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

Stewart · 27th Apr 2007, 12:38 pm

Kat raises an interesting point here about the method of tuning. I wonder if that does have an influence? Certainly the sets I have that are most stable use permeability tuning... If this B&O does use a capacitor is it somewhere that will get warm in 10 mins? To require 500 kHz of re-tuning I would have thought a significant expansion would be required and the tuning capacitor would have to be very hot indeed?

From memory many of the free-standing valve FM tuners used capacity tuning so it cannot be too bad? (although this may well have been combined with AFC)

I would be inclined to look at some of the components in the tuner myself. I’m sure whilst some sets may need a little while to settle this need not be the case. One of my first restorations was a Bush VHF54 - “ok” as sets go, but hardly the last word in circuit design either. Finding out of spec components throughout the rest of the set I dived inside the tuner head too (based around two EF80s – great to have something to use all those spares up!). It was all quite a fiddle and, with hindsight over-the-top, I decided I never wanted to go inside it again – and replaced everything – resistors, caps; the lot!

This was before the age of the digital camera so lots of sketches were made so that component position and lead routing could be easily replicated. Capacitors were selected with good stability characteristics, (and good resistors too) - my theory being that I could never hope to reproduce any original correcting / complementary temperature coefficients in place with the originals, but if the replacements were good enough then I may get away with it.

The result? A set that can sit for months unused - or receive a daily blast – that unless a duster has knocked the tuning dial (some chance!) will warm up bang on tune from the last time…and stay there.

27th Apr 2007, 12:38 pm	#8
Stewart Retired Dormant Member Join Date: Jul 2003 Location: N.E. Surrey, UK. Posts: 361	Re: FM tuning drift - any ideas? Kat raises an interesting point here about the method of tuning. I wonder if that does have an influence? Certainly the sets I have that are most stable use permeability tuning... If this B&O does use a capacitor is it somewhere that will get warm in 10 mins? To require 500 kHz of re-tuning I would have thought a significant expansion would be required and the tuning capacitor would have to be very hot indeed? From memory many of the free-standing valve FM tuners used capacity tuning so it cannot be too bad? (although this may well have been combined with AFC) I would be inclined to look at some of the components in the tuner myself. I’m sure whilst some sets may need a little while to settle this need not be the case. One of my first restorations was a Bush VHF54 - “ok” as sets go, but hardly the last word in circuit design either. Finding out of spec components throughout the rest of the set I dived inside the tuner head too (based around two EF80s – great to have something to use all those spares up!). It was all quite a fiddle and, with hindsight over-the-top, I decided I never wanted to go inside it again – and replaced everything – resistors, caps; the lot! This was before the age of the digital camera so lots of sketches were made so that component position and lead routing could be easily replicated. Capacitors were selected with good stability characteristics, (and good resistors too) - my theory being that I could never hope to reproduce any original correcting / complementary temperature coefficients in place with the originals, but if the replacements were good enough then I may get away with it. The result? A set that can sit for months unused - or receive a daily blast – that unless a duster has knocked the tuning dial (some chance!) will warm up bang on tune from the last time…and stay there.