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22nd Feb 2005, 9:10 pm | #1 |
Octode
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Transformer 'T4' for Bush TV24 Required
I realise that this is a bit of a long shot, but I was wondering if anyone out there had a spare transformer as used in the frame blocking oscillator section b of V15 (ECL80). Winding (a) on mine is o/c! [Needles to say this is quite effective in providing total frame colapse! Bah humbug ]
I am looking at Trader Service Sheet 1091/T38. The transformer in question sits on the tag strip mounted on the outside of the chassis on the volume control side (small job with flying wires). I you do, and are willing to part with it then I'd really love to hear from you for sure. Here's hoping ..... Regards, Andy |
23rd Feb 2005, 9:24 am | #2 |
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Re: Transformer 'T4' for Bush TV24 Required
Hi Andy
If you cannot find one, it should be not too bad a job rewinding it, and not too critical in this application. Many other TVs of this era used a blocking osc here.
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23rd Feb 2005, 6:54 pm | #3 |
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Re: Transformer 'T4' for Bush TV24 Required
Thanks for both of your replies.
The data sheet quotes dc resistance readings of 540 and 480 ohm. I guess, judging by the size of the transformer this means that the wire is pretty thin (and loads of turns). If I start to unwind the thing, I bet the first winding off will be the surviving one (someone's law is surely to apply). I have no mechanical aid to asist me with a re-wind and if I do it by hand I bet I'll end up with one big fat ugly mess. That's not to say that I'm frightened of having a go, but you know how these things go, you start to unwind, forget where you are with the counting and then by the time you've worked out how much wire you need in one continuous length you'll forget which way it was wound with respect to the other winding etc etc. I started to tackle a LOPTx overwind once and wish I'd never started. I think I still have the remains in a cardboard box somewhere! Why the high resistance windings anyway guys? Because of this there is nothing available from RS, Farnell or CPC that comes close. Of course there's the other matter concering the ratio .... mmmmmm... what to do what to do ... Thanks for the tip on the insulated tape - I must confess I never knew what it was called. Goodness knows where you'd get some from these days though. Also, sorry to show my ignorance here, but what is meant by a blocking oscillator? I keep hearing the term, but I'm still on the learning curve I'm afraid. Many thanks again - much appreciated. Regards, Andy |
24th Feb 2005, 9:31 am | #4 |
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Re: Transformer 'T4' for Bush TV24 Required
Hi Andy, Scientific Wire Co may be able to supply, or look in your area for transformer manufacturers, its surprising how many ar out there. They may be able to help with the wire and / or tape. The best way to estimate turns is to look up you chosen wire gauge in the wire tables and work out how many turns you can fit into the volume of the bobbin. 80% of this is a reaslistic figure for hand winding. You should then be able to cross check against resistance values based on the mean length of turn. In this application the resistance is not critical. A reduced inductance will simply reduce lower frequency reproduction slightly.
Ed |
24th Feb 2005, 9:37 am | #5 |
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Re: Transformer 'T4' for Bush TV24 Required
Hi Andy
I'll try to answer some of your questions. Rewinding is possible with nothing more than a hand drill. I would not bother too much about counting turns. The reason for the fine wire is that these are fairly high impedance circuits. The blocking osc was a favourite for frame oscillators. It is a bit like an LC oscillator with a long time constant in the grid side. To simplify a bit, it relies on a capacitor in the grid circuit discharging slowly through a resistor network until the valve starts to conduct; the rising anode current through the transformer drives the grid positive, the grid current charging the capacitor. Saturation is soon reached - as the anode current ceases to rise, the feedback to the grid winding ceases, and the negatively charged capacitor drives the valve into cut-off, where it remains until the capacitor discharges and conduction starts again. There is a sawtooth waveform on the grid, a pulse on the anode. The Philips K70 chassis used the frame OP Tx as the blocking osc transformer. HTH
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24th Feb 2005, 11:38 am | #6 |
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Re: Transformer 'T4' for Bush TV24 Required
Hi Andy
I've honestly no idea as to whether one could conveniently be pressed into service here, but I have a couple of Radiospares FB5s, replacement type for the fairly contemporary Pye FV1 and related models. They're a small transformer to be sure, fixing centres an inch and a half apart: DC resistances of the windings look to be around 140 and 400 ohms. Any opinions? Paul |
24th Feb 2005, 11:44 am | #7 |
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Re: Transformer 'T4' for Bush TV24 Required
Hi Paul_RK,
What do you mean with: "for the fairly contemporary Pye FV1 and related models" Kind regards Darius |
24th Feb 2005, 12:53 pm | #8 | |
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Re: Transformer 'T4' for Bush TV24 Required
Quote:
I think he means that the FV1 (and other similar Pyes) were made in approximately the same year as the Bush TV24. Nick. |
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24th Feb 2005, 2:53 pm | #9 |
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Re: Transformer 'T4' for Bush TV24 Required
Thanks, Nick: yes, that's just what I meant.
Paul |
25th Feb 2005, 6:53 pm | #10 |
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Re: Transformer 'T4' for Bush TV24 Required
Thanks again everyone!! I now understand.
I wonder if the transformer in question is 1:1 ratio? Anyhow an FB5s sounds like a good one to try. Whether it would 'take out' an ECL80 due to the lower resistance I don't know. What if a step-up (or down) is important? Eeeee by 'eak, nowt's easy is it. Decisions decision ..... Cheers all, Andy |
26th Feb 2005, 9:50 am | #11 | |
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Re: Transformer 'T4' for Bush TV24 Required
Quote:
Usually about 3:1 or more, the '3' side in the grid circuit. A lower impedance should not damage the valve - it spends the scan cut off, and only conducts during flyback.
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26th Feb 2005, 11:50 am | #12 |
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Re: Transformer 'T4' for Bush TV24 Required
Thanks again folks.
Yes I can immagine that the number of turns would be high (seems crazy really but there go). Well done to that chap for rewinding it - do tell him that he's earned my gold star for the week!! One thing though, if (and it's a bit if) both primary and secondary windings are made using the same spec wire, then the data sheet puts the highest dc resistance winding on the anode side of things (as opposed to the grid). Doesn't this place the higher number of turns on the anode side of the circuit, or am I barking up the wrong tree? Could it simply be that less turns are acheived using more wire the further away from the centre of the core you get? If we go with what I've just said and assume that more turns are on the grid side, this this would imply that the winding for the anode side of the circuit would be the first to come off if unwound. Does this make sence? Now if this logic holds up, then since my o/c winding is the one in the anode side, it should come off first (and I should be able to preserve the other winding and binding tape etc). Mmmmm ... I might have a go (only might mind you). Cheers all, Andy |
26th Feb 2005, 9:17 pm | #13 |
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Re: Transformer 'T4' for Bush TV24 Required
Hi Andy, the other factor affecting resistance is the wire diameter. It was usual to wind the grid winding with much finer wire (to cram as many turns as possible into a small space). The anode winding would be rated to carry to average valve anode current, not usually that large. From memory of the odd one that I have re-wound the gauges used were about 43 and 46SWG.
Winding wire is usually run at 3A per sq mm. HTH Ed |
26th Feb 2005, 11:14 pm | #14 |
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Re: Transformer 'T4' for Bush TV24 Required
Righto - thanks again Ed. This is all good stuff.
Regards, Andy |
1st Mar 2005, 6:42 pm | #15 |
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Re: Transformer 'T4' for Bush TV24 Required
A photo of the offending brute ...
The wax caps have all now gone by the way. Cheers, Andy |
1st Mar 2005, 10:05 pm | #16 |
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Re: Transformer 'T4' for Bush TV24 Required
What a nostalgia-laden photograph. Thanks, Andy. Was the chassis really this clean (wouldn't surprise me), or have you buffed it up ?
Cheers Steve |
2nd Mar 2005, 7:25 pm | #17 |
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Re: Transformer 'T4' for Bush TV24 Required
Apart from a very light dusting, that's exactly how it was!
Cheers, Andy |
12th Mar 2005, 7:47 pm | #18 |
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Re: Transformer 'T4' for Bush TV24 Required
Hello,
I have a Transformer which should do you its on a Bush TV22 if you are interested. Also if you can get hold of a Thorn 1500 set the transformer will do but you will need to fix it in as it is physically differant. Regards, Keith. Last edited by garfy; 12th Mar 2005 at 7:52 pm. |
12th Mar 2005, 8:45 pm | #19 |
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Re: Transformer 'T4' for Bush TV24 Required
Wow .. that's great stuff!
The Bush TV22 one sound just the ticket. How do we proceed from here? Cheers, Andy |
17th Mar 2005, 10:58 pm | #20 |
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Re: Transformer 'T4' for Bush TV24 Required
Are your there garfy?
Regards, Andy |