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Old 16th May 2022, 1:47 pm   #1
Avid_Nerdlinger
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Default Putting RMB on a pedestal

I have been slowly attending to the restoration of a Roberts Mains Battery which I got off eBay for £41. So far so good. I added four 1n4007s to MR2 to boost the LT up to 1.4V, stitched the leather back on to the handle, new flex and repaired an OC pot (see before pics) with graphite varnish. After reassembly and testing just now I found that the reception was still too quiet and there was maybe a bit too much hum so there's more work to do but I was surprised to find a glaring design flaw/feature that the radio sits on the mains cable which comes out of a hole in the bottom of the rear door, either requiring a very very thin flat mains cable or maybe a circular mat or pedestal.

I usually just try to make old things useful again but I'm not sure this model was ever really successfully used on mains. The best I could do off the top of my head would be to change the flex to a flat one and sit it on a couple of round souvenir Jack & Victor coasters but I'd be interested to know how other people approached allowing the turntable to rotate or at least remain level while on mains. It would be a shame to have to modify the original design by cutting into the case.
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Old 16th May 2022, 2:11 pm   #2
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Default Re: Putting RMB on a pedestal

I think that the original mains flex was flat/oval, double-insulated PVC, probably 2 x 0.5mm sq. or 2 x 0.75mm sq. (or the Imperial equivalent). Your orange replacement does look rather more substantial, but that could be the photo of course.

Did it rotate OK with the original?
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Old 16th May 2022, 2:33 pm   #3
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Default Re: Putting RMB on a pedestal

0.75mm seems remarkably thin, I'll look into getting something like that. My recycled orange flex is a bit more heavy duty.

I binned the original early on as there was only a few inches of it bodged on to a length of round 3-wire mains flex. The original was flat but a lot bigger than 2 x 0.75mm, might not have been THE original.
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Old 16th May 2022, 3:08 pm   #4
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Default Re: Putting RMB on a pedestal

I see now that the 0.5mm and 0.75mm refer to conductor diameter. So the 3A flex with 0.5mm wires plus insulation would probably just about fit, might not rotate much.
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Old 16th May 2022, 3:50 pm   #5
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Default Re: Putting RMB on a pedestal

Quote:
Originally Posted by Avid_Nerdlinger View Post
I see now that the 0.5mm and 0.75mm refer to conductor diameter. So the 3A flex with 0.5mm wires plus insulation would probably just about fit, might not rotate much.
Not conductor diameter, but conductor cross section area (which is why the units are mm sq).
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Old 16th May 2022, 4:21 pm   #6
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Default Re: Putting RMB on a pedestal

Some woven sleeving over the orange might be kinder on the eyes.

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Old 16th May 2022, 4:27 pm   #7
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Default Re: Putting RMB on a pedestal

A typical 2192Y flat twin 3 Amp mains cable would have cross-section dimensions close to 3.5mm by 5.5 mm with 0.5 mm sq conductors. A 0.75 mm sq conductor cable would be rated as 6 Amp carrying capacity and decidedly OTT in this application.
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Old 16th May 2022, 4:37 pm   #8
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Default Re: Putting RMB on a pedestal

I would have thought ghat on a set of that vintage, single insulated flat twin PVC (perhaps?) would have been original equipment. Only rated as bell wire nowadays......
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Old 16th May 2022, 4:55 pm   #9
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Default Re: Putting RMB on a pedestal

Fairly sure these had double insulated pvc flex, like you’d find on a modern double insulated tv, desk lamp or whatever, but with the old colours ie red and black. No cheap bell-wire stuff on these expensive Roberts!
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Old 16th May 2022, 4:58 pm   #10
Avid_Nerdlinger
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Default Re: Putting RMB on a pedestal

Quote:
Originally Posted by Radio Wrangler View Post
Some woven sleeving over the orange might be kinder on the eyes.

David
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Quote:
Originally Posted by 60136 Alcazar View Post
A typical 2192Y flat twin 3 Amp mains cable would have cross-section dimensions close to 3.5mm by 5.5 mm with 0.5 mm sq conductors. A 0.75 mm sq conductor cable would be rated as 6 Amp carrying capacity and decidedly OTT in this application.

3.5mm thick flex would also tilt the radio a bit and stop the turntable. I'll probably have to get my chisels out and feed the flex out the back of the door.

Quote:
Originally Posted by Herald1360 View Post
I would have thought ghat on a set of that vintage, single insulated flat twin PVC (perhaps?) would have been original equipment. Only rated as bell wire nowadays......
makes sense, I read somewhere they originally plugged into light sockets.
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Old 16th May 2022, 5:03 pm   #11
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Default Re: Putting RMB on a pedestal

Quote:
Originally Posted by Nickthedentist View Post
Fairly sure these had double insulated pvc flex, like you’d find on a modern double insulated tv, desk lamp or whatever, but with the old colours ie red and black. No cheap bell-wire stuff on these expensive Roberts!
The remnant of the "original" was double insulated.
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Old 16th May 2022, 5:22 pm   #12
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Default Re: Putting RMB on a pedestal

There is enough room on mine for a 6mm round 3 core mains lead to pass underneath. There is 1/4 inch clearance between the radio base and ridge on the turn table. It does 'stall' the movement very slightly, but only because the cable does not lie flat. I wonder if your turn table is damaged?

Lighting shops carry huge ranges of 0.5mm flex in various colours. Ask about the cable diameter though, there is no common standard.

Or you could re-purpose a flat 2 core computer cable?
Or maybe remove the turn table and place a thin gasket between it and the radio base to increase the 'lift'?

Obviously the cable you have now is just too thick especially as the bend round the exit hole actually increases the thickness.
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Old 16th May 2022, 5:34 pm   #13
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Default Re: Putting RMB on a pedestal

My turntable rotates well enough, it did need cleaning out with Servisol to get it moving to begin with. Mine does not protude from the base as much as yours though, I get a couple of mm clearance. Yours might have been modified with a spacer added under the turntable... which might be a good solution...
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Old 16th May 2022, 6:09 pm   #14
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Default Re: Putting RMB on a pedestal

Personally, I hate the Roberts turntables - they make tuning the radio or adjusting the volume into a two-handed task - which is annoying to those of us who spend most of of our radio-time tuning up and down the wavebands seeking to hear [rather than listening-to] stations.

I'd suggest perhaps attaching a quartet of the little self-adhesive rubber nubs available from the likes of Screwfix and intended for fitting to kitchen-cabinet doors.

https://www.screwfix.com/p/door-cush...-49-pack/18213

to raise the radio sufficiently high so there's room for the flex to emerge and they will also lift things to the point where the turntable is deactivated and you can cruise the wavebands with just one hand...
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Old 16th May 2022, 6:17 pm   #15
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Default Re: Putting RMB on a pedestal

Here's a crop of a pic I took of one of my RMBs during restoration, showing the original black/red, oval-sheathed PVC mains flex.

I replaced it with modern 3 x 0.5mm^2 "2183Y" flex so that I could have a mains earth connection. Like this, apparently 5.6mm in diameter: https://cpc.farnell.com/pro-elec/pel...isexcsku=false

Note that thin mains flexes like this need a sensible fuse to protect them, typically 3A (though I sometimes fit 2A or 1A).
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Old 16th May 2022, 8:18 pm   #16
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Default Re: Putting RMB on a pedestal

Even Screwfix stock 2183Y, a £12.70 reel should do about 25 radios

https://www.screwfix.com/p/time-2183...50m-drum/507pf
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Old 18th May 2022, 12:54 pm   #17
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Default Re: Putting RMB on a pedestal

A short between screen and ground on V1 turned out to be the valve itself which has no markings and so is probably a DK91, a bit suspect but there are no other signs of bodgery and the seller said it hadn't been switched on since the 60s. While waiting for delivery of a DK92 and talking of bodgery I used a beermat as a spacer between turntable and case and replaced the flex with a flat one from a cassette lead. The turntable is not very thick so this made it protrude near enough all the way out but I am satisfied with this solution.
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Old 18th May 2022, 2:08 pm   #18
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Default Re: Putting RMB on a pedestal

Personally I always use an earthed mains lead now, especially when going to the hassle of changing it anyway. There are probably some chassis where this should not be done, so it cannot be done without checking the chassis and schematic, but for the sake of an M4 bolt, a crimped or soldered cable shoe, and some toothed washers, it is a lot of security for minimal effort.

On the subject of replacing cables, has anyone used this cable fixing connector? It just requires a 10mm hole, and cables up to 6.8mm are fixed securely, without those horrible knots.
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Old 18th May 2022, 2:23 pm   #19
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Default Re: Putting RMB on a pedestal

Quote:
Originally Posted by Avid_Nerdlinger View Post
...replaced the flex with a flat one from a cassette lead.
That's more like it
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Old 18th May 2022, 6:50 pm   #20
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Default Re: Putting RMB on a pedestal

Both of these Roberts RMB have their original plastic mains leads. They are now the best part of 70 years old and as can be seen show no signs of abrasion at the point of exit from the cabinet.
As to the earthing situation, adding an earth wire to the chassis appears to increase the general background hash, it certainly does at my location but may not in your locality, just a point worth checking if you have earthed the chassis and then discover a rise in general noise.

I have a number of RMB portables and I consider them to be First class and well built receivers, the peak of Roberts quality and that also stands for the brilliant M/B R55.
Of course they were quite expensive at the time but the good quality power supply was well worth the extra. John.
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