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Old 2nd Feb 2018, 10:49 am   #1
Keith
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Default Bush TV22 restoration

Having repaired the LOPT (https://www.vintage-radio.net/forum/...d.php?t=143229) I'm now starting to tackle the other problems in this set.
I am attempting to rewind the blocking oscillator transformer which has any open circuit secondary. As I don't have a coil winder I'm using the hand drill method. I have successfully done this on an early RAF microphone transformer but this one might be a challenge too far! I was hoping to re-use the original wire but am discovering multiple breaks due to green spot, etc, so I think I will have to replace it. I have a large reel of 40 swg which I'm hoping will be suitable. Having failed to count the turns as I pulled off the old wire, can anyone let me have winding information for this transformer please. I shall give it my best shot, but if all else fails, at least I am aware that there are rewinding experts in the society!
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Old 2nd Feb 2018, 10:58 am   #2
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Default Re: Bush TV22 restoration

Keith,

If you have a sample of the original wire and know the diameter of the copper, and acquire its resistance from R = pL/A (or the wire tables) it helps and if you can measure the geometry of the space the wire occupied (regardless of any insulation with it), you can from that estimate the average turn length. Then if somebody has the original DC resistance value of the winding, you can work out the number of turns that were there in the first place. (R = ohms, L = length, A = Cross sectional area of the wire, p = 1.72 x 10E-8 for copper off hand)

Failing that , somebody may have the turns number data on file.

Hugo.

Last edited by Argus25; 2nd Feb 2018 at 11:15 am.
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Old 2nd Feb 2018, 11:28 am   #3
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Default Re: Bush TV22 restoration

I think this must have come from another forum member, but I can't remember who...
Attached Files
File Type: pdf BUSH TV22 FRAME TRANSFOMERS DETAILS.pdf (6.2 KB, 123 views)
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Old 2nd Feb 2018, 6:37 pm   #4
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Default Re: Bush TV22 restoration

Hi Richard,

Thanks very much for the info. I'll let you know how I get on (I may be some time!).
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Old 3rd Feb 2018, 3:38 am   #5
Ed_Dinning
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Default Re: Bush TV22 restoration

Hi Keith, blocking osc transformer is approx. 2000t and 3000t of 0.071 mm wire. I'm away from my records at present so approx. figures only.
It is easier to make up some cheeks and glue them to the former before rewinding.

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Old 3rd Feb 2018, 6:11 am   #6
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Default Re: Bush TV22 restoration

I make 0.071 mm to be 45 or 44 wire gauge, correct? It will be tricky with a hand drill, that's pretty thin wire.
I bought a cheap,17 or so, 2 speed manual winder from China and it copes with thin wire very well.
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Old 3rd Feb 2018, 6:48 am   #7
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Default Re: Bush TV22 restoration

Keith,

Most vertical blocking oscillators are in the range of 1:1.5 to 1:4 ratio. American sets favored a close to 1:4 ratio. These small transformers are readily available on US ebay too.

If you did use one with a higher ratio, it is necessary to reduce the value of the associated time constant capacitor.

In the case of the TV22, I think it is best to do what you are doing and attempt a rewind of the existing transformer stack. You can fashion a new bobbin for the stack fairly easily, that way when its finished it will look more original than any substitute transformer.

It is quite a lot of fun winding small transformers. I set up a small lathe with an electronic turns counter to wind such items. Picture attached.

Hugo.
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Old 3rd Feb 2018, 11:15 am   #8
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Default Re: Bush TV22 restoration

Nothing unusual in finding an O/C blocking TX, I have found these to be a common failure with the TV22/TV24.
If you are unsuccessful with your rewind, try forum member jerryw22.
He supplied me with a new bobbin for my set at a very reasonable cost.
It was quite straightforward to dismantle the lams and rebuild with the new bobbin, the rebuilt TX works a treat


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Old 3rd Feb 2018, 3:17 pm   #9
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Default Re: Bush TV22 restoration

Thanks for all the very useful advice/info. I too had concluded that I will need to add cheeks to the existing former. This will enable me to wind "bulk" windings rather than the beautiful original layers which I cannot hope to replicate.
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Old 5th Feb 2018, 6:19 pm   #10
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Default Re: Bush TV22 restoration

While I am waiting for 43swg wire to arrive I'm replacing leaky wax capacitors and excessively high resistors. I'm curious about the function of the series RC between the heater tap on the dropper and HT line. The capacitor (0.75uF) is missing at present and the 1200 ohm resistor has got extremely hot in the past, the paxolin board underneath it now being blackened and delaminated.
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Old 5th Feb 2018, 7:25 pm   #11
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Default Re: Bush TV22 restoration

Its for hum reduction.
https://www.vintage-radio.net/forum/...ad.php?t=40881
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Old 5th Feb 2018, 7:28 pm   #12
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Default Re: Bush TV22 restoration

Hum bucking is what I have understood but I can't see that it will work all that well as mixing 50Hz AC with the half wave rectified ripple is only going to get rid of the fundamental. Leaving it out seems the common solution.
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Old 5th Feb 2018, 7:34 pm   #13
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Default Re: Bush TV22 restoration

Quote:
Leaving it out seems the common solution.
I agree, all it seems to do is increase power consumption.
Both my TV22's and my TV24 work well without this in circuit.

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Old 5th Feb 2018, 7:51 pm   #14
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Default Re: Bush TV22 restoration

I looked up once what the .75uF does and came to the same conclusion - nothing except explode!

What I do now is slide the sleeving up and cut an inch or so out of the .75's leadout to isolate it. Then the sleeving can be slipped back over the cut leadout ends. It leaves the set looking original.
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Old 5th Feb 2018, 11:32 pm   #15
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Default Re: Bush TV22 restoration

Thanks gents,
I wondered at first if it was a modulation hum filter but the values were much too large. Interesting that it makes little difference - presumably it did back in the day. I guess excessive 50Hz ripple could produce apparent brightness modulation due to beating with the frame period if this was not locked to the mains frequency.
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Old 6th Feb 2018, 4:16 am   #16
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Default Re: Bush TV22 restoration

It would be interesting to know if this arrangement was selected to exactly cancel the ripple, or partially (under) cancel it.

The fact that a specific value like 0.75uF was used suggest it was carefully set up for optimal hum cancellation.

The implication is that; then would become critical that the dual filter electrolytic capacitor had both the same uF value as the original, and about the same ESR. So if the hum cancelling network is retained, one should bear this in mind when rebuilding the dual electrolytic, to make the uF value as close to the original as possible for the two sections.
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Old 6th Feb 2018, 4:37 am   #17
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Default Re: Bush TV22 restoration

Quote:
Originally Posted by Keith View Post
..... rather than the beautiful original layers which I cannot hope to replicate.
Back in the late 1970's I required to replace a layer wound line blocking oscillator in a TV which used an MW31-74 CRT. It had a fairly thin stack of iron laminations (many American line blocking osc transformers had ferrite cores even from the late 1940s).

In any case, it was important that not only the inductance was correct, but the winding self capacitance so that the self resonance and flyback time would be unchanged. At that point I didn't twig to the fact that I could have just jumble wound it for the correct number of turns and added external capacitance. So I was hell bent on having it layer wound, with the same geometry paper and the same wire & layers.

I visited a local transformer crowd and pleaded with them to layer wind it. (They thought I was a lunatic). In any case I persuaded them to do it and to my astonishment they perfectly layer wound it, no cheeks on the bobbin, with fine layers of paper. But it cost a small fortune. Even then the company manager remarked "I haven't seen a transformer wound like this in years what a masterpiece". I still have it somewhere.

With a frame blocking osc transformer the self winding capacitances are rarely an issue, more often too low than high and external capacity is easily added if required to get the correct flyback time.

As far as I am aware, there is only one common type of "transformer" still perfectly wound with layers and fine wire and this is the standard auto oil filled ignition coil, otherwise this technique has all but vanished.

Last edited by Argus25; 6th Feb 2018 at 4:57 am.
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Old 6th Feb 2018, 10:50 am   #18
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Default Re: Bush TV22 restoration

The .75uf capacitor was employed by Bush for a good number of years.
The value was .5uf in later receivers, the last model to employ it being the TV115 series. It had vanished by the time the TV125 dual standard receiver was in production, around 15 years in total.

With it disconnected it is possible to observe a very slight ripple, very slight that would never be noticed by the customer.

I think Bush were the only company to use this circuit feature. Was it part of the TV11 circuit? My manual is not to hand at moment. John.
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Old 6th Feb 2018, 1:30 pm   #19
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Default Re: Bush TV22 restoration

You can just disconnect the capacitor for now.

It reduces the ripple on the HT line from about 0.7v rms to about 0.15 rms. The set will work fine without it, but you can see the picture expand and contract slightly as the Aurora generated 50Hz beats gently with the mains 50Hz. With it in circuit this effect becomes virtually invisible.

However given the amount that the picture size varies anyway with scene brightness it's not essential. If you do replace it ensure adequate voltage rating. It has ~185vdc on one side and ~200vac rms on the other. so must be able to tolerate (200*1.4) + 185 = 465vdc. Which is the worst case during a negative AC cycle.
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Old 6th Feb 2018, 11:56 pm   #20
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Default Re: Bush TV22 restoration

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Originally Posted by wd40addict View Post
so must be able to tolerate (200*1.4) + 185 = 465vdc. Which is the worst case during a negative AC cycle.
I replaced the one in my set with a 0.68uF 1kV rated polypropylene type. I think any capacitor that gets mains AC voltages, or close, on it should be 1kV rated, to ensure safety. The reason is that voltage spikes come along at times riding on the mains wave and they are generally sourced from a low impedance, so the effects can be very destructive, if the insulation in the capacitor fails. I also have seen failures in 600V & 630v rated poly caps with mains applied, where you would think they should be just fine and also the common X2 rated caps that are generally not reliable either and I no longer use them. Repeated voltages spikes break down the metalized paper and the capacitance falls with time, that is if they don't smoke first. Probably a DC 800V rated cap would be fine for applied mains AC voltages but its an odd rating and 1kV is more common.

Luckily modern capacitors are smaller than their vintage counterparts so the size often matches.

Last edited by Argus25; 7th Feb 2018 at 12:01 am.
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