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Old 5th Sep 2020, 3:11 pm   #21
Jim - G4MEZ
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Default Re: Bush VHF94 - too much drift on FM?

I have two VHF64s here and neither drift at all, even my very scruffy one!
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Old 5th Sep 2020, 3:14 pm   #22
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Default Re: Bush VHF94 - too much drift on FM?

Maybe a few pics of the VHF tuner from members' sets suffering/not suffering drift. As has been pointed out earlier, temperature coefficients of capacitors can be critical for stable operation. As it doesn't appear to be a one-off I wonder if it is an endemic problem in certain models using a particular make of capacitor(s)?

Regards,
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Last edited by Philips210; 5th Sep 2020 at 3:20 pm.
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Old 5th Sep 2020, 4:42 pm   #23
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Default Re: Bush VHF94 - too much drift on FM?

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Originally Posted by PaulR View Post
Maybe the "fault" is the combination of valves and components and their precise position in that particular radio. Just a thought.
That's my view too. Good design of a free-running 100MHz valve oscillator is a bit of a black art.
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Old 5th Sep 2020, 6:11 pm   #24
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Default Re: Bush VHF94 - too much drift on FM?

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Originally Posted by Philips210 View Post
Maybe a few pics of the VHF tuner from members' sets suffering/not suffering drift. As has been pointed out earlier, temperature coefficients of capacitors can be critical for stable operation. As it doesn't appear to be a one-off I wonder if it is an endemic problem in certain models using a particular make of capacitor(s)?

Regards,
Symon
All the normal 'dodgy' caps have been changed, but I suppose the mica caps may be problematic.
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Old 5th Sep 2020, 6:35 pm   #25
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Default Re: Bush VHF94 - too much drift on FM?

I didn’t think there were mica caps, there is a mixture of different temperature coefficient ceramic caps.
SC are silver ceramics
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Old 5th Sep 2020, 6:38 pm   #26
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Default Re: Bush VHF94 - too much drift on FM?

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I didn’t think there were mica caps, there is a mixture of different temperature coefficient ceramic caps.
SC are silver ceramics
Are those the ones with the P and N prefixes?
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Old 5th Sep 2020, 6:56 pm   #27
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Default Re: Bush VHF94 - too much drift on FM?

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Quote:
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I didn’t think there were mica caps, there is a mixture of different temperature coefficient ceramic caps.
SC are silver ceramics
Are those the ones with the P and N prefixes?
Yes they are, the trimmers are silver ceramic but no note of temp compensation.

Before replacing any frequency dependants parts confirm nothing is loose, earthing, coil windings etc.
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Old 5th Sep 2020, 7:03 pm   #28
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Default Re: Bush VHF94 - too much drift on FM?

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Quote:
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Quote:
Originally Posted by Nuvistor View Post
I didn’t think there were mica caps, there is a mixture of different temperature coefficient ceramic caps.
SC are silver ceramics
Are those the ones with the P and N prefixes?
Yes they are, the trimmers are silver ceramic but no note of temp compensation.

Before replacing any frequency dependants parts confirm nothing is loose, earthing, coil windings etc.
Thanks, so who can supply those caps with the prefixed temperature coefficients type numbers?
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Old 5th Sep 2020, 7:44 pm   #29
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Default Re: Bush VHF94 - too much drift on FM?

There are some here but I have not checked if they are the same values or if they can supply the UK, I have had a quick search but not found others.

https://www.surplussales.com//Capaci...mpDogbone.html


Are the tuner components clean, from memory those trimmers can stop the oscillator and just moving them a fraction clears the problem. Could they be the cause and a small fraction of a turn cure it.

Those n an p caps will probably be difficult to find so try everything else first.
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Old 23rd Sep 2020, 10:08 am   #30
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Default Re: Bush VHF94 - too much drift on FM?

Current situation:

Used yesterday for several hours (fully warmed up) and after several retunes was working perfectly on a my local channel, 94MhZ. The set was switched off in that state.

Switching on this morning without touching the tuning knob, 94MhZ was coming in predominant together with a touch of R4 (94 was strongest). But after just a few minutes R4 took over completely. Gradually the two levelled out (both stations on together) then after about 50 minutes 94 was winning the battle again. This suggests a sort of S shaped tuning drift curve going on.

I'm of the mind that there may be some kind of physical, mechanical thing going on with the tuning system. AM is conventional tuning cap and cord tuning, while FM is permeability tuned on rods. Are the associated springs gradually easing/tightening?

I previously removed the shielding can from the ECC85 and this did make a small improvement to the amount of drift.

Or maybe I need to just try more ECC85s.
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Old 24th Sep 2020, 12:49 pm   #31
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Default Re: Bush VHF94 - too much drift on FM?

My Bush VHF61 drifts. Set to my local station of 94.9 it will be about there from cold switch on, then drift away and then come back to spot frequency about 20 mins later if left alone and not touched. I'm wondering if those who say they have sets that don't drift at all are those that tune around all the time and don't do the 'switch off the day before and switch on again the next day on the same station WITHOUT touching anything'. Grundig 1328 and PYE Fenman - no drift at all.
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Old 24th Sep 2020, 12:56 pm   #32
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Default Re: Bush VHF94 - too much drift on FM?

My VHF61 and 62: fair amount of drift as you describe.

VHF64: none whatsoever as far as I can discern (and it's used daily, sometimes just stuck on Radio 4, sometimes flitting around other stations on FM).

VHF90A: loads of drift as it warms up, needs a definite tweak on the tuning knob after about 5-10 mins.
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Old 24th Sep 2020, 7:57 pm   #33
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Default Re: Bush VHF94 - too much drift on FM?

My VHF62 needs to be retuned a couple of times within 15 minutes of being switched on from cold, but will stay on frequency for hours after that. The only other observation that I'd make is that the effects of drift would be exacerbated by poor alignment or badly out of balance EABC80 diodes- i.e if the s-curve isn't symmetrical. Also check that your HT isn't coming up slowly due to a tired rectifier; varying voltage will cause varying frequency.
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Old 24th Sep 2020, 9:42 pm   #34
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Default Re: Bush VHF94 - too much drift on FM?

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The only other observation that I'd make is that the effects of drift would be exacerbated by poor alignment or badly out of balance EABC80 diodes- i.e if the s-curve isn't symmetrical.
I can't see how that would be causing the drift in question here, or even an effect similar to it.

The EZ80 rectifier in my Bush VHF61 had no emission when I got it, so I fitted silicon diodes and a resistor, as I'd run out of EZ80s at the time...and it still drifted. I've since fitted a new EZ80...and it still drifts. Anyway, the OP says he's monitored the HT and it doesn't vary, perhaps only by a volt with mains fluctuation.
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Old 27th Sep 2020, 3:39 pm   #35
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Default Re: Bush VHF94 - too much drift on FM?

So it looks like there's no definitive answer to this then?

And there was me thinking that surely someone on here would have solved the issue by now.

I have to admit that I haven't particularly looked for the answer myself.
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Old 27th Sep 2020, 8:05 pm   #36
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Default Re: Bush VHF94 - too much drift on FM?

That's right. And still - apparently - lots of VHF64/94s drift on FM. It's a common fault, must be a common reason for it. It's beyond my technical skills unfortunately. I'm more of a dry joints, dirty switches, poor valves and bad capacitors man myself. Well, I do myself down a tad there but when the technology level gets too involved, I'm afraid I'm foundering.
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Old 27th Sep 2020, 8:09 pm   #37
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Default Re: Bush VHF94 - too much drift on FM?

I wonder why those Bush sets drift but the Cossor 524 types are dead steady. At least that is what I have found
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Old 27th Sep 2020, 11:37 pm   #38
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Default Re: Bush VHF94 - too much drift on FM?

Hi.

From what I can tell, the VHF61/62 uses the same type of VHF box as in the VHF64/94. The critical capacitors in the oscillator circuit C7, C8, C9, C10 and C13 share the same part numbers in both the 61/62 and 64/94 receivers. It's not that common to find a drifting ceramic capacitor so I'd start by looking elsewhere. The ECC85 really needs to be proved to be blameless. Maybe borrow one from a similar set known to be OK?

The HT voltage to the VHF box seems to be fairly stable but I wonder about the anode load resistors in the box? R4 (6k8) in the oscillator anode load may be worth replacing. I don't know whether the RF amp anode load resistor, R2 (2k2) could be responsible. Also, the oscillator's grid resistor, R3 (100k), would that be a possible candidate for drift?. R2, R3 and R4 are carbon composition resistors and as they're in a VHF circuit would best be replaced like for like as carbon film types maybe sufficiently inductive as to upset the alignment. Not sure though how significant that will be in practice so film types may well be OK. It would be good to get others views on that first.

On the other hand, one or more of the capacitors mentioned above could be to blame as could trimmer capacitors TC1 and TC2.

A mechanical fault with the drive cord to the permeability tuner is a definite possibility as Steve mentions in post #30. Maybe the return spring is a little weak?
Any replacement components fitted to the VHF tuner box should be configured as per the originals ie positioning and lead lengths to preserve alignment as far as possible.

I also wondered if it would be practical to use a VHF GDO (gate dip oscillator) or (grid dip oscillator) to check the resonant frequency when cold and after say 10 minutes use.

Also interesting to note that in post #32, Nick mentions his VHF64 has no discernable drift, so it would appear some sets are drift free and that the circuit should be fundamentally OK. One would expect that from Bush anyway.

It's certainly a tricky problem to solve and would be good to know the solution.

Regards,
Symon.

Last edited by Philips210; 27th Sep 2020 at 11:45 pm.
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Old 28th Sep 2020, 4:29 am   #39
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Default Re: Bush VHF94 - too much drift on FM?

Some great ideas there Simon, thanks. I've got other stuff on the bench at the moment but I'll give those a go in due course.
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Old 28th Sep 2020, 2:21 pm   #40
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Default Re: Bush VHF94 - too much drift on FM?

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The HT voltage to the VHF box seems to be fairly stable but I wonder about the anode load resistors in the box? R4 (6k8) in the oscillator anode load may be worth replacing. I don't know whether the RF amp anode load resistor, R2 (2k2) could be responsible. Also, the oscillator's grid resistor, R3 (100k), would that be a possible candidate for drift?
Yes, good point, but it's worth noting that a radio often (mine in particular) doesn't just drift in one direction and stay there. It actually starts off in one place and then drifts off so far and then returns to where it started once fully warm and is then stable for the rest of the time the set is in operation. When switching the set on first thing the next day it's often near as dammit on frequency from the night before, but quickly drifts off and as long as it's just ignored for half an hour, it'll come back to spot frequency on its own and then stay there, so it goes round in a full circle.
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