Bush VHF61. No VHF FM.

[Graham Quinlan]

I'm new to valve radio restoration and have acquired a Bush VHF61. MW and LW both sound fine, good volume, lovely 'valvy' sound quality, no noise etc, but the VHF only produces distortion and motorboating at certain frequencies e.g. just above 100mhz (Classic FM?). I'd really like to get the VHF working but given my limited resources I've decided to take the 'if it ain't broken don't fix it' approach with replacing the capacitors.

I've read various posts about this but can anyone recommend which ones to replace to get the VHF working again, given that I only want to replace the minimum necessary? I did a valve swap with the two EF 89s which confirmed the originals seem ok, and I read that replacing the capacitors around these valves should do the trick, but which ones exactly?

I have the original Bush servicing manual which I downloaded from ***********************, but the capacitor data sheet has the website address plastered over the data for some of the VHF capacitors, namely C11 and C13. Can anyone please confirm the capacitance, type, working DC voltage and tolerance for these please? I suspect they are silver mica and silver ceramic, in which case this is not an issue, but it would be nice to be sure.

If anyone could recommend a source of suitable capacitors where I don't have to buy huge bulk bags just to get one or two, that would be appreciated, or maybe someone would be willing to sell me the ones needed.

The control knobs are in poor shape with shrunken plastic and the brass trim dropping off; I understand this is normal for this model. The photographic component diagram in the servicing manual shows 'two tier' knobs, presumably from a similar model. Would these be a suitable replacement?

By the way, I've renewed the mains lead with an earth in accordance with recommended methods, in case anyone is concerned I'm going to fry myself!

Any other suggestions or offers of help to look at it would be welcome.

Thanks,

Graham Quinlan

[Nuvistor]

C11 and C13 are silvered ceramic, C13 is N750, I would not change them unless you know they are faulty.
I would change the paper tubular and metalised paper types, used for decoupling or coupling, at the age they are they will all probably be failing.
Have you changed the ECC85 valve, probably the first component to try.

Edit. The manual is available from the service link at the top of the page.

[Graham Quinlan]

Hi Frank, thanks for your reply.

I do have a spare ECC85 but I don't have the equipment or knowledge to test it - I guess there's no point trying it otherwise. I've found a reasonably priced tested one on the WWW so I may buy that and see if it makes any difference.

I suppose this is the quickest and simplest way forward for now, rather than replacing the caps, which I realise I will have to do if the valve is not the cause of the problem. I'll post an update as and when. Thanks for your advice.

Graham Quinlan

[paulsherwin]

All the caps between 0.1uF and 0.001uF are suspect. Lots of people don't agree with wholesale recapping, but if you don't do it then you will find fault diagnosis very difficult. Leaky caps can produce all sorts of strange symptoms, and it's effectively impossible to test them. The symptoms you describe sound more like bad cap problems than a tired ECC85.

[Nuvistor]

Agree Paul it does sound like capacitors, I just thought if an ECC85 was available it would be worth trying, certainly not worthwhile buying one yet.

[Station X]

Originally Posted by Graham Quinlan

I do have a spare ECC85 but I don't have the equipment or knowledge to test it - I guess there's no point trying it otherwise.

Graham Quinlan

There's no reason not to try your spare untested ECC85 in the set.

If it doesn't cure the problem you've lost nothing

[paulsherwin]

Absolutely, it only takes seconds. Substituting valves is always the first thing to try if spares are to hand, not because it's the most likely fault but because it's so quick and easy.

[Tim]

I would suspect the Hunts moldseals round the VHF side and on the mixer section of the ECH81.
It wouldn't hurt to test/change the VHF demodulator capacitor.

[Graham Quinlan]

Thanks for all your advice guys.

I tried my spare ECC85 and it made no difference, though I did briefly hear some distorted music as it was still heating up, before it reverted to motorboating. I guess I'll have to bite the bullet and replace the relevant caps.

Can anyone recommend a supplier who is happy to sell small quantities?

Graham Quinlan

[Graham Quinlan]

A PS to my previous message: Some of the replies I've received contain terminology I'm not familiar with, being a beginner. What is coupling/decoupling and what is a demodulator? Layman's explanations (as far as possible) appreciated.

Thanks,

Graham Quinlan

['LIVEWIRE?']

Coupling, in the electronic sense, means the transferring of a signal (speech &/or music) from one stage to another. E.G, the component often referred to as 'that' capacitor couples the output from the anode of a preamplifier valve to the (control) grid of the output valve. Decoupling means separating one stage of a radio or amplifier from another, in practice by a series resistor followed by a capacitor connected between the HT (High Tension) supply and earth. A Demodulator recovers the audio from the received signal and is placed between the Intermediate frequency stages and the audio amplifier.

[ms660]

Originally Posted by Graham Quinlan

A PS to my previous message: Some of the replies I've received contain terminology I'm not familiar with, being a beginner. What is coupling/decoupling and what is a demodulator? Layman's explanations (as far as possible) appreciated.

Thanks,

Graham Quinlan

Coupling in the context of what's been said....Capacitors...eg: a capacitor which couples (connects) one stage to another stage.

Decoupling...Again capacitors but these are used to decouple (shunt) unwanted signal frequencies from a particular point in the circuit to chassis (usually) they're also known as bypass capacitors.

Demodulator....Sometimes called the detector, the demodulator (or detector) in effect extracts the audio modulation (voice, music etc) from the transmitted carrier.

So...Looking at the Trader sheet 1301 for your receiver:

Coupling capacitor....C62 and C65 are examples of coupling capacitors, C62 couples the audio output from the demodulator (detector) that's been selected to the grid of the audio amplifier valve via the volume control etc, C65 couples the anode of that valve to the control grid of the audio output valve.

Decoupling capacitor....C37 and C46 are examples of decoupling capacitors, those decouple the screen grids of V3 and V4, in other words they shunt to chassis an unwanted signal that might want to appear on the screen grids, without sufficient decoupling feedback can occur, sometimes in a circuit feedback is wanted, but in other circuits it's unwanted, in the case of screen grids feedback is generally unwanted but there are exceptions.

Demodulator....There are two demodulators in your receiver, one for AM and the other for FM, the AM one is the circuit with the single diode, the FM one is the circuit with the two diodes.

Most folks would call the AM demodulator the AM detector and the FM demodulator the FM discriminator, although with the latter sometimes it's also referred to as the ratio detector if it's of that type which this one is.

Hope that helps.

EDIT: Post crossed.

Lawrence.

[mark_in_manc]

Mine does this too, and I've yet to attempt a big re-cap on it. Tell you what, if you end up with spare caps due to having to buy more than you need, I'll buy a set off you

[Neil Breward]

Unlike many others, these excellent sets were well supplied with decoupling capacitors. Unfortunately the Hunts 0.04mfd 'micromold' types are notorious for either going leaky or (especially) open-circuit as they age. The obvious targets are the ones in the HT feeds to the IF transformers and the screen grids of the EF89 IF valves and the ECH81 frequency changer (IF amp on FM). There is also one in the HT feed to the VHF tuner, which is perhaps the prime target.

Also, with its HT feed decoupler open-circuit it is almost impossible to correctly balance the FM Discriminator transformer. The other component worth replacing is the small 5mfd electrolytic capacitor which is the FM discriminator's 'reservoir' capacitor.

Persevere with this set. They are excellent performers on FM when they are sorted out.

Pity Bush didn't build a push-pull output stage into them though!

Neil