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Old 18th Sep 2018, 8:42 pm   #1
G6Tanuki
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Default A Roberts R24...

Bought at a car-boot-sale at the weekend. I only paid £2! Even if it hadn't been a worker it would still have been worth buying just for the knobs and the telescopic antenna.

There was a long-dead maroon Ever Ready PP9 inside. And the mains-lead came complete with a PAT label dating from 1999. The plug is one of the slimline "Legrand" type that have been discussed several times within this forum.

It was kinda grubby round the edges but half an hour with a range of car-cleaning products lifted the grime off the tan Rexine and removed the ingrained finger-goo from the concentric milled rings in the knobs/buttons. Unlike many such radios this one has escaped the oh-so-common "Decorator's mate" experience and there were precisely zero spots of white emulsion anywhere to be seen on it!

Plugging in and switching on, it worked, sort-of: intermittently. Then I realised the 'figure-of-8' mains connector wasn't pushed fully home in the socket on the back: a bit of a shove and it went in another 1/4 inch and things were looking up.

Except.... on FM the tuning is dreadfully 'scratchy' - I'm suspecting the Polyvaricon is unwell. I'm reluctant to apply any of the usual organic-solvent-type cleaners because in the past I've had such things reduce plastic components to a gloopy mass with the texture of cheese.

Is it possible to dismantle Polyvaricons and rebuild them? If so, I'd consider removing some of the dielectric 'leaves' from the MW/LW tuning-gangs (and sacrificing those wavebands) if it meant I got good FM tuning.

It's also got a bit of a hum: I'm thinking the main smoothing-electrolytic (which, from what I can see is a typical 1970s grey-sleeved Hong Kong-style item like the other electrolytics) has lost its capacitance. No fear, I've got a nice new Rubycon waiting.

When it works, it's not a bad radio: a bit lacking in treble (you wouldn't want to listen to much Xylophone or marimba music on it). I guess it represents Roberts' attempt to keep in business by having the insides made in the Far East as an economy measure. I'm guessing it dates from the very-early-1980s since it only goes up to 104MHz and has Radios 1 and 2 sharing the bottom end of the FM band, which surely would have changed following WARC '79 and the expansion of FM broadcasting into the above-96MHz area previously occupied by police/fire/ambulances?
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Old 18th Sep 2018, 10:49 pm   #2
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Default Re: A Roberts R24...

I have got one that has been used for a long time in the background.
The FM tuning has been getting a little bit scratchy but I suspect the caps in the AFC circuit around the RF section or the one on the discriminator. There is no hum.
I would be interested in the results of your investigation.
The tuning would jam up solid if the dielectric was faulty so that is unlikely.
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Old 18th Sep 2018, 10:55 pm   #3
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Default Re: A Roberts R24...

Do not spray switch cleaner into the tuning capacitor. The RF alignment will never be the same.
What you are possibly experiencing is poor contacts on the moving vanes. Constant use may rub the wipers and contacts clean.
Give it a try on a small part of the band and see if the crackling starts to diminish after a while.

Mike
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Old 21st Sep 2018, 3:06 pm   #4
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Default Re: A Roberts R24...

I definitely won't be using any kind of liquid cleaner! I've been caught that way with solventy-things and plastics that definitely don't play well together. A gentle whiff of dry Nitrogen (I've got half a cylinder full of it left over from the race car) is about as far as I will go.

But first, as you suggest, a bit of knob-twiddling on an unpopular end of the waveband [88 to 95MHz] will be tried - like you say, it could have a tarnished wiper. I'll also fire up my scanner and listen 10.7MHz away from the frequency the R24 is tuned to - so I can hear the LO signal and see if it's 'clean' - that way I'll be able to tell if the scratchiness is due to the LO jumping around or the signal-frequency tuned-circuit being the problem.
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Old 21st Sep 2018, 3:22 pm   #5
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Default Re: A Roberts R24...

Quote:
Originally Posted by crackle View Post
Do not spray switch cleaner into the tuning capacitor. The RF alignment will never be the same.
I have done this as a last resort and it does work. Just spray some Servisol into the trimmer holes on the back of the cap, slosh it around, operate the cap vigorously then drain as much cleaner as possible out of the trimmer holes. The alignment will be way out initially and the radio may not work at all, but the cleaner will dry out over the next couple of weeks and the alignment will return to normal.

This isn't risk free of course, but I haven't had any disasters, and a radio with a very crackly tuning cap isn't much use anyway so there's little to lose.

The R24 dates from 1983-86 so is very late to only tune to 104.
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Old 21st Sep 2018, 4:01 pm   #6
G6Tanuki
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Default Re: A Roberts R24...

I'll be trying the wiggling-the-tuning thing first though!

And yes it does seem really odd that a radio introduced in 1983 would still only go to 104MHz: I guess Roberts didn't hear about WARC '79 in time!
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Old 24th Sep 2018, 7:15 pm   #7
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Default Re: A Roberts R24...

I have had some success cleaning these. I remove them from the radio take off the plastic cover and blow them out with air duster. They seem to form a powder like coating on the capacitor plates. I have also had to wash them through with isopropyl which has done the trick. He’s a video of me tackling one in a Roberts R747. https://youtu.be/TaijbJB6U5A. I recently had to replace one in a Roberts RFM3 as the dialectric disintegrated when I tried the air duster. You can make a new one fit but the spindle fit is critical. There is also an article here http://antiqueradios.com/forums/view...p?f=4&t=261835 but not for the faint hearted if you fancy stripping one down.

Good luck

Graham
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Old 25th Sep 2018, 6:36 pm   #8
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Default Re: A Roberts R24...

Tuning-wiggling seems to be having some improvement, though I haven't hit it with the dry Nitrogen yet!
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Old 26th Sep 2018, 8:03 pm   #9
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Default Re: A Roberts R24...

Working on the 'one-change-at-a-time' principle I've replaced the "Tracon"-branded 1000uF main smoothing-cap in the power-supply with a 4700uF 25VW low-ESR 105C Rubycon (which occupies about the same volume; isn't technological progress wonderful?) - this has totally eliminated the intrusive background hum: I can now turn the volume up to full and there's only just a faint 'breathing' background noise in the absence of a signal.

I've also done away with the silly 'turntable' thing on the underside, and fitted a quartet of proper rubber feet (repurposed from a late-1990s 'portable' SCSI hard-drive enclosure) so it now sits stably and squarely. Turntables might have made sense in the days when MW/LW reception with ferrite-rods was a thing, but when you're only interested in FM reception they make accurate tuning a real pain.
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Old 26th Sep 2018, 11:20 pm   #10
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Default Re: A Roberts R24...

Rather than switch cleaner use IPA
Some caps can cope with it others can't !
Spay some in via the trimmer caps ,slosh it around but don't move the shaft until it has all evaporated .warm air ( not to hot ) from a hairdryer will speed things up
Sometimes caps get damaged because the IPA causes the insulation to grip on the metal vanes which causes the damage
If it's duff anyway it's worth a try
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Old 27th Sep 2018, 12:33 pm   #11
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Default Re: A Roberts R24...

Bit of a cheek I know but I have a baseless Roberts that's would really like a turntable. Please PM if the R24's turntable is completely homeless.

Thanks

Alan
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Old 28th Sep 2018, 7:50 pm   #12
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Default Re: A Roberts R24...

Quote:
Originally Posted by ajgriff View Post
Bit of a cheek I know but I have a baseless Roberts that's would really like a turntable. Please PM if the R24's turntable is completely homeless.

Thanks

Alan
PMing you now....
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