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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 28th May 2022, 2:52 pm   #21
Avid_Nerdlinger
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Join Date: Apr 2019
Location: Durness, Sutherland, UK.
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Default Re: Putting RMB on a pedestal

So unlike both the Roberts and "Trader" schematics this RMB has a centre-tapped LT secondary winding on the mains transformer and a 2 diode rectifier. I had replaced the tired selenium rectifier with 4 1n4007s which seemed to work OK but after a hour or so the transformer was worryingly hot. Then I noticed the two wires soldered to the transformer case and realised my assumption that MR2 was 4 diodes with a body to case ground connection was incorrect. Replaced with just 2 diodes and a dropper.

On closer inspection found that the wax in 3 of the variable inductor/transformer cores had been tampered with so I followed the parts of the alignment procedure off the trader sheet applying to them. One of the IFTs had been tuned way out... retuning this and fitting a DK92 brought it up to decent performance.

It's a shame I had to replace MR2 with silicon but I left it in situ disconnected and I haven't had to replace any other components. It's remarkably well preserved. Sounds great.
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Old 28th May 2022, 3:27 pm   #22
Heatercathodeshort
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Location: Warnham, West Sussex. 10 miles south of DORKING.
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Default Re: Putting RMB on a pedestal

I have no doubt that you have checked this but check that the filament voltage does not exceed 1.4V on mains. The valves are delicate filament wise and can easily be over run with a silicon diode in place of the old selenium rectifier. I wonder how many of these were manufactured. Quite large numbers appear to have survived in excellent condition. John.
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