Local help for a VHF80c - UK Vintage Radio Repair and Restoration Discussion Forum

TrevorM6TSE · 26th Jul 2009, 9:58 pm

Hi All,

OK, so I've hit a block on this one! I've been restoring a Bush VHF80c for some time now and I've got it electrically nearly 100% apart from one VERY annoying thing...

When it's cold and you turn it on it works fine on LW and MW, but on VHF you get background white noise and a crackle but no stations. As it warms up fully the stations come in, but the "crackle" keeps taking them away and leaves you with just white noise. After about 30 mins it will settle down. If you leave it off for a few weeks it takes even longer to fully work.

I've changed all suspect caps, cleaned valve bases, changed the UCC85, moved the two UF89's round and had it on the bench and prodded around to try and produce the fault to no avail. I've cleaned the wavechange switch as well. I've also been in the FM front end and resoldered and checked in there.

So, I'm at a loss! Could I have a fault in a IF can or similar? Dodgy valve?

I'm looking ideally for another member in the Derbyshire/Notts area to have a look for me and see what I've missed.

Any help greatfully received.

Trev

'LIVEWIRE?' · 27th Jul 2009, 12:09 pm

I have a similar problem with a VHF90C, which is an FM Only Radio. This had been dropped, but I can see no evidence of any damaged or broken parts, dislodged cores, etc.
Like you, I have replaced capacitors & resistors, changed the UCC85, etc., to no avail. What are we missing?

Leon Crampin · 27th Jul 2009, 12:33 pm

On the Bush sets with Plessey slider wavechange switches (including VHF61 and less often, the '64), crackling is almost always tracking in the switch sections from HT (or thereabouts) to some adjacent contact.

These switches have very close spacing between contacts - they are interleaved front and rear - and the paxolin grade is rather poor. Sometimes there is a workaround; one of my VHF61s had a bad crackle cused by HT tracking to the switch in the AF feedback network. Luckily, there were enough unused spare contacts for a (rather devious) fix.

Obviously none of this can apply to the '90C - but check all Hunts have been evicted. I don't have the circuit of the '90C to hand, but I did once have an inexplicable crackle on a Murphy U502 which was caused by a CZ1 thermistor with a crack in it. Freezer is often the way to find this type of problem.

Leon.

TrevorM6TSE · 27th Jul 2009, 8:00 pm

Hi,

I did think about the wavechange, but in the VHF80 and 81 it's quite a big three button push switch with all the elements open and visible. I've cleaned it using serisol switch cleaner, but it still does it

I've also prodded the wave change a few times, but it makes no noticeable change.

Anything else I should check? It's driving me nuts!

Steve_P · 27th Jul 2009, 8:31 pm

Get your Signal Generator and couple it loosely to the IF out of the VHF box. When there's no VHF, do you still get the 10.7 Mhz?

Cheers,

Steve P.

TrevorM6TSE · 28th Jul 2009, 5:41 pm

Quote:

Originally Posted by Steve_P

Get your Signal Generator and couple it loosely to the IF out of the VHF box. When there's no VHF, do you still get the 10.7 Mhz?

Cheers,

Steve P.

Hi Steve,

Unfortunately I've not really got the gear to do this I'm Afraid

Ideally I'm looking for a local member to take a look for me.

cheers

PJL · 28th Jul 2009, 11:31 pm

I looked at the repair notes on a VHF81 I did a while back and the reservoir capacitor in the ratio detector was a problem. It's a small electrolytic.

I have also had IF capacitor problems on a couple of Bush radios but as I recall it was the 465KHz not 10.7MHz IF.

I guess you have already tried wriggling valves in their sockets and the wavechange buttons to see if this has any effect?

You kind of already have a signal generator as you are getting white noise. I don't have a circuit but perhaps someone can suggest a few places where a small capactior could be put between grid and chassis/cathode to work back through each stage to see where the noise is coming from.

Steve_P · 29th Jul 2009, 10:52 am

Take DC measurements on the valves too. Might show something up.

Before and After the fault occurs and corrects itself. If applying a meter probe clears the the problem then that is a big clue.

Might be worth using freezer spray - carefully....

Cheers,

Steve P.

TrevorM6TSE · 1st Aug 2009, 5:04 pm

Hi All,

Well I've spent a number of hours on this set now and it's still doing it. I've changed a number of resistors, checked voltages etc and it all seems well. It'll play fine for a few minutes then crackle and just white noise. Moving the wavechanger does nothing, tapping it does nothing. I've been in the FM front end and re soldered, prodded, etc and still the same!

I'm at a loss with it now, in fact I've had to wlak away from it as I was beginning to lose my temper!

Anyone local fancy having a look? If not, it's going in the loft never to be seen again!

Top Cap · 1st Aug 2009, 5:26 pm

Have you another VHF radio to hand that you can use as a test receiver? You could try tuning it to around 88MHz then place it close to the Bush. Tune the Bush to somewhere around 98MHz and you should hear a strong carrier signal appear on the other test receiver which is the local oscillator of the Bush. Note the tuning position on the Bush where you hear this carrier and set the Bush to receive a local station. When the station fades due to the fault, check to see if you can still get the test receiver to pick up the Bush local oscillator as before. If you cannot, then the local oscillator is shutting down for some reason, at least it may help narrow the fault for you.
Les

matthewhouse · 1st Aug 2009, 5:28 pm

I have a Marconi set doing this at the moment, one minute it works, the next just white noise. The fault on my set is a faulty ceramic trimmer in the FM box. It looked ok but as soon as I prodded it, it fell apart.

Phil G4SPZ · 1st Aug 2009, 5:52 pm

The tip involving a second set to generate a beat note is a good one, as is the use of freezer aerosol spray. Alternating with a hairdryer, this can be a quick way of localising faults with a thermal cause.

It sounds almost as if the local oscillator is suddenly going way off tune. I had a similar fault in the VFO of a Heathkit ham-bands transceiver once, where the frequency would suddenly jump a few kHz, and it turned out (after exhaustive replacement of caps) to be corrosion on one of the soldered ends of the oscillator coil.

26th Jul 2009, 9:58 pm	#1
TrevorM6TSE Heptode Join Date: May 2003 Location: Old Tupton, Derbyshire, UK. Posts: 540	Local help for a VHF80c Hi All, OK, so I've hit a block on this one! I've been restoring a Bush VHF80c for some time now and I've got it electrically nearly 100% apart from one VERY annoying thing... When it's cold and you turn it on it works fine on LW and MW, but on VHF you get background white noise and a crackle but no stations. As it warms up fully the stations come in, but the "crackle" keeps taking them away and leaves you with just white noise. After about 30 mins it will settle down. If you leave it off for a few weeks it takes even longer to fully work. I've changed all suspect caps, cleaned valve bases, changed the UCC85, moved the two UF89's round and had it on the bench and prodded around to try and produce the fault to no avail. I've cleaned the wavechange switch as well. I've also been in the FM front end and resoldered and checked in there. So, I'm at a loss! Could I have a fault in a IF can or similar? Dodgy valve? I'm looking ideally for another member in the Derbyshire/Notts area to have a look for me and see what I've missed. Any help greatfully received. Trev __________________ I'd just like to say "Good luck, we're all counting on you"

27th Jul 2009, 12:09 pm	#2
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Local help for a VHF80c I have a similar problem with a VHF90C, which is an FM Only Radio. This had been dropped, but I can see no evidence of any damaged or broken parts, dislodged cores, etc. Like you, I have replaced capacitors & resistors, changed the UCC85, etc., to no avail. What are we missing?

27th Jul 2009, 12:33 pm	#3
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Local help for a VHF80c On the Bush sets with Plessey slider wavechange switches (including VHF61 and less often, the '64), crackling is almost always tracking in the switch sections from HT (or thereabouts) to some adjacent contact. These switches have very close spacing between contacts - they are interleaved front and rear - and the paxolin grade is rather poor. Sometimes there is a workaround; one of my VHF61s had a bad crackle cused by HT tracking to the switch in the AF feedback network. Luckily, there were enough unused spare contacts for a (rather devious) fix. Obviously none of this can apply to the '90C - but check all Hunts have been evicted. I don't have the circuit of the '90C to hand, but I did once have an inexplicable crackle on a Murphy U502 which was caused by a CZ1 thermistor with a crack in it. Freezer is often the way to find this type of problem. Leon.

27th Jul 2009, 8:00 pm	#4
TrevorM6TSE Heptode Join Date: May 2003 Location: Old Tupton, Derbyshire, UK. Posts: 540	Re: Local help for a VHF80c Hi, I did think about the wavechange, but in the VHF80 and 81 it's quite a big three button push switch with all the elements open and visible. I've cleaned it using serisol switch cleaner, but it still does it I've also prodded the wave change a few times, but it makes no noticeable change. Anything else I should check? It's driving me nuts! __________________ I'd just like to say "Good luck, we're all counting on you"

27th Jul 2009, 8:31 pm	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Local help for a VHF80c Get your Signal Generator and couple it loosely to the IF out of the VHF box. When there's no VHF, do you still get the 10.7 Mhz? Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

28th Jul 2009, 11:31 pm	#7
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Local help for a VHF80c I looked at the repair notes on a VHF81 I did a while back and the reservoir capacitor in the ratio detector was a problem. It's a small electrolytic. I have also had IF capacitor problems on a couple of Bush radios but as I recall it was the 465KHz not 10.7MHz IF. I guess you have already tried wriggling valves in their sockets and the wavechange buttons to see if this has any effect? You kind of already have a signal generator as you are getting white noise. I don't have a circuit but perhaps someone can suggest a few places where a small capactior could be put between grid and chassis/cathode to work back through each stage to see where the noise is coming from.

29th Jul 2009, 10:52 am	#8
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Local help for a VHF80c Take DC measurements on the valves too. Might show something up. Before and After the fault occurs and corrects itself. If applying a meter probe clears the the problem then that is a big clue. Might be worth using freezer spray - carefully.... Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

1st Aug 2009, 5:04 pm	#9
TrevorM6TSE Heptode Join Date: May 2003 Location: Old Tupton, Derbyshire, UK. Posts: 540	Re: Local help for a VHF80c Hi All, Well I've spent a number of hours on this set now and it's still doing it. I've changed a number of resistors, checked voltages etc and it all seems well. It'll play fine for a few minutes then crackle and just white noise. Moving the wavechanger does nothing, tapping it does nothing. I've been in the FM front end and re soldered, prodded, etc and still the same! I'm at a loss with it now, in fact I've had to wlak away from it as I was beginning to lose my temper! Anyone local fancy having a look? If not, it's going in the loft never to be seen again! __________________ I'd just like to say "Good luck, we're all counting on you"

1st Aug 2009, 5:26 pm	#10
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Re: Local help for a VHF80c Have you another VHF radio to hand that you can use as a test receiver? You could try tuning it to around 88MHz then place it close to the Bush. Tune the Bush to somewhere around 98MHz and you should hear a strong carrier signal appear on the other test receiver which is the local oscillator of the Bush. Note the tuning position on the Bush where you hear this carrier and set the Bush to receive a local station. When the station fades due to the fault, check to see if you can still get the test receiver to pick up the Bush local oscillator as before. If you cannot, then the local oscillator is shutting down for some reason, at least it may help narrow the fault for you. Les __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

1st Aug 2009, 5:28 pm	#11
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Local help for a VHF80c I have a Marconi set doing this at the moment, one minute it works, the next just white noise. The fault on my set is a faulty ceramic trimmer in the FM box. It looked ok but as soon as I prodded it, it fell apart.

1st Aug 2009, 5:52 pm	#12
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: Local help for a VHF80c The tip involving a second set to generate a beat note is a good one, as is the use of freezer aerosol spray. Alternating with a hairdryer, this can be a quick way of localising faults with a thermal cause. It sounds almost as if the local oscillator is suddenly going way off tune. I had a similar fault in the VFO of a Heathkit ham-bands transceiver once, where the frequency would suddenly jump a few kHz, and it turned out (after exhaustive replacement of caps) to be corrosion on one of the soldered ends of the oscillator coil. __________________ Phil Optimist [n]: One who is not in possession of the full facts