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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I finally found a TV with an AC flyback in it (well, my new girlfriend found it in her parent's loft).
It's quite an unusual AC flyback though, being wound on a pair of E-cores (I think).
The associated tripler is also quite unusual, as it is not potted.
These came from a Thorn/BRC 1500 chassis. (Googling has shown that this was a very popular monochrome TV chassis in the UK and elsewhere and was still in production at the end of the '70s)
I was planning on powering it using a Mazilli ZVS driver, but I'm going to have difficulty winding a centre-tapped primary on it, as it appears to be wound on a pair of E-cores.
It was connected to the tops of two (yes, two) valves (vacuum tubes) in it's original application (the two at the top right in the photo), so I may try and run it in it's original configuration, as I still have all the parts required.
I've looked for a circuit diagram for it online, but have so far been unsuccessful.
If anyone has a circuit diagram for the Thorn/BRC 1500 chassis (or a link) I'd be very grateful. (This TV was 'badged' as a Ferguson, DER, Ecko, and several other names, and was supplied as a rental model by Radio Rentals, Redifusion and others).
Also, if anyone can suggest other ways of powering it (existing primary, etc) I'd also be grateful.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The larger of the two valves in the top right hand corner is the Line Output valve. (US: "sweep tube.") The smaller of the two is called the 'Booster Diode,' or occasionally the 'Efficiency Diode.'
This is a very common configuration for TVs of that era.
The ceramic rod diodes were intended to allow complete serviceability. If one of them failed, which they often did, it could be replaced without having to replace an entire module.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Proud Mary wrote ...
The ceramic rod diodes were intended to allow complete serviceability. If one of them failed, which they often did, it could be replaced without having to replace an entire module.
Expect no more than one or two milliamps of them.
could he use this feature to replace the crummy old diodes, with better modern ones?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
Proud Mary wrote ...
The ceramic rod diodes were intended to allow complete serviceability. If one of them failed, which they often did, it could be replaced without having to replace an entire module.
Expect no more than one or two milliamps of them.
could he use this feature to replace the crummy old diodes, with better modern ones?
I only posted the photo of the tripler for it's curiosity value.
I'm not planning to use it (at least I can't think of a use for it at the moment)
I imagine not potting it also saved on manufacturing costs (apparently most of these sets were for the rental market, so ease of repair was probably a large factor here)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Here is a simplified valve Line Output circuit. In your case, you have the solid state tripler in place of the thermionic EHT diode, but basic operation is the same.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Thanks for the schematic, PM.
I may try re-connecting the flyback, and powering the board up, to measure voltages, etc., but I've no idea if the TV was functional before I aquired it.
I won't have much time to do this for the next week or two, though.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Most - if not all - the valve heaters in this design are connected in series and powered straight off the mains by the dreadful multi-tap wirewound 'dropper resistor' at the top edge of the board. So if one heater has failed, none of them will light up.
This board is typical of what was called the 'live chassis' technique - there being no mains isolation transformer. Watch out!
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Unusual AC flyback and tripler.
