as it warmed up could affect linearity, couldn't it? Having said that, I've tried several ECL80s and they all do it.

Steve

[Steve_P]

Just look at the top end of the waveform and see if it changes as the set warms up.
Does it happen with the cabinet off?

If it does not seem to occur with the scope on the secondary of the Frame Output Transformer, go forward. If it does, go back.

How about the smoothing block? Have you changed it and are the new caps up to the job. Only a slight bit of 50Hz here, along with the 50Hz on the frame stage, could cause some funny results.

R36 and C34.

C60 and R63.

The thermistor on the valve heaters?

Cheers,

Steve P.
Who is just as stuck here....

[Panrock]

I've worked through the suggestions but have now come to the conclusion there's not much I can do about this slight expansion of the height as the set warms up. It seems due to a combination of factors.

There was a little 'gunk' underneath on the ECL80's holder and this has cleaned. The pots had been served but have been cleaned again.

I've changed the ECL80 again. All the bits Steve_P suggested had been changed anyway.

The majority of it seems to be due to warm-up in the LOPT. When cold, the overwind's resistance is 843 ohms, when hot: 942 ohms. The eht reduces by about 0.4Kv when hot. Artificially increasing the eht back to its original value as measured (by turning down the brightness) largely restores the picture to its original shape, ie. the height is affected more than the width.

Sadly, because of the compact bakelite cabinet, I'll best advised to run the LOPT with the can lid on. This won't help cooling. However, pressing my finger on the LOPT laminations shows it running as cool or cooler than any I've encountered.

I guess it may be just 'one of those things' about the TV22. It only really matters if you closely monitor the roundness of the Test Card circle for the first 20 minutes. Before the days of the Aurora it probably wouldn't have been noticed.

How do the other TV22s out there perform in this respect?

Steve

[dseymo1]

We sometimes tend to forget that it was normal when sets of this vintage were new, to allow 20 minutes or so for the set to warm up before watching. For 'critical' viewing, the presets often needed a little tweaking too, so I suspect that the height creep is quite normal.
Slightly off-topic, I once had a beautifully built Solartron valve 'scope, which worked brillantly, but took an hour or so to stabilise before critical measurements could be made. Similarly, although I never saw one in the flesh, I had a brochure for a very early (1960s) Philips domestic VTR, again mainly valve-based, which recommended switching on an hour before use to allow the servos to stabilise.

[Top Cap]

Regarding the frame creep, I remember as a young lad reading a piece by Smithy (In your workshop) where he encountered this due to heating of the frame coils. In his particular set there was a thermistor mounted inside the windings to compensate for frame amplitude variation. However I am not too familiar with the TV22 and I bet such a thermistor was not fitted.
Looking at the work done to restore these old sets, I am filled with guilt when I think of the sets I dismantled as a young lad and scrapped them just to get hold of the centering magnets . One never realised at that time that old tellies would be worth anything in the future but most were dumped just because the tubes went low, they were so expensive to replace then. Also it was a known fact that a new tube was most likely to kill off the LOPT so most people bought a new set.

[Panrock]

A postscript on this frame creep, which now appears to have been solved.

Basically it was caused by subtle changes in all the higher value resistors in the frame timebase and the height pot as the set warmed up, even though these resistors were the correct value when cold and didn't get obviously warm in use.

Once these resistors were changed for new, and the height pot bridged with 2M to reduce its variance (with suitable re-adjustment of course) the set remains stable from four minutes after switch-on.

Steve