Bush VHF94 - too much drift on FM?

[stevehertz]

I've just finished a full restoration and realignment of a Bush VHF94 (same as 64). From switch on to fully warmed up it drifts about 560Kc on FM. That seems excessive. What should I expect for a set with no AFC? Anyone with the same set able to comment?

Before people say check this or that component, all potentially dodgy caps have been changed and all valves tested out of the set.

[ms660]

BBC Bush VHF64 tests here if it's any use:

https://www.bbc.co.uk/rd/publications/rdreport_1958_19

Lawrence.

[Nickthedentist]

Mine has no noticeable drift at all.

[stevehertz]

Originally Posted by ms660

BBC Bush VHF64 tests here if it's any use

Great document, thanks. Didn't know that the BBC did reports on commercial sets!

[Nuvistor]

Hi Steve,
Off topic but a list of BBC tech docs.
http://downloads.bbc.co.uk/rd/pubs/r...eport-list.pdf

[Philips210]

Thanks to Frank and Lawrence. I was unaware of such documents being available. A quick look through the listings and I see colour television trials in the early/mid 1960s. Also stereo radio broadcast tests as well. Very interesting stuff.

Regards
Symon

[Graham G3ZVT]

Originally Posted by stevehertz

Before people say check this or that component, all potentially dodgy caps have been changed and all valves tested out of the set.

Don't discount the possibility that a brand new capacitor in the frequency changer area might increase the amount of drift, if its temperature coefficient is significantly different from the original.

[Philips210]

I think I be inclined to try another known good ECH81 valve. I have had problems with these in AM/FM sets with intermittent start up of the LO when the radio is switched to FM.
I wonder if the HT voltage to the frequency changer is drifting. If so could that account for oscillator drift?

Regards,
Symon

[Freya]

I have had lazy ECH81s which gradually increase emission, creating the same effect.

[PaulR]

My VHF 61 drifts quite a bit despite changing every Hunts cap but my Cossor 524 is rock solid. I have another VHF 61 to restore, it will be interesting to see whether that drifts

[Leon Crampin]

Fit a blackened screening can to the ECC85. This will reduce drift to acceptable limits in most cases by limiting its temperature rise. Sometimes, selecting an appropriate ECC85 will help - the best one I have is, surprisingly, branded "Edicron".

The ECH81 will have no bearing on FM tuning drift as it functions as an IF amplifier on this band. AM performance of these sets is mediocre, to say the least.

Leon.

[stevehertz]

Originally Posted by Philips210

I think I be inclined to try another known good ECH81 valve. I have had problems with these in AM/FM sets with intermittent start up of the LO when the radio is switched to FM.
I wonder if the HT voltage to the frequency changer is drifting. If so could that account for oscillator drift?

HT is pretty rock solid, I checked it over an hour period. Very minor (1v or so) changes that were probably attributable to mains fluctuations.

[stevehertz]

Originally Posted by Leon Crampin

Fit a blackened screening can to the ECC85. This will reduce drift to acceptable limits in most cases by limiting its temperature rise. Sometimes, selecting an appropriate ECC85 will help - the best one I have is, surprisingly, branded "Edicron"

Thanks Leon. I recall you have done a lot of work on these sets. I'll give that go.

[FERNSEH]

Nickthedentist wrote: "Mine has no noticeable drift at all."
The VHF64 I'm working is the same but other VHF Bush radios I have drift quite badly, the two VHF90 sets are the worst.

Leon Crampin wrote: "AM performance of these sets is mediocre, to say the least."
With two IF stages and the best frequency changer valve of all one would have expected much better performance.
The Bush VH80 is a much better performer.

DFWB.

[unitelex]

Originally Posted by PaulR

My VHF 61 drifts quite a bit despite changing every Hunts cap but my Cossor 524 is rock solid. I have another VHF 61 to restore, it will be interesting to see whether that drifts

Ditto, I have a VHF61 I restored already, and another one waiting. I will also do the same drift comparison I might also try blackening the can on the ECC85.

Best Regards
Chris

[stevehertz]

I've fitted a black screening can, no difference. Will see if I've got another ECC85 to try.

[PaulR]

I am watching this thread with interest!

[Tractionist]

Originally Posted by Nickthedentist

Mine has no noticeable drift at all.

Ditto !

[stevehertz]

'No drift' messages heartening to hear. Would rather have a fault that needs to be located than a problem with no solution (the problem is, finding the fault!).

[PaulR]

Maybe the "fault" is the combination of valves and components and their precise position in that particular radio. Just a thought.