Scratchy noise when tuning fm radio? - UK Vintage Radio Repair and Restoration Discussion Forum

David Gardener · 22nd Jan 2020, 1:21 pm

Hi, I have a Ferguson 3T18 boombox from the early eighties. As I tune it to another fm station it produces a scratchy noise ( the same kind of noise as a scratchy volume pot makes). Is dirt in the main variable capacitor responsible for this? If so can I simply give it a shot of Servisol to clean it?
Cheers.

'LIVEWIRE?' · 22nd Jan 2020, 2:53 pm

If, as most were, it's a plastic (Polyvaricon) tuning capacitor, I'd be very wary of squirting Servisol into it. TBH I don't know of a reliable way of cleaning these devices. Maybe other members do?

OldTechFan96 · 22nd Jan 2020, 6:31 pm

I've had this happen in full size tuning capacitors when the metal veins are touching or are dirty.

The Polyvaricon will probably be a sealed unit and would need to be removed to clean. It might only need blowing through with compressed air.

Are you comfortable stripping your unit right down?

Sideband · 22nd Jan 2020, 7:32 pm

Earthing springs on the variable cap can cause this. In open types, these can tarnish and then the moving vanes cause a crackle or scratchy noise. It all depends on what type of variable cap is fitted and if it's possible to dismantle it to clean.....and if it can be reassembled afterwards....

Pfraser · 22nd Jan 2020, 7:39 pm

I had this problem with a 'Joysonic' cheap 'n' cheerful CB/FM/Airband portable. My Roberts R9962 currently has it.

I suspect it's due to tarnishing of the shaft and contacts in the capacitor, given that it's sealed. For temporary improvement, try tuning moderately briskly across the full range, several times.

I've never had the confidence to remove & open up a polyvaricon tuning capacitor.

Graham G3ZVT · 22nd Jan 2020, 8:11 pm

The 3T18 is pictured on this site
http://www.stereo2go.com/topic/index...92314111653340

Here is a detail from an image attributed to "soul.music"

Click image for larger version

Name: tuning.jpg
Views: 255
Size: 70.0 KB
ID: 197593

I have a vague memory of partially dismantling a similar cap and finding some tiny washers that grounded the stator and cleaning them, but I wouldn't rely on whether it was real or imagined, and in any case I couldn't say if I made it worse or better.

Nickthedentist · 22nd Jan 2020, 9:57 pm

I've cleaned these with Servisol before, in a bathroom set which became very noisy and awkward to tune.

It cured it, but others have reported disaster, so try it at your own risk!

N.

David Gardener · 22nd Jan 2020, 10:46 pm

Is it possible to drill a small hole in the case of the variable capacitor and squirt some servisol inside, and if that fails can I purchase a new one anywhere that would fit and work, if so would it need some setting up or would it simply be a matter of soldering the new one in place?
Cheers.

Nickthedentist · 22nd Jan 2020, 10:52 pm

No hole needed, I can see plenty of gaps in the pic in post 6, and often, the squishy plastic cover can be completely removed too.

Goldieoldie · 22nd Jan 2020, 11:08 pm

Don't use sevisol !
Much better to use IPA
Most caps are ok with IPA but I have had one where the IPA reacted with the insulators .
If you go for it ,squirt a small amount in with capacitor fully open ( high frequency end )
Don't twiddle the shaft as the insulators might buckle up .
Shake it around and then drain
Dry with cold air from hair dryer ,don't twiddle until you are sure all the IPA has evaporated
Then turn gently backwards and forwards .
Cheers
Pete

David Gardener · 22nd Jan 2020, 11:16 pm

Quote:

Originally Posted by Pfraser

I had this problem with a 'Joysonic' cheap 'n' cheerful CB/FM/Airband portable. My Roberts R9962 currently has it.

I suspect it's due to tarnishing of the shaft and contacts in the capacitor, given that it's sealed. For temporary improvement, try tuning moderately briskly across the full range, several times.

I've never had the confidence to remove & open up a polyvaricon tuning capacitor.

I've just tried tuning briskly up and down a few times and it seems to have fixed the problem at least for now.

crackle · 23rd Jan 2020, 10:22 am

Exercising by twiddling the tuning back and forth is the best way. If cleaners like servisol are squirted in the tuning will go way out and may never resolve itself.
Mike

Mr 1936 · 23rd Jan 2020, 12:29 pm

The Polyvaricon type capacitors have alternating metal vanes and plastic dielectric sheets, plus a little bit of air in the tiny gaps. The dielectric and the reduced thickness allows the while thing to be considerably miniaturised. Older forum members might recall the solid dielectric variable capacitors used for reaction controls etc, which used thin paxolin sheets. Lower Q but smaller and cheaper than air spaced types.

If squirting in a solvent based product, the risk is that it will wick into the tiny gap between the vanes and sheets. Solvents have some surprisingly extreme dielectric properties, so initially the tuning may go way off. The solvent part will probably eventually evaporate, but the residual lubricant (which will also have dielectric properties) may lodge in there permanently.

Why do I know this ? I was once responsible for the design of a PCB containing those silver coloured polystyrene "swiss roll" capacitors. Close tolerance and quite expensive. We couldn't work out why the oscillator frequencies were way off, and production were blaming the designer. Then someone tested a board which hadn't been cleaned in "Arklone" after soldering. Everything was fine on that one. It turned out that the ends of the capacitors werent sealed, and sucked up the solvent nicely......

I also find that a bit of "exercise" can cure crackly tuning etc. When I used to mend electronic things for people, the commonest complaint was tape decks which wouldn't record, due to the switch not having been used for ages. A dozen changes from play to record was usually enough to get things going even without switch cleaner.

poppydog · 23rd Jan 2020, 11:09 pm

I have a dulci air band radio, a far eastern made thing from the early 70s that is cheap and cheerful, is really super sensitive, but has really scratchy sealed poly tuning caps. No amount of exercising will clear the scratching. I will at some point try to remove and clean them, maybe with a low pressure airline, I wouldn't recommend spraying anything in them.
See the circus I had here

-

https://www.vintage-radio.net/forum/...d.php?t=146051

poppydog

David Gardener · 24th Jan 2020, 8:33 am

The answers on this thread will make me more cautious in future when spraying Servisol to clean pots. I may even use a contact cleaner with a syringe so I only get the cleaner where it is required. Going off topic here but is Alcohol effective at cleaning pots and switches?

'LIVEWIRE?' · 24th Jan 2020, 9:09 am

In my experience, Servisol rarely damages pots. - I can only recall one instance in the best part of 50 years where this has happened, but other components, particularly variable capacitors, may be adversely affected by the use of Servisol or similar products. As with all things read the instructions before using the product

Luxman1050 · 25th Jan 2020, 2:09 pm

I find servisol good for cleaning contacts although I would not advocate spraying into pots as I have found it temporarily fixes the problem but as many say the pot has started going noisy again. All it does is gives a quick clean but you still have the deposits of carbon dust etc still left inside. If you can open them up and clean that's the way to go plus you can also see if there's too much wear or minor damage.
Visible switches just undo and again clean by hand.
All seems like a lot of work but worth the effort.
Don't just spray all over the place asking for trouble.

Nickthedentist · 25th Jan 2020, 4:58 pm

Useful thread here: https://antiqueradios.com/forums/vie...it=polyvaricon

This confirms what I thought. A solvent and lubricant is needed, but in very well-defined point only! I can't see that a blast with an air line will achieve much.

David Gardener · 26th Jan 2020, 11:49 am

Quote:

Originally Posted by Nickthedentist

Useful thread here: https://antiqueradios.com/forums/vie...it=polyvaricon

This confirms what I thought. A solvent and lubricant is needed, but in very well-defined point only! I can't see that a blast with an air line will achieve much.

Wow! I'm not sure I would have the patience or dexterity to that. Just imagine how frustrating it would be to lose some of those tiny washers on the carpet! Just out of interest are these variable polyvaricon capacitors easy to source new?

Luxman1050 · 26th Jan 2020, 12:10 pm

Two good tricks to locate anything that has fallen off a part you are cleaning servicing onto floor or carpet that's at home off course.
Main seem bit stupid but it works for me two methods cordless vacuum just go all over carpet and you'll hear it as its sucked up or use a large flash light and shine all over and eventually you'll see it. I've not lost a single part yet using those methods.

Worth a try.

22nd Jan 2020, 1:21 pm	#1
David Gardener Triode Join Date: Nov 2017 Location: Rhayader, Powys, Wales, UK. Posts: 11	Scratchy noise when tuning fm radio? Hi, I have a Ferguson 3T18 boombox from the early eighties. As I tune it to another fm station it produces a scratchy noise ( the same kind of noise as a scratchy volume pot makes). Is dirt in the main variable capacitor responsible for this? If so can I simply give it a shot of Servisol to clean it? Cheers.

22nd Jan 2020, 2:53 pm	#2
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Scratchy noise when tuning fm radio? If, as most were, it's a plastic (Polyvaricon) tuning capacitor, I'd be very wary of squirting Servisol into it. TBH I don't know of a reliable way of cleaning these devices. Maybe other members do?

22nd Jan 2020, 6:31 pm	#3
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,178	Re: Scratchy noise when tuning fm radio? I've had this happen in full size tuning capacitors when the metal veins are touching or are dirty. The Polyvaricon will probably be a sealed unit and would need to be removed to clean. It might only need blowing through with compressed air. Are you comfortable stripping your unit right down?

22nd Jan 2020, 7:32 pm	#4
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	Re: Scratchy noise when tuning fm radio? Earthing springs on the variable cap can cause this. In open types, these can tarnish and then the moving vanes cause a crackle or scratchy noise. It all depends on what type of variable cap is fitted and if it's possible to dismantle it to clean.....and if it can be reassembled afterwards.... __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

22nd Jan 2020, 7:39 pm	#5
Pfraser Pentode Join Date: Sep 2018 Location: Devon, UK. Posts: 152	Re: Scratchy noise when tuning fm radio? I had this problem with a 'Joysonic' cheap 'n' cheerful CB/FM/Airband portable. My Roberts R9962 currently has it. I suspect it's due to tarnishing of the shaft and contacts in the capacitor, given that it's sealed. For temporary improvement, try tuning moderately briskly across the full range, several times. I've never had the confidence to remove & open up a polyvaricon tuning capacitor.

22nd Jan 2020, 8:11 pm	#6
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,713	Re: Scratchy noise when tuning fm radio? The 3T18 is pictured on this site http://www.stereo2go.com/topic/index...92314111653340 Here is a detail from an image attributed to "soul.music" I have a vague memory of partially dismantling a similar cap and finding some tiny washers that grounded the stator and cleaning them, but I wouldn't rely on whether it was real or imagined, and in any case I couldn't say if I made it worse or better. __________________ -- Graham. G3ZVT

22nd Jan 2020, 9:57 pm	#7
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: Scratchy noise when tuning fm radio? I've cleaned these with Servisol before, in a bathroom set which became very noisy and awkward to tune. It cured it, but others have reported disaster, so try it at your own risk! N.

22nd Jan 2020, 10:46 pm	#8
David Gardener Triode Join Date: Nov 2017 Location: Rhayader, Powys, Wales, UK. Posts: 11	Re: Scratchy noise when tuning fm radio? Is it possible to drill a small hole in the case of the variable capacitor and squirt some servisol inside, and if that fails can I purchase a new one anywhere that would fit and work, if so would it need some setting up or would it simply be a matter of soldering the new one in place? Cheers.

22nd Jan 2020, 10:52 pm	#9
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: Scratchy noise when tuning fm radio? No hole needed, I can see plenty of gaps in the pic in post 6, and often, the squishy plastic cover can be completely removed too.

22nd Jan 2020, 11:08 pm	#10
Goldieoldie Octode Join Date: Jul 2014 Location: Petersfield, Hampshire, UK. Posts: 1,043	Re: Scratchy noise when tuning fm radio? Don't use sevisol ! Much better to use IPA Most caps are ok with IPA but I have had one where the IPA reacted with the insulators . If you go for it ,squirt a small amount in with capacitor fully open ( high frequency end ) Don't twiddle the shaft as the insulators might buckle up . Shake it around and then drain Dry with cold air from hair dryer ,don't twiddle until you are sure all the IPA has evaporated Then turn gently backwards and forwards . Cheers Pete

23rd Jan 2020, 10:22 am	#12
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Scratchy noise when tuning fm radio? Exercising by twiddling the tuning back and forth is the best way. If cleaners like servisol are squirted in the tuning will go way out and may never resolve itself. Mike

23rd Jan 2020, 12:29 pm	#13
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 521	Re: Scratchy noise when tuning fm radio? The Polyvaricon type capacitors have alternating metal vanes and plastic dielectric sheets, plus a little bit of air in the tiny gaps. The dielectric and the reduced thickness allows the while thing to be considerably miniaturised. Older forum members might recall the solid dielectric variable capacitors used for reaction controls etc, which used thin paxolin sheets. Lower Q but smaller and cheaper than air spaced types. If squirting in a solvent based product, the risk is that it will wick into the tiny gap between the vanes and sheets. Solvents have some surprisingly extreme dielectric properties, so initially the tuning may go way off. The solvent part will probably eventually evaporate, but the residual lubricant (which will also have dielectric properties) may lodge in there permanently. Why do I know this ? I was once responsible for the design of a PCB containing those silver coloured polystyrene "swiss roll" capacitors. Close tolerance and quite expensive. We couldn't work out why the oscillator frequencies were way off, and production were blaming the designer. Then someone tested a board which hadn't been cleaned in "Arklone" after soldering. Everything was fine on that one. It turned out that the ends of the capacitors werent sealed, and sucked up the solvent nicely...... I also find that a bit of "exercise" can cure crackly tuning etc. When I used to mend electronic things for people, the commonest complaint was tape decks which wouldn't record, due to the switch not having been used for ages. A dozen changes from play to record was usually enough to get things going even without switch cleaner.

23rd Jan 2020, 11:09 pm	#14
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	Re: Scratchy noise when tuning fm radio? I have a dulci air band radio, a far eastern made thing from the early 70s that is cheap and cheerful, is really super sensitive, but has really scratchy sealed poly tuning caps. No amount of exercising will clear the scratching. I will at some point try to remove and clean them, maybe with a low pressure airline, I wouldn't recommend spraying anything in them. See the circus I had here- https://www.vintage-radio.net/forum/...d.php?t=146051 poppydog

24th Jan 2020, 8:33 am	#15
David Gardener Triode Join Date: Nov 2017 Location: Rhayader, Powys, Wales, UK. Posts: 11	Re: Scratchy noise when tuning fm radio? The answers on this thread will make me more cautious in future when spraying Servisol to clean pots. I may even use a contact cleaner with a syringe so I only get the cleaner where it is required. Going off topic here but is Alcohol effective at cleaning pots and switches?

24th Jan 2020, 9:09 am	#16
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Scratchy noise when tuning fm radio? In my experience, Servisol rarely damages pots. - I can only recall one instance in the best part of 50 years where this has happened, but other components, particularly variable capacitors, may be adversely affected by the use of Servisol or similar products. As with all things read the instructions before using the product

25th Jan 2020, 2:09 pm	#17
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Scratchy noise when tuning fm radio? I find servisol good for cleaning contacts although I would not advocate spraying into pots as I have found it temporarily fixes the problem but as many say the pot has started going noisy again. All it does is gives a quick clean but you still have the deposits of carbon dust etc still left inside. If you can open them up and clean that's the way to go plus you can also see if there's too much wear or minor damage. Visible switches just undo and again clean by hand. All seems like a lot of work but worth the effort. Don't just spray all over the place asking for trouble.

25th Jan 2020, 4:58 pm	#18
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: Scratchy noise when tuning fm radio? Useful thread here: https://antiqueradios.com/forums/vie...it=polyvaricon This confirms what I thought. A solvent and lubricant is needed, but in very well-defined point only! I can't see that a blast with an air line will achieve much.

26th Jan 2020, 12:10 pm	#20
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Scratchy noise when tuning fm radio? Two good tricks to locate anything that has fallen off a part you are cleaning servicing onto floor or carpet that's at home off course. Main seem bit stupid but it works for me two methods cordless vacuum just go all over carpet and you'll hear it as its sucked up or use a large flash light and shine all over and eventually you'll see it. I've not lost a single part yet using those methods. Worth a try.