Bush TV12B Restoration

[mark pirate]

Well the Bush has made it on to the bench, I have a feeling that I have a lot of work ahead!

A bit of background info first.
I purchased the set nearly a year ago, but did not collect it until July whilst on holiday. I had already posted a wanted ad for a replacement tube, which thanks to a forum member came in the guise of a complete Pye LV30 chassis whose cabinet was disposed of due to extensive woodworm.

I was intrigued as to how a Sutton coldfield set had ended up in deepest Devon, the seller told me that he had purchased the set in Birmingham many years ago as a display piece.

The CRT was already broken when he acquired it. I had a good look at it several months ago and carefully applied power via my variac after bridging the CRT heater.

The results were promising, there was both a line whistle and EHT.
The audio output was also working, and touching the aerial socket with a screwdriver showed the receiver to be passing a signal to the speaker.

I have heard that the LOPT can be a problem with these sets, so it seems wise to replace any components that may cause damage to it first.

I have all the service info and replacement caps to hand, but if anyone is familiar with this set, any hints or tips will be more than welcome.

Mark

[line sync]

Hi Mark
I restored one of these approx 15 years ago and although it worked reliably I was never really happy with it .
I`m hoping I get inspired by your restoration and so get my TV12 working properly .
I had no LOPT problems with my set but seem to remember I had to replace some of the rubber wire.
If your using a crt with external dag coating then you wont need the visconal eht capacitor because this would need replacing.
This chassis is well made using tag boards like the tv22 which I think is a far better way of designing a chassis.
Hope you enjoy the restoration Mark.

Robin

[mark pirate]

Hi Robin, The replacement CRT is a MW22-18. This has the same base and a dag coating, so the original visconal will no longer be needed, but left in place on the chassis.

The rubber wiring on this set is amazingly still supple and thankfully will not have to be replaced.

I have now removed the cabinet and given the chassis a good look over, the chassis is certainly very grungy, there is some corrosion to the aluminium and some surface rust.
Obviously stored in poor conditions before the previous owner got it.
The mask is fixed to the CRT unlike the TV22, it is very cracked and crazed.
The good news is that I can smash the old tube to release it if need be.

I do know that this set has been in a warm and dry environment for the last two decades though, so any damp in the wound components will be long gone.

It is certainly very interesting to compare it with the TV22, the family resemblance is quite obvious, but this is a very different set in many ways.
Unlike the TV22, this has separate EHT and line output transformers.
The valve lineup is similar to the Pye D18T, no less than 10 EF50's!

I have done a resistance check on the blocking transformer, and it is spot on to the Bush service sheet.

It uses the same back to front speaker as fitted to the DAC10 radio, I am not sure why though. There is plenty of room for a conventional speaker, maybe magnetic shielding was an issue?

A few more photos of the innards.

Mark

[McMurdo]

Hi Mark, this was the first pre-60's TV I restored and found it straight forward enough having previously only restored 60's hybrids.
One annoyance was that although I tried to reform the smoothing caps, and indeed it worked ok for about half an hour, once the set got warm one of them vented violently and sprayed the underside of the main deck with disgusting boiling wax which got everywhere; they're positioned ideally to do that, be warned!

Mine is sometimes left untouched for a year or more and then gets dragged out, it's always worked first time, although tapping it reveals bad valve holder connections.

I have the original non-dag tube and it runs ok with the visconol (note the spelling!) disconnected.

[beery]

Hi Kevin,
I thought is was spelt vis-con-all

Cheers
Andy

[mark pirate]

Hi Kevin, I hope my electrolitics will be OK, their bases are nice and flat with no sign of leaks or venting. Thanks for the warning though.

Reading through the Bush service info has been very interesting, it appears that later sets had some modifications to increase the EHT.
I seem to have the later EHT transformer with the yellow spot, but further mods included the EHT bleeder resistors. These were changed from 6x 10 Meg resistors to 7x 27 Meg +/_ 20%, so it appears that mine is a later set.

They have increased in value but are still in spec bar one, which appears to be O/C. I will have to order some replacements, so before I go any further it would make sense to check my way through the set and make a list of any that are out of spec and add them to my order.

I am carefully cleaning the top deck at present, it is smothered in what appears to be thick black coal dust. In fact this is the most filthy TV chassis that I have seen

The EHT wiring from the CRT and top cap of the PL38 will need replacing, but so far the rest of the rubber wiring looks to have survived well.

The photos below show the EHT bleeder resistors, I will be replacing all the components on the board including a new EY51.
You can see the black muck in the photos of the RF unit, Yuk!

Mark

[wd40addict]

I wonder why they fitted bleeders when the TV22 doesn't have them, maybe a hang over from mains EHT?

[beery]

Good question.
I've had a quick look at the circuit and it seems that the bleeder is tapped to provide the A1 supply.
Although there is an efficiency diode (providing big improvement on EHT regulation compared to the TV1), the recovered voltage is not really exploited, it does not even add to the voltage available to the PL38, but merely props up the line oscillator.
Looking at the valve voltage readings on trader sheet 971, it would seem perfectly reasonable to hook the A1 to the efficiency diode (I might try it on my TV11), so maybe it is to do with EHT/focus regulation.

Cheers
Andy

[mark pirate]

It seems 27 meg ohm resistors are hard to find, no results from CPC search
They will have to be fitted before I power it up. I suppose I can get on with recapping the timebases in the meantime...

Mark

[brianc]

The resistors on your bleeder panel are 33M, Mark!

[Jac]

And why replace those resistors when they are still fine Mark?
Did you already measure them?

Jac

[ukcol]

Originally Posted by mark pirate

It seems 27 meg ohm resistors are hard to find, no results from CPC search
They will have to be fitted before I power it up. I suppose I can get on with recapping the timebases in the meantime...

Mark

Hi Mark

I have some H.V. film resistors in the following values ....

200 M ohms
100M ohms
50 M ohms
10 M ohms
3M3 ohms.

They are physically larger than the 33M resitors in your picture.

I had a look at Trader sheet 0971 to see what tappings there were in your EHT bleeder network. I was hoping to suggest a combination of the above values as replacements. However the resitors in the EHT bleeder chain are 10M ohms on the Trader sheet.

Anyway, if you can make up a chain using the resistor values above and you are not concerned about exact originality PM me with your address and I'll send you some.

[mark pirate]

The resistors on your bleeder panel are 33M, Mark!

Must be my poor photography Brian, they are red, red, blue (22M)
According to the Bush factory manual, mine has the factory modifications as I listed in post #5.

Further mods included the EHT bleeder resistors. These were changed from 6x 10 Meg resistors to 7x 27 Meg +/_ 20%.

The resistors fitted are definitely factory original 22Meg
Is it that critical I wonder?

Mark

[ukcol]

Your 154M ohm chain could be replaced with a 100M + 50M H.V. film type.
The offer is still there if you want it.

[mark pirate]

Thanks for the kind offer Col, I would rather keep the original look, but will let you know if I change my mind.

I will probably go with the value that is already fitted (22M). I have found these with a 3.5Kv rating available from CPC.

Mark

[brianc]

Mark - you don't need very high voltage resistors for that chain. There is only about 1kV across each resistor and this would almost certainly be within the specs for a modern 2W resistors (the originals are 1/2W Eiries!). That would save a lot of money!

[mark pirate]

Hi Brian, the new resistors are metal film 500mW 2.5Kv rated and are only 14P each, so will not break the bank

I might as well replenish my stocks of resistors by adding to the order a few other higher value types, I still need to go through the set to check the rest of the resistors before placing an order.

I will try to find the time to remove the mask from the CRT, it is in pretty poor condition. Not sure how to go about repairing it yet, but it will require some sort of flexible filler before it gets repainted.

Mark

[beery]

Hmm, a nasty looking mask.

Soak it in warm soapy water I think, but you still might need a flat bladed screwdirver and a knife to release it. Best to start from the front and free that first.
This is the same mask as used on the PYE D16T and Invicta T102.

I'm not sure about repairing it. You can buy things like Devcon GP Putty, which is for repairing conveyer belts, but it is black in colour. Silicone sealants tend to be a bit too soft, but it you do try that route, try to the get electiral type which cures without producing acetic acid in the process.
I've also just spotted Raytech Magic Rubber, which is a form of potting compound.

I suggest you find some old natural rubber parts (or masks that are too far gone) to use as test pieces

Regards
Andy

[McMurdo]

Some time ago I ordered some special tyre paint for one of the vintage cars. It was for filling or improving the look of cracked but otherwise-sound sidewalls. This was black (well..very dark grey) and dried like liquid latex; I know because I got it over my hands and it was like peeling off a tough outer skin. I wonder if something like that might work, built-up. A white thick latex solution would surely be available from a theatrical supplier or ebay.

My TV12 is slowly starting to crack its finish around the picture end of the tube and I'd thought about something similar. There's already a thread on the forum about the type of creamy green-white paint to use.

[mark pirate]

Soak it in warm soapy water I think, but you still might need a flat bladed screwdirver and a knife to release it. Best to start from the front and free that first.

I will give that a try, if it is really stuck fast I can always take a hammer to the old CRT

Liquid Latex or non acidic silicone looks like the stuff to try, a wet finger should smooth it in to the cracks nicely.

There's already a thread on the forum about the type of creamy green-white paint to use.

Thanks for the heads up, I will search for it.
It is interesting that the TV22 mask was screwed to the cabinet rather than the tube itself, my early TV22 has the same off white colour mask. The later sets used a pale green.

If the absolute worse happens, I do have a spare mask from a TV22 I could use.

Mark