AC128s on convergence boards - why? - UK Vintage Radio Repair and Restoration Discussion Forum

Mikey405 · 5th Jul 2007, 4:57 pm

Hi everyone.

Does anyone know why lots of old convergence boards used AC128s with the base and emitter connected together?

Thanks everyone.

From Mike.

ppppenguin · 5th Jul 2007, 5:05 pm

I didn't know they did!

What you are making is a medium power germanium diode. I don't think many germanium rectifiers were made, only low power signal diodes such as OA81 and OA47. So if you wanted a low voltage drop diode capable of handling a bit more current then a strapped AC128 seems sensible.

ukcol · 5th Jul 2007, 5:13 pm

Hi Mike

Just to add to what Jeffrey said.

If memory serves me correctly they were used to give a DC component to certain of the waveforms and as the convergence circuits were low impedance a germanium device, with the low forward voltage drop that Jeffrey referred to, was desirable.

Welsh Anorak · 5th Jul 2007, 9:11 pm

Hi
I always thought that it was something to do with temperature compensation, but the above explanations seem far more rooted in fact!
Glyn

5th Jul 2007, 4:57 pm	#1
Mikey405 Octode Join Date: Jul 2006 Location: Solihull, West Midlands and Beaford, Devon Posts: 1,626	AC128s on convergence boards - why? Hi everyone. Does anyone know why lots of old convergence boards used AC128s with the base and emitter connected together? Thanks everyone. From Mike. __________________ G7TRF

5th Jul 2007, 5:05 pm	#2
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: AC128s on convergence boards I didn't know they did! What you are making is a medium power germanium diode. I don't think many germanium rectifiers were made, only low power signal diodes such as OA81 and OA47. So if you wanted a low voltage drop diode capable of handling a bit more current then a strapped AC128 seems sensible.

5th Jul 2007, 5:13 pm	#3
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: AC128s on convergence boards Hi Mike Just to add to what Jeffrey said. If memory serves me correctly they were used to give a DC component to certain of the waveforms and as the convergence circuits were low impedance a germanium device, with the low forward voltage drop that Jeffrey referred to, was desirable.

5th Jul 2007, 9:11 pm	#4
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,918	Re: AC128s on convergence boards Hi I always thought that it was something to do with temperature compensation, but the above explanations seem far more rooted in fact! Glyn