Bush TV22

Well I have now got the tube heaters reading 6.3V. I turned the set on its side tapped the neck a couple of times and with the DMM connected watched in amazement as the voltage rose from 5v to 6.3v!

I think I mentioned earlier that when I first switched the set on the HT was 190V. I now find that it is only 160V. Something must be loading the HT. The boost HT is 200V (280V)  and the first anode is only 75V my other Bush TV 22 gave a reading of 150V. I disconnected the EHT supply to the tube but the voltages stayed the same.I also replaced the 470pF (C34) across the LOpT as recommended by LL Johns but still no "first light"! Tomorrow I think I will try and replace some of the low value capacitors around the line osc stage.

Funny thing is the sound amplifier is now working and I get a mild hum through the speaker.

Its end of play time for a while. I was concerned about the low metrosil voltage. So I measured winding b (trader sheet) resistance was only 27 ohms, should be 30 ohms, it was that when I stock the transformer together! So I will have to see about rewinding the Tx or getting a replacement!

3 ohm's won't make much difference surely? Quite normal variation I would expect to be honest.

I am sure Chris has done a write up of ringing a LOPTX to test it. I am not in a position to search just now, I will look later when home.

https://www.radios-tv.co.uk/333-2/

I gave it a ring test after "sticking it" back together before putting it  in the set.

I also measured the resistance of the windings and B  was exactly 30 ohms. It is now 27 ohms and what is more it fails the "ring" test!

The low HT must have been caused by the LOpT as it now as risen to 188V with no transformer fitted. So I suspect I have got shorted turns on the B winding, so play time over until I sort out a replacement or substitute transformer.

I've never taken too much notice of transformer winding resistances when they are given. It only takes one shorted turn to cause a transformer to fail and one turn won't make any difference to the resistance of the winding....not on a standard meter at any rate. At best they can be used as a guide. A ring test is the best way to check if a transformer has any chance of working.

Well the shorted turn could be between layers so we could have quite a few turns that are shorted out not just two wires next to one another. In my case I would imagine we have 3 ohms of windings putting a  3 ohm load on the TX.

Well yes of course that is possible in which several ohms difference could indicate a problem.  What I was trying to point out is that anyone checking a transformer by resistance, shouldn't necessarily worry about a few ohms difference since one shorted turn will kill a transformer but not result in any significant difference in winding resistance.

In any case you have 'double confirmed' it's faulty by carrying out a ring test.

Thought it might be of use to have this posted in this TV22 thread for those following now and in the future. As far as I'm aware its not been posted here before. Also available in the data library.

*Measured resistance and inductance derived from measurements taken from three different Bush TV22 line output transformers. These measurements were made with an Atlas LCR 40 component analyser which has a specified accuracy of +/- 1%

Does anyone have the winding and coil size details for the TV22 LOpT?  I may have a go at rewinding the primary.

First post gives wire size and number of turns for the primary and over wind.

https://www.vintage-radio.net/forum/printthread.php?t=37396

Well worthwhile reading thread in the previous post, gives some details of how it was rewound.

Chris, I tried to edit the above post but it stated I was out of time.

Thanks for that. I have been doing some searching and have come up with the following data. Using  ET 1091 for winding layout.

Winding (a)  = 310 turns

Winding (b) = 1040 turns

Winding (d) = 630 turns

Winding (e) =  60 turns  all above using 0.22-0.25mm dia wire.

Winding (c) = 2960 turns  EHT overwinding using 0.1-0.12 dia wire.

I have dismantled the LOpT. The laminates (see photo) are in a bad condition. Some of them are rusted together. Would laminates from a mains transformer be suitable replacements?

I unwound the top layer primary winding which was reading 27 ohms. When it was all over the floor I measured it again and it was back to 30 ohms!. There was 9 layers, so it must have had a short between 2 layers that took the resistance down to 27 ohms!

I don’t know about other laminations, I have never had to deal with repairing them, spares were plenty, time was short so it was out with the old, in with the new when I was in the trade.

I would just remove the rust from the lamination's and try them . I would expect they should be OK or at least good enough to test the rewound transformer.

Frank

Well I thought I would have a go at rewinding the LOpT. I used a plastic phone card as the former. I have done 3 windings each separated. The wire has come from a transformer primary winding. It is all hand wound. So I am now looking for a suitable outer former that the eht overwinding will go on. The original isin a bad state and broken. So it is still work in progress.

I would try using the old EHT winding if it is at all serviceable as winding the EHT winding will not be easy.

The original EHT winding will have been wave wound. If you don't wave wind, it's self capacitance will be much higher and most lightly will inhibit the winding from producing the required voltage.
One possible way around the self capacitance would be to use multiple windings and diode split them. The other problem is the insulation between layers. Keeping the number of layers high and the windings narrow helps. Some form of additional insulation will be also needed I usually use coil dope.

Frank

I have read in a post on another forum were the EHT overwinding was  handwound using 37 layers each of 80 turns and that seemed to work. The width of the winding was twice that of the original. The EHT overwinding I have seemed to be OK when I first powered the set up as I got a spark from the overwinding. When it was dismantled it read as O/C.  I unwound some of the windings and found the break. The EHT winding now has a resistance of 760 ohms, before it was 840 ohms. So I would imagine it would produce some EHT maybe 10% low.

I am only experimenting. The split diode method sounds interesting. Fit a diode say ever 300 turns and no need for a EY51 or the heater winding? Another thought was a voltage quadrupler taken from the anode of the PL38?

I am still waiting for a few parts to arrive before I start on what I really want to do is, to build the "Hedgehog" standards converter. The post from China seems to take an age!

In this thread post #46 there are some photos of a diode split winding. Which consists of two windings with a diode between them. The original rectifier is still used. The former is made from acrylic sheet.
There is a high potential between the windings so good insulation is required.

Frank