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Old 12th Apr 2018, 12:40 am   #1
beery
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Default Re-framing a Bush T36

Hi all,

A while back my Bush T36 suffered frame collapse that ended in the failure of the frame output transformer.
I finally got the set working yesterday. Here is the story of how...

I'll start with a little info on the T36. I have to say that I really don't like the design of the set. I'm not sure what Bush were thinking (maybe it was hastilly put together for the start of ITV), the frame timebase is a backwards step compared to the TV22A, the dual band reciever chassis is quite nasty and in terms of access for servicing it is a disaster.
As much as I hate this set, I have it as a recreation of the first TV in my family. This particular set is made with the chassis of a TUG36C which were fitted in an empty T36 cabinet together with a few parts of our original set that my Dad had kept. All the waxies were re-stuffed in the restoration.

So with all that in mind I took the chassis out and soon discovered the failed frame transformer. Ususally a transfomer from a different set can be made to fit, but this set has high impedance scan coils, so I had no choice but to rewind it.

Bush transformers don't have end cheeks on their coil formers, so two identical bobbin ends were 3D printed from PLA to enable the coil former to mount in the AVO coil winder.

The turns were counted to be 6000 turns of 43 SWG for the primary and 1000 turns of 35 SWG for the secondary.
The wire was originally layer wound with a thin layer of waxed paper between each layer. When re-winding I had to use yellow transformer tape between each layer, however it is slightly thicker than the paper tape and only sticky on one side, so the new windings tended to slip off the end of the bobbin...
After much stressing, I managed to wind the primary and indeed also did a second primary for my Bush TV11A (the primaries are the same).

Whilst winding the secondary it became apparent that I could not fit all the turns on, so I removed the secondary. I then removed the last layer of the primary to reveal a full layer of turns underneath (hence a nice even basis for the secondary). About 100 turns were removed, so the secondary turns were also reduced to maintain the 6 to 1 ratio.
However still the secondary would not quite fit, so the last layer of this was removed, slightly upsetting the winding ratio, but still close enough.

I put the transformer back together, put it in the set and switched it on.
I was greated with a perfectly proportioned raster. However after a while the picture started to bounce, curl up at the bottom, vary in height and occasionally collapse, which is what was going on before the transformer originally.

Clearly there was still another fault that put the newly rewound transformer at risk.

To be continued...

Cheers
Andy
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Old 12th Apr 2018, 1:16 am   #2
Argus25
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Default Re: Re-framing a Bush T36

I must say I like the way you took the time and patience to layer wind the replacement windings. They look really great, well done.

I wonder if the frame output tube is getting turned on hard by a fault causing + voltage to its grid, like a leaky cap or a problem in the drive circuit....I assume the original primary winding went O/C (the secondary wire looks quite good and not over-heated)

It would be good to find a plastic tape replacement for the super thin paper used on these old transformers, its quite a bit thinner than any transformer tape I have found so far.
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Old 13th Apr 2018, 12:28 am   #3
beery
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Default Re: Re-framing a Bush T36

Hi Argus25,

Quote:
Originally Posted by Argus25 View Post
I must say I like the way you took the time and patience to layer wind the replacement windings. They look really great, well done.
Thanks, I wanted to have a go at layer winding, to learn what it was like. I'm building up to having a go at the frame oscillator transformer for my HMV 905 (I have a very similar transformer from an HMV 1804 in it at present).
I suppose however that to rewind the transformer in the Bush, I could have 3D printed a whole new bobbin with end cheeks and not bothered with layer winding...

Quote:
I wonder if the frame output tube is getting turned on hard by a fault causing + voltage to its grid, like a leaky cap or a problem in the drive circuit....I assume the original primary winding went O/C (the secondary wire looks quite good and not over-heated)
Yep, you are very warm there. All will be revealed in the next instalment...

Cheers
Andy
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Old 13th Apr 2018, 7:34 am   #4
rogerdup
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Default Re: Re-framing a Bush T36

Hi guys,
Andy, change all those old wax capacitors at once.
Have a nice week end!
Regards,
Roger
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Old 15th Apr 2018, 7:55 pm   #5
beery
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Default Re: Re-framing a Bush T36

Hi all,
here is the final part of the story of the frame timebase.

Quote:
change all those old wax capacitors at once.
Haha Roger. I know wht you mean though. It is about 10 years since I carried out the initial restoration and the solder joints on the rebuilt wax capacitors have dulled down, so I have to remind myself that I have already re-stuffed them.

Back to the story.
I reluctantly took the set appart again in order to find what was going on. Taking into account what I have just said above, I knew that I could rule out the waxies when it came to fault finding.

I checked all the resistors and found a few slightly out of tollerance, but nothing that would really cause a fault.
I then checked some of the bypass electrolytics, 2 associated with the boost supply and also the frame output cathode bypass. They checked out ok with a capacitance meter, but I decided that since it was so difficult to get the chassis out of the set, that I would change them anyway. I also decided to replace the audio cathode bypass as it was next to the other 3. It turned out that the audio cathode bypass cap was turning resistive.

Regarding electrolytics. I find that HT voltage large can types are the best at re-forming, but the physically smaller the capacitors are, the worse they respond. Low voltage capacitors seem to be particularly bad.

Anyway, I sliced each capacitor open by sawing them around where they had been clamped. I removed the old innards, washed everything out and dried the casings with a hairdryer. I then drilled a small hole below each solder tag to bring out the leads of the new capacitors. After re-stuffing them they were glued back together with 5 minute epoxy and put back in the set.

I was not convinced I had found the fault though. I then checked the potentiometers and found that the interlace control was resting on a dead spot. I did not have a 100K pot to hand so I opended it up and wiped it with an IPA soakced cloth. This did the trick, but I noticed that one end of the pot had never been connected, so this meant that if the wiper failed to connect the HT feed to the frame osccillator would be wrong. A wire link between the wiper and the unused end of the pot was a simple fix.

I put the set back together and powered it up. It worked for a while but started to fold over at the bottom after 10 minutes or so. I remembered that when the fault first showed up up tried changing the ECL80 frame timebase valve before the output transformer gave up.
I put a different ECL80 in the set and hey presto, it now works perfectly

The first photo shows a rebuilt capacitor that is awaiting having its leads cropped and soldered to the original solder tags.

The second photo shows the 4 rebuilt capacitors back in the chassis.

The third photo shows the interlace control pot with a wire link added from one end to the wiper.

The fourth photo shows the frame timebase ciircuit. On the top left you can see the interlace control, VR9. If VR9 fails then the circuit will not oscillate properly. It is worth noting that the blocking oscillator transformer failed a couple of years ago, it is actually the same as the one in the TV22, but with a tag board on top to disguise it.

The fifth photo shows the result.

Cheers
Andy
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Old 16th Apr 2018, 12:22 pm   #6
beltinge bore
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Default Re: Re-framing a Bush T36

Hello Andy
I marvel at the quality of your work!
I know I should be working on the EKCO TSC48 that I bought from you but by a strange coincidence I have been battling with the Frame section on a Bush T36!
I would agree with you that it is not one of Power Road’s W4 best products (a mix of quite old fashioned frame CCT and various other bits and pieces).....and a real pain to work on!
Mine was a Xmas present from my nephews and came from a forum member in Welwyn Garden City. It had some caps replaced with a variety of different types. I fired it up gently and it produced a very strained raster so I began with the main chassis (boost cap etc/the usual suspects).
The result was a dim but usable picture with some frame problems. A Philips 17/47U chassis and tube (no cabinet) were offered on eBay just down the road in Dover for £1.50 buy it now. I was just going to use the tube (Mazda Labelled MW43/69 with Mullard M on the cap - Never seen that before) but curiosity got the better of me and many Philips tar caps and resistors later I had a working chassis and what is a very good tube with no where to put it so to speak!
I put the tube on the Bush chassis and it is brilliant. I have carried on evicting waxies and all the 1000pf little black slugs from the RF chassis. At one stage I had a bizarre fault with a barrel shaped squat raster. I feared the worst (scan coils) but it turned out to be C21 8uf O/C. This led me to change the other small value electrolytics although they all tested OK. I now have good sound and a nice picture with good frequency response……Except for Frame fold over at the top! I have tried and tested everything in the area including NOS ECL 80’s. Height is OK frame hold rock steady both frame Lin pots test OK and have an effect. The interlace control tests and works OK. Any ideas?
Regards Steve.
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Old 20th Apr 2018, 8:05 pm   #7
beery
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Default Re: Re-framing a Bush T36

Hi Steve,
I would suggest double check everything and then also check the two 3K3 resistors across the scan coils. You have to unsolder one end of each resistor to measure them.

If you think about it, top foldover is caused by incorrect biasing of the output stage, slow/late flyback or unwanted ringing.

And for those who are not familiar with these sets (you really don't want to be!), what Steve and I are talking about in terms of access is:-
The chassis can't be run out of the set without extension leads to the CRT and scan coils etc.
To remove the chassis you have to:-
1 Remove a bracket on the receiver chassis (luckily this bracket is missing on my set).
2 Remove both the band 1 & 3 tuning slugs
3 Disconnect the band switch shaft (the knob end) from the shaft coupler
4 Unscrew four screws underneath the chassis (you need to roll up your sleeves and removed your watch to get your arms in)
5 Discharge the CRT and remove anode cap
6 Disconnect speaker
7 Loosen side control panel retaining nut and slide out the control panel
8 Disconnect CRT base
9 Disconnect the two chassis to cabinet screen bonding wires
10 Remove two screws fixing the chassis to the rear of the set.
11 Pull out the chassis just enough to gain access to the scan coil plug
12 Unplug scan coils
13 Remove chassis

To remove the reciver chassis from the main chassis:-
1. Disconnet vision tag
2. Disconnect synch tag
3. Unplug receiver supply plug
4. Unplug contrast control
5. Remove band switch shaft coupler
6. Remove two screw securing the vision chassis and pull the chassis out.

As you can see this must have been a nightmare for repair technicians, they can't have made any money fixing them. Thankfully the chassis was short lived and replaced with the superb TV53 and TV56 etc.

Cheers
Andy
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