Roberts' tuning caps. - UK Vintage Radio Repair and Restoration Discussion Forum

Viscount · 29th Jun 2022, 6:55 pm

I have two Roberts radios both with the same problem, scratchy tuning caps in other word sounds like a dirty pot. Is there a way of cleaning these? Radios are an R24 and an R72.

G6Tanuki · 29th Jun 2022, 7:59 pm

Tuning-capacitors can - if exposed to dampness - get corrosion on the aluminium vanes which result in random short-circuits between the vanes as the capacitor is turned.

The 'classic' way to clear these is to use a vacuum-cleaner to provide an extractive suckage while setting the capacitor so the plates are maximally-enmeshed, then slipping a very thin [1 thou?] feeler-gauge blade down into each and every gap to dislodge the corrosion-lumps.

But don't also exclude a contact-problem in the springy fingers that ground the moving spindle to the capacitor body; old, congealed grease mixed with decades of accreted dust can make the contact here intermittent. The best way to deal with this is to take the capacitor out and give it a blast in an ultrasonic cleaning-bath.

Viscount · 29th Jun 2022, 8:55 pm

the prob i have is that the R24 and R727 use those square plastic units and not the vane type ,sorry maybe should have mentioned that in the op

G6Tanuki · 29th Jun 2022, 9:38 pm

Ah, Polyvaricons!

They're generally considered 'no user-serviceable parts inside' - I've got a couple of radios whose Polyvaricons have gone scratchy, and have just learned to live with it.

But - it's always worth checking that there are no dry solder-joints on the little leadouts that connect the capacitor to the rest of the circuit-board; I've had this happen in the past and the mechanical movement of the capacitor when the tuning was adjusted was enough to cause the dry-joint to rustle. Reflowing the solder fixed _that_ particular fault.

Viscount · 30th Jun 2022, 7:15 am

Polyvaricons ,so thats what they are called?,i will check for dry joints,thanks for that info G6Tanuki

Heatercathodeshort · 30th Jun 2022, 8:47 am

I suffered loads of problems with these. Never discovered a way of reliably bringing them back to life. Many developed this fault within two years together with noisy waveband switches and volume controls. I would add that these were in dry domestic situations.
Size does matter. John.

29th Jun 2022, 6:55 pm	#1
Viscount Heptode Join Date: Jan 2018 Location: Folkestone, Kent, UK. Posts: 711	Roberts' tuning caps. I have two Roberts radios both with the same problem, scratchy tuning caps in other word sounds like a dirty pot. Is there a way of cleaning these? Radios are an R24 and an R72.

29th Jun 2022, 7:59 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Roberts' tuning caps. Tuning-capacitors can - if exposed to dampness - get corrosion on the aluminium vanes which result in random short-circuits between the vanes as the capacitor is turned. The 'classic' way to clear these is to use a vacuum-cleaner to provide an extractive suckage while setting the capacitor so the plates are maximally-enmeshed, then slipping a very thin [1 thou?] feeler-gauge blade down into each and every gap to dislodge the corrosion-lumps. But don't also exclude a contact-problem in the springy fingers that ground the moving spindle to the capacitor body; old, congealed grease mixed with decades of accreted dust can make the contact here intermittent. The best way to deal with this is to take the capacitor out and give it a blast in an ultrasonic cleaning-bath. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

29th Jun 2022, 8:55 pm	#3
Viscount Heptode Join Date: Jan 2018 Location: Folkestone, Kent, UK. Posts: 711	Re: Roberts' tuning caps. the prob i have is that the R24 and R727 use those square plastic units and not the vane type ,sorry maybe should have mentioned that in the op

29th Jun 2022, 9:38 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Roberts' tuning caps. Ah, Polyvaricons! They're generally considered 'no user-serviceable parts inside' - I've got a couple of radios whose Polyvaricons have gone scratchy, and have just learned to live with it. But - it's always worth checking that there are no dry solder-joints on the little leadouts that connect the capacitor to the rest of the circuit-board; I've had this happen in the past and the mechanical movement of the capacitor when the tuning was adjusted was enough to cause the dry-joint to rustle. Reflowing the solder fixed _that_ particular fault. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

30th Jun 2022, 7:15 am	#5
Viscount Heptode Join Date: Jan 2018 Location: Folkestone, Kent, UK. Posts: 711	Re: Roberts' tuning caps. Polyvaricons ,so thats what they are called?,i will check for dry joints,thanks for that info G6Tanuki

30th Jun 2022, 8:47 am	#6
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Roberts' tuning caps. I suffered loads of problems with these. Never discovered a way of reliably bringing them back to life. Many developed this fault within two years together with noisy waveband switches and volume controls. I would add that these were in dry domestic situations. Size does matter. John.