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Old 6th Oct 2010, 2:04 pm   #21
G8HQP Dave
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Default Re: Fuzzy sound on Bush VHF80C

I have a spare UCC85, but unlikely to be that as the fuzz is present on AM too. I have wondered about h-k leak in UF89 or UCH81, but I haven't any spares for those yet and I would expect the onset to be quicker than 20 mins running.

I think the next job is to recheck the UL84 grid voltage over a long period. After that it may be a case of hunting for a cracked ceramic capacitor which is allowing RF to stray.
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Old 6th Oct 2010, 5:59 pm   #22
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Default Re: Fuzzy sound on Bush VHF80C

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Originally Posted by G8HQP Dave View Post
I have wondered about h-k leak in UF89 or UCH81, but I haven't any spares for those yet
Don't forget there are two UF89's in this set. You could try swapping them round and see if the fault alters.

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Old 6th Oct 2010, 6:53 pm   #23
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Default Re: Fuzzy sound on Bush VHF80C

Already tried that. No change. I was planning to have another go at it today but I got sidetracked by other things. Maybe tomorrow.
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Old 6th Oct 2010, 8:44 pm   #24
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Default Re: Fuzzy sound on Bush VHF80C

This 'fuzzy' sound is a puzzle. Just out of interest, have you tried an alternative speaker. You've also mentioned that AM is fine (post 12). You've changed the electrolytic in the discriminator...positive goes to chassis...is this correct?.

It's so unusual for one of these to give so much trouble.

Seems also that sometimes it's worked OK. I once had a small radio (not one of these) that sounded dreadful when first switched on due to a duff speaker but once it had got warm after about ten minutes, the sound improved. The heat was enough to 'cure' the scratchy speaker. Just wondering if the speeaker is affected by heat. If it is, you may be able to re-centre the cone. Unfortunately I had to replace the speaker.

Virtually any good speaker will do for testing even if it's not 3 ohms. It will prove the point one way or the other.


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Old 6th Oct 2010, 10:21 pm   #25
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Default Re: Fuzzy sound on Bush VHF80C

No, I haven't yet tried an alternative speaker. I'm interested in your finding that a little heat can cause/cure a speaker problem.

Yes, the discriminator electrolytic is the right way round. The sound is fine at first.

When the problem first occured I just assumed it was a gassy UL84 drawing too much current and overloading the PSU. As the output transformer has hum cancellation the effect of HT hum might be heard as intermodulation rather than hum. The coupling capacitor did fail, after being OK at first. Last time I checked there was a minor grid current problem but not enough to cause trouble. There is nothing else which can pull a lot of current, and nothing seems to be getting unduly hot.

The funny thing is, whenever I attach test leads for DVM there are hints of instability which may or may not be related to the fuzzy sound. Inevitably in a radio like this the input and output can't be far apart, and a UL84 is quite happy to amplify RF if it sees any.

Perhaps I should admit that I do not have an isolation transformer, so I am proceeding by running it with the back on and being careful with DVM test leads. I can only attach the scope to the isolated tape output. I am getting to the point where I will have to shell out for a transformer, then hopefully I can have a better look inside while it is working.

The circuit is well-designed but nothing unusual, so I am puzzled that this fault is taking so long to find. I keep finding and fixing things, but no change!
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Old 7th Oct 2010, 12:49 pm   #26
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Default Re: Fuzzy sound on Bush VHF80C

Minor update/mystery:

I have rechecked the UL84 grid voltage. It gently rises to about 0.28V then settles down to 0.25V - I assume that what is happening is that at first some gas is released from the metal as it warms up, then the getter mops up some of it. It was some time later than the fuzz appeared, so no evidence that the UL84 is taking excessive current or causing the problem.

I quickly swapped the UL84 for a spare. The spare one looks used, but in better nick than the resident one. Grid voltage now zero, which confirms that the small rise seen before is due to the resident UL84 not a faulty replacement capacitor. Fuzz still present. The spare UL84 has more hum though. This is the mystery: when I switched off the hum didn't stop but merely faded at the same rate as the audio. Two possibilities: the hum is picked up by the test leads (it is much less with the leads removed), or the DP switch is faulty so live is still present and coming via h-k leakage in the spare UL84. If the former, why do the leads pick up much less hum before the fuzz starts? Is it that the anode impedance of the UABC80 triode has changed?
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Old 7th Oct 2010, 1:21 pm   #27
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Default Re: Fuzzy sound on Bush VHF80C

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Originally Posted by G8HQP Dave View Post
I'm interested in your finding that a little heat can cause/cure a speaker problem.
It's not the first time I've experienced this. Another set had the opposite effect....the sound got worse as it warmed up, a little HMV1373 I think which I no longer have. This one had a UCL83 as the output valve and the speaker was mounted at the same end of the chassis as this and the dropper. The combined heat was enough to distort the speaker frame. I couldn't cure it although a temporary improvement was obtain by wadding some cotton wool between the cone and the frame. In the end I replaced the speaker with one from a scrap Bush TR82C which was an exact fit.


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Old 7th Oct 2010, 1:42 pm   #28
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Default Re: Fuzzy sound on Bush VHF80C

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Originally Posted by G8HQP Dave View Post
Perhaps I should admit that I do not have an isolation transformer, so I am proceeding by running it with the back on and being careful with DVM test leads. I can only attach the scope to the isolated tape output. I am getting to the point where I will have to shell out for a transformer, then hopefully I can have a better look inside while it is working.
Well at the risk of running the wrath of other contributors on here, you could disconnect the earth from your scope and make sure that the chassis of the radio is actually neutral. You can then connect your scope without getting any nasty bangs or tripping the house circuit breaker. As long as the chassis is neutral there is no real danger and an isolating transformer won't stop you from getting a shock, it just means that you can connect earthed test equipment.

These sets are nice to work on out of the cabinet because everything is accessible on a flat chassis. Tracing hum should not be too difficult. If it was me, I would short the triode grid of the UABC80 valve to chassis and see if the hum stopped/reduced. If yes, then it's in this area. If no then do the same with the control grid of the UL84. If yes, it's between the UABC80 and the UL84. If no then it's the output valve or power supply. I think the volume control casing is earthed on these. Check that this is in order.


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Old 8th Oct 2010, 5:17 pm   #29
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Default Re: Fuzzy sound on Bush VHF80C

I now suspect the speaker. Although it is barely warm to touch (with the power removed!) a separate speaker temporarily hooked up in parallel does not seem to have the fuzz. I found this after monitoring the HT voltage to the bulk of the set - no change so the fuzz is not associated with any change in DC conditions.

Given that a quick look a couple of days ago showed no visible problem, is there anything I can do to the speaker? Or do I need to find a replacement?

BTW the existing speaker is labelled RS, so maybe this set has already killed one?

Last edited by G8HQP Dave; 8th Oct 2010 at 5:28 pm. Reason: mention RS
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Old 9th Oct 2010, 4:45 pm   #30
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Default Re: Fuzzy sound on Bush VHF80C

Light pressure one side of the speaker cone causes a voice coil rub, elsewhere does not. I suspect poor alignment, which gets worse with temperature. I am having a go with the technique described here. I put in four shims, and one of them is definitely tighter than the others. Now waiting for it to dry.
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Old 12th Oct 2010, 6:46 pm   #31
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Default Re: Fuzzy sound on Bush VHF80C

Update: first go seemed to make little difference so I decided to try again, as I had nothing to lose. The second time I used slightly thicker shims on the side which was rubbing, and was a bit more generous with the water. Seems better. Reglued the dust cap using PVA. Checked with temporary lash-up with speaker sitting outside set. Tinny but OK.

I put it all back together temporarily, but tried not to overtighten the nuts holding the speaker in place. I have just had it running for 40 mins. There is perhaps a slight hint of the fuzz, although difficult to tell as BBC R4 audio on the news is sometimes distorted anyway. Certainly better than before, as obvious fuzz was appearing after 20 mins running. A few days use should tell whether the problem has gone away.
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Old 12th Oct 2010, 10:12 pm   #32
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Default Re: Fuzzy sound on Bush VHF80C

Someone on here must have a scrap speaker from one of these. There are so many about, try putting a request in the 'Parts Wanted' section.



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