UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - FM tuning drift

Chris55000 · 30th Aug 2007, 10:37 pm

Hi!

Well done to Member Studio263 on catching that one - even my 1963-edition Quarrington "Radio And Television" doesn't pick up on excessively-low HT as a cause of VHF tuning drift!

As well as the lowered HT, the output valve would be running excessively hot as well, and the extra heat from this would affect the temperature of the VHF-front-end components to a greater extent thean the original set'd designer may have allowed for! For this reason whenever I've worked on any set using a B8A/B9A based tetrode/pentode or triode-pentode, I've always added extra decoupling for the O/P valve G2/previous anode - 10K 2W from HT to G2/triode anode decoupled with 8-10uF to earth won't affect volume too much but it does provide a noticeable hum reduction and can greatly help O/P valve life!

Chris Williams

30th Aug 2007, 10:37 pm	#16
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,868	Re: FM tuning drift - any ideas? Hi! Well done to Member Studio263 on catching that one - even my 1963-edition Quarrington "Radio And Television" doesn't pick up on excessively-low HT as a cause of VHF tuning drift! As well as the lowered HT, the output valve would be running excessively hot as well, and the extra heat from this would affect the temperature of the VHF-front-end components to a greater extent thean the original set'd designer may have allowed for! For this reason whenever I've worked on any set using a B8A/B9A based tetrode/pentode or triode-pentode, I've always added extra decoupling for the O/P valve G2/previous anode - 10K 2W from HT to G2/triode anode decoupled with 8-10uF to earth won't affect volume too much but it does provide a noticeable hum reduction and can greatly help O/P valve life! Chris Williams