Bush VHF94 - too much drift on FM?

[PaulR]

That is odd. Mine just drifts a certain way then only returns when it cools down

[Philips210]

Hi.

I've found some carbon composition resistors can be a little more temperature dependent than say carbon film types. Looking inside the VHF box, the only resistor which is reasonably close to the ECC85 base socket is R3 (100k), the oscillator's grid resistor. So this might be most likely to be affected by temperature change as the valve warms up.

Also, the critical ceramic caps (C7, C8, C9, C10 and C13) with defined temperature coefficients, appear to be well away from the valve base.

After disproving the ECC85 then I'd be inclined to replace R2, R3 and R4 one at a time and note results after each replacement. I wouldn't mind betting they're out of tolerance anyway, especially R3.

Regards,
Symon

[Philips210]

I also wondered about the ECC85's base socket. If made from ceramic then I wouldn't envisage any problems but if of a plastic material then maybe a potential area to investigate? I would have thought a ceramic valve base should be standard practice for VHF circuits.

Regards,
Symon

[Techman]

Regarding the valve itself...

See my post on this in your wanted thread, but basically the same valve in the Grundig 3028 and their variants that you've got a stack of (remember the picture you posted), doesn't ever seem to cause a drift problem in those sets, so why would it cause a problem in the Bush?

You've got one in each of those Grundig sets to try, should you want to!

I think we can rule out the valve, unless anyone thinks differently for some reason that I've not thought of at this present time.

[mark2collection]

Steve,

Having played with a troublesome VHF61 recently, the ceramic trimmers within the VHF module (as previously mentioned) are worth checking, and in the case of the set I was working on (crackled an awful lot), one of the red fibre washers on one of the ceramic trimmers had become conductive, dragging the HT down & causing the crackle.

Strange, but true, and certainly worth very close examination. I also cleaned the trimmers with isopropyl to remove any dried lube or nicotine (tar?) which had caused them to yellow. Now ice-white, & with a new nylon washer, the performance is very stable and crackle-free.

It's probably worth using contact cleaner on the trimmers too, marking their current location, a quick squirt & adjust through 180 degrees in each direction. Personally, I wouldn't use Servisol Super 10 in this application, too oily. Something like Ambersil contact cleaner, as it leaves a very low residue behind.

Mark

[simpsons]

Food for thought. The BBC report on the Bush FM front end says that there is little or no drift, 13 Khz after 30 mins and so the engineering design must be sound. The report also says that varying the mains input ie HT only shifted the front end by 8Khz.

In previous threads on FM drift, use of freezer spray has been mentioned to track down such faults but this idea is fraught with problems not the least that ice is wet!!

May I suggest compressed air spray, used to clean keyboards, could help?

The blast of air, now quite cold having been subject to thermodynamics, can be directed toward an individual component.

Chris

[stevehertz]

Thanks for the ongoing suggestions guys. I'm not 'on to' this set at the moment, my worktable is currently cluttered up with something else, but it shouldn't be long before I'm back on it. We should have a conclusion eventually.