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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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3rd Sep 2020, 11:39 am | #1 |
Dekatron
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Bush VHF94 - too much drift on FM?
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.
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3rd Sep 2020, 11:48 am | #2 |
Dekatron
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Location: Cornwall, UK.
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Re: Bush VHF94 - too much drift on FM?
BBC Bush VHF64 tests here if it's any use:
https://www.bbc.co.uk/rd/publications/rdreport_1958_19 Lawrence. |
3rd Sep 2020, 1:19 pm | #3 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
Mine has no noticeable drift at all.
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3rd Sep 2020, 4:09 pm | #4 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
Great document, thanks. Didn't know that the BBC did reports on commercial sets!
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3rd Sep 2020, 4:22 pm | #5 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
Hi Steve,
Off topic but a list of BBC tech docs. http://downloads.bbc.co.uk/rd/pubs/r...eport-list.pdf
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Frank |
3rd Sep 2020, 4:41 pm | #6 |
Nonode
Join Date: May 2007
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Re: Bush VHF94 - too much drift on FM?
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 |
3rd Sep 2020, 5:00 pm | #7 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
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.
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3rd Sep 2020, 5:20 pm | #8 |
Nonode
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Re: Bush VHF94 - too much drift on FM?
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 |
3rd Sep 2020, 5:21 pm | #9 |
Octode
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Re: Bush VHF94 - too much drift on FM?
I have had lazy ECH81s which gradually increase emission, creating the same effect.
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3rd Sep 2020, 5:24 pm | #10 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
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
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Paul |
3rd Sep 2020, 5:33 pm | #11 |
Octode
Join Date: Nov 2005
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Re: Bush VHF94 - too much drift on FM?
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. |
3rd Sep 2020, 5:34 pm | #12 | |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
Quote:
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3rd Sep 2020, 5:37 pm | #13 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
Thanks Leon. I recall you have done a lot of work on these sets. I'll give that go.
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3rd Sep 2020, 8:12 pm | #14 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
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. |
3rd Sep 2020, 8:19 pm | #15 | |
Hexode
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Re: Bush VHF94 - too much drift on FM?
Quote:
Best Regards Chris |
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4th Sep 2020, 11:53 am | #16 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
I've fitted a black screening can, no difference. Will see if I've got another ECC85 to try.
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4th Sep 2020, 1:45 pm | #17 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
I am watching this thread with interest!
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Paul |
5th Sep 2020, 8:23 am | #18 |
Heptode
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Re: Bush VHF94 - too much drift on FM?
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5th Sep 2020, 11:39 am | #19 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
'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!).
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5th Sep 2020, 12:42 pm | #20 |
Dekatron
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Re: Bush VHF94 - too much drift on FM?
Maybe the "fault" is the combination of valves and components and their precise position in that particular radio. Just a thought.
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Paul |