BBC VT/4 and VT/5 Balance indicators? - UK Vintage Radio Repair and Restoration Discussion Forum

Chris55000 · 21st Mar 2019, 8:14 am

Hi!

Just a quick thought, as I've not yet studied the BBC Valve Testing papers in detail yet, since (I believe!) the purpose of the vibrating galvo was to indicate a balance–point, could a "magic eye" employed as per Leak"s trough–line FM tuner be used as an alternative?

I have read several sources, which I can't quickly locate at the moment, that suggest such indicators have a very sharp and sensitive "null" indication!

(Another reason to get a very bright Russian–made one perhaps? – only one shadow is needed for a null–point indicator!)

Chris Williams

David Simpson · 21st Mar 2019, 9:54 pm

Hello Chris, you may remember my VT/4 VT/5 thread last year. I never acquired one, but had intended using a Pye Mirror Galv. had I found a VT/4 & its Vib. Galv. was u/s. I'm not a physics lab expert, but I seem to recall that the Pye could detect currents less than 1uA, having seen an ex lab tech doing some experiments. I suspect that a vib. galv. might not be so accurate.
What is the spec of the M/eye in the Leak. Certainly the old EM type of indicators, used in a lot of vintage radios & T/Eq., have target currents of a few hundred uA in the obsolete types, and approx. 2 - 4 mA in later ones. I do have a lovely big 25-0-25uA MC panel meter which again I might have experimented with had I acquired a TV/4.
Anyway, I'm glad someone else is still interested in these old pre-war BBC valve testers. Mind you, with a DC HT of only 100V available in the VT/4 & 100V RMS available in the VT/5, they're not much use these days as workable valve testers, Ia/Vg - wise.

Regards, David

Synchrodyne · 21st Mar 2019, 11:27 pm

The original Leak Troughline tuning indicator may have been based upon the NBS circuit, which was intended to be a null indicator. An outline of the NBS circuit is provided in the attachment.

Leak claimed an indication that was within 2 to 5 kHz of the centre-channel position.

Cheers,

Chris55000 · 22nd Mar 2019, 12:07 am

Hi!

"Radio Constructor" magazine reviewed this circuit in either 1959 or 1960 and I seem to remember the R.C. circuit was drawn with thermionic, rather than semiconductor rectifiers, and Leak used little "Westectors" in their circuit if I remember correctly!

The use of this circuit in one of the BBC valve–testers means the whole cathode/heater circuit of the m.e. would be floating at either the d.c. anode H.T. voltage (VT4) or the transformer a.c. secondary voltage, (VT5) but now that small 6V transformers are readily and cheaply available there's no difficulties in providing a suitable isolated m.e. heater supply!

I'll have to read those BBC papers again and give the idea a go!

Chris Williams

21st Mar 2019, 8:14 am	#1
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,864	BBC VT/4 and VT/5 Balance indicators? Hi! Just a quick thought, as I've not yet studied the BBC Valve Testing papers in detail yet, since (I believe!) the purpose of the vibrating galvo was to indicate a balance–point, could a "magic eye" employed as per Leak"s trough–line FM tuner be used as an alternative? I have read several sources, which I can't quickly locate at the moment, that suggest such indicators have a very sharp and sensitive "null" indication! (Another reason to get a very bright Russian–made one perhaps? – only one shadow is needed for a null–point indicator!) Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

21st Mar 2019, 9:54 pm	#2
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,851	Re: BBC VT/4 and VT/5 Balance indicators? Hello Chris, you may remember my VT/4 VT/5 thread last year. I never acquired one, but had intended using a Pye Mirror Galv. had I found a VT/4 & its Vib. Galv. was u/s. I'm not a physics lab expert, but I seem to recall that the Pye could detect currents less than 1uA, having seen an ex lab tech doing some experiments. I suspect that a vib. galv. might not be so accurate. What is the spec of the M/eye in the Leak. Certainly the old EM type of indicators, used in a lot of vintage radios & T/Eq., have target currents of a few hundred uA in the obsolete types, and approx. 2 - 4 mA in later ones. I do have a lovely big 25-0-25uA MC panel meter which again I might have experimented with had I acquired a TV/4. Anyway, I'm glad someone else is still interested in these old pre-war BBC valve testers. Mind you, with a DC HT of only 100V available in the VT/4 & 100V RMS available in the VT/5, they're not much use these days as workable valve testers, Ia/Vg - wise. Regards, David

22nd Mar 2019, 12:07 am	#4
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,864	Re: BBC VT/4 and VT/5 Balance indicators? Hi! "Radio Constructor" magazine reviewed this circuit in either 1959 or 1960 and I seem to remember the R.C. circuit was drawn with thermionic, rather than semiconductor rectifiers, and Leak used little "Westectors" in their circuit if I remember correctly! The use of this circuit in one of the BBC valve–testers means the whole cathode/heater circuit of the m.e. would be floating at either the d.c. anode H.T. voltage (VT4) or the transformer a.c. secondary voltage, (VT5) but now that small 6V transformers are readily and cheaply available there's no difficulties in providing a suitable isolated m.e. heater supply! I'll have to read those BBC papers again and give the idea a go! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!