Sobell 511-P radio restoration

[Charles Wallis]

A neighbour gave me this early 50's 3 band superhet AC mains type radio. He said it had last worked in the sixties and had been kept in a loft ever since. It was it was in a filthy state, covered with dust; see Pic 1. Interestingly the set had no sign of tampering with all original components, including many waxy capacitors.

Out with the DMM and the mains leads were open circuit, though it was quickly obvious that the switch was open circuit whatever its position but the transformer primary seemed OK. I soldered a new mains lead to bypass the faulty switch. It had a half wave metal rectifier complete with cooling fins that was also open circuit (pic 2). I disconected the HT secondary from the rectifier for safety, and gingerly applied power via a bulb limiter to test the heater circuits. Somewhat to my surprise all 4 valves lit up. So something was working. The HT winding seemed to have AC on it. I then put in a 1N4007 diode across the rectifier, a 330 ohm dropper and a new electrolytic smoother to replace C24, the dud looking first HT smoother. Rather more nervously I applied power via the bulb limiter, which did not light up, just got a bit warm. Sure enough, the radio crackled into life with semi reasonable reception on medium wave. It was a goer!

I checked all the valve DC voltages and they were roughly where the data sheet said they should be. I went through the resistors with the DMM and most were Ok with a few exceptions. I set about cleaning the set up as it was so filthy and then replaced all those tubular waxies. There were many smaller flat wax capacitors which I gather tend to be OK so I left them alone.
The mains switch began to respond to masses of Servisol and lots of clicking and eventually went from always open circuit to switching correctly, I presume it had been jammed with gunk.

I made a massive mistake in that I thought C23, the second HT smoother was also a dud and disconnected it without much thought. When I turned on the radio it went motor boating and then a high pitched scream with no reception at all. What had I done! Looking up motor boating the main
cause seemed to be lack of decoupling, which is of course what I had inadvertently done. I soldered in new electrolytic and it was all cured. So the 1948 date stamped cap had been OK all along! I then put in a new tag board on which to properly mount the the power supply components.

I put in a new mains lead via the now working switch with proper earth to chassis and a 500mA line fuse for safety, mounted where the old smoothing cap had been, easily accessible from the back of the set.

The last job was to restring the tuning knob to the tuning capacitor and dial with some special chord from a sewing shop. This was very fiddly but worked eventually. The cabinet looked like Bakelite but was actually plastic, hence the 511-P code. I polished that up with car polish and some Mr Sheen and it came up nicely (pic 4).

The set now worked reasonably well on all bands, LW, SW and MW though the latter is plagued by interference between stong channels. I am not sure what that is. It does need a good external aerial to work though, as it has no internal ferrite rod.

[joebog1]

Nice looking restoration!!!!, especially the metal chassis and IFT's etc, almost looks brand new.

Joe

[joebog1]

Errhh hmm, I thought carefully about this and have decided to say it. I approve of the new mains cable but just knotting the cable is not very nice. The cable can still be rotated and that can rip wires off, or break tagstrips. A much nicer and neater method would be to use a P clip and screw it to the chassis so the cable cannot move.

I hope you are not offended with my comment.

best regards
Joe

[mark2collection]

Good to see another 511 revived, it came up really nice.

I did the wooden version a while back, can been seen here https://www.vintage-radio.net/forum/...d.php?t=152634 along with mains cable modifications.

Joe has a point regarding the mains lead, for a ground I used an eyelet/nut/both/washer combo taking up a spare hole in the chassis and a P-Clip/Grommet at the entry point.

They sound nice, mine has the 6V6 output valve, 8" driver & works great on all bands, especially good on SW. Had been 'got at' by the previous owner but now works a treat.

Hope yours bring many happy hours of enjoyment, can't be many of these around.

Mark

[slidertogrid]

What a nice looking set! Well done for saving it.

Rich

[Hybrid tellies]

Thats a nice looking little radio. Great write up and repair.

[Charles Wallis]

Originally Posted by joebog1

Errhh hmm, I thought carefully about this and have decided to say it. I approve of the new mains cable but just knotting the cable is not very nice. The cable can still be rotated and that can rip wires off, or break tagstrips. A much nicer and neater method would be to use a P clip and screw it to the chassis so the cable cannot move.

I hope you are not offended with my comment.

best regards
Joe

Not at all Joe! I did think about the mains cable issue but could not source a P clip so just knotted it, aware of its limitations. However if you look carefully at my picture you will see a black cable tie that I looped around it to stop the said rotation issue. Not perfect I know but will do for now!