Radio Sobell 516

The circuit, should members wish to discuss, while waiting for the repair to start.

• V1 6K8G
• V2 6K7G
• V3 6Q7G
• V4 6V6G
• V5 5Z4G

Posted by: @sideband

 I'm contemplating contacting the seller to arrange collection. Guildford is only 40 miles from me and it might save a disaster...

A wise option, if I may say. 😉 

A few interesting items in the circuit,

Bandpass aerial tuned circuits for MW to aid selectivity, presume coupling must be inductive between L4 and L5, I can’t see any capacity coupling unless it’s stray.

Muting on weak MW/LW stations and reduce inter station noise, switched out on SW, a small bias on the detector diode to produce the muting.

Negative feedback on the audio stage, I suppose this was quite common by 1947.

Gram input, high impeadance type pickup required.

A good Sobell design. When they went over to PCB's and used double-sided print, they somewhat fell out of favour with the Trade (or was that during the GEC/Sobel merger). Printed circuits were bad enough when they first appeared but double-sided?

It will be the first Sobel in the collection. I expect it to perform rather well.

Posted by: @pye625

Posted by: @sideband

 I'm contemplating contacting the seller to arrange collection. Guildford is only 40 miles from me and it might save a disaster...

A wise option, if I may say. 😉 

Well I'll have to keep my fingers crossed. The seller had already dispatched it by the time I contacted them this morning. 😲 

It's arrived in one piece! No damage at all and I'm not surprised....it took me a good 15 minutes to get into the package! When I had finally extracted the radio I was pleased with the overall condition and whizzed it straight into the Mancave before the missus could see it (fortunately she was still at work....). I haven't even looked at it yet but looking through the vent holes in the back it all seems present and correct. The mains lead has been cut off and I suspect it hasn't been used for very many years. Certainly something to keep me occupied while waiting for the rewound line transformer for the Royal Star.

Only one (very poor) picture taken hastily on my phone camera as unpacked. It looks a lot better than in

the picture.

If only everyone took as much care over packing things up! I agree, I much prefer to collect delicate online purchases wherever possible, even if a fair bit of time and distance is involved. The worry with any vintage electronic device then being that an entirely well-meaning seller greets you with, "you know, this has been in the shed since Churchill died but I've had it plugged it in for a few hours so you can see it working...." 😱 

I had some playtime tonight and managed to have a good look at the Sobell. I'm very pleased with the overall condition. Very dusty, very original. The only evidence of any previous work is a Radiospares volume control/switch and probably a replacement 6K7 I.F valve as it has an Armed Forces replacement (but I didn't take note of the number). Everything else looks original with all the waxies still fitted. At the moment all I've done is remove the chassis from the cabinet and give it an initial vacuum out to remove the worst of the dust. I ran the valves through the Avo two-panel tester just to assess the general condition and all but the 6K8 give good readings. The 6K8 reads low on the triode section but it will probably work in practice. Before I removed the chassis, the only other check I made was to connect the testmeter across the primary of the output transformer. There was an encouraging crackle from the speaker and the resistance indicated about 300 ohms so a good chance that output transformer is OK.

Next time I work on the set, some basic checks around the mains transformer and then I'll start 'de-waxing' it!

First shows a much better picture of the radio, second with the back removed, third the dusty chassis removed, forth after an initial vacuum, fifth the large rather attractive tuning scale....it has a course/fine tuning drive, sixth the underside showing all original components (apart from the volume control).

Plenty of room underneath and looks very easy to work on.

That's a very nice looking and distinctive radio and looks to have lived its life in pretty benign conditions judging by the state of the case and chassis. I wouldn't be surprised if it springs to sprightly life after checking and dewaxing. A keeper!

Is that tubular 6K7 a Rogers (Canadian)? They seemed to have their act together as regards getting the (dark grey in their case) metallising to adhere reliably to the glass, far more so than some makers this side of the pond. Though it may have involved chemicals, e.g. HF, that would make a modern H + S person turn blue and fall over....

Hi TS. Yes it is a Rogers Canadian 6K7 (their equivalent of.....I'll get the number next time I work on the set.....it tests 100% anyway).

I liked the look of the set from the first time I saw one. Some Sobel sets don't appeal at all but this one is the exception. The front gold grille appears to be metal so I'll see if I can find a close-matching gold spray when it comes to doing the cabinet. I think this must have been quite an expensive set in it's day. I notice also that the mains transformer has the voltages and ratings marked on each winding. It has choke smoothing rather than resistive so again a touch of quality. I think mid 30's to late 40's were really the 'glory years' of domestic radio design. This one is 1947. As the 50's approached, many sets became more basic. I'm also expecting it to be very lively when revived.

It's a pity the Sobel emblem is a bit scuffed but hopefully I can reproduce a replacement....or just leave it as is as part of the history.

Is the chassis mounted on rubber grommets?

Looks very clean and lots of room under the chassis to make repairs easier.

Posted by: @nuvistor

Is the chassis mounted on rubber grommets?

Looks very clean and lots of room under the chassis to make repairs easier.

Yes there are two at the rear and two at the front. You can see them in the last picture showing the underside.

Lots of room under the chassis and it will be easy to work on.

As requested, the alignment procedure

Thanks Chris. Hopefully I won't need it but always useful to have.

I had some more time to assess the Sobell tonight. Just doing cold checks I have determined that:

The mains transformer is (most likely) good according to resistance checks.

The output transformer is good

Now to the things that do need changing:

All the waxies (of course),

C38,C39, C30, C33 are O/C so will have to be replaced

C36, C37 (smoothing) is swollen although it measures reasonably it will be changed,

R16 output valve cathode bias resistor (270 ohm) has gone high to 450 ohms and is charred. (I suspect a leaky coupling capacitor has caused it's demise).

Now there appears to have been a factory change around the tone control. It's shown in the negative feedback network (R19). However it is wired differently in my set. I discovered this when looking for the output valve grid leak which was absent! The tone control is now used as the grid leak replacing R14 (the track is now wired as R14). The capacitor is retained between slider and the earthy end of the control so now it is just acting as a passive tone control rather than an active one in the negative feedback circuit. R18 is not present (effectively linked out) and there is an extra 47k in series with the grid stopper R15 bringing the value up to 69K. Exactly why this was done would be known only to Sobell but it may have been due to some instability. I've heard that some 6Q7's can be rather lively so perhaps it was found that HF oscillation was taking place with some valves so the feedback circuit was taken out.  The increased value of the grid stopper may also bare out the instability theory.

Next job will be to start evicting the waxies. I may need to order some more 0.1uF capacitors.....

All the other resistors seem to be well within tolerance so really not too much work.

Watch this space....

This evening I evicted all the waxies, two cathode bypass electrolytics and a 270 ohm resistor. Replacements have been fitted (with the caveat that I may restuff the two old electrolytics).

Now I thought I had a double 16uF capacitor for the smoothing but now realise I used it in another restore. So I need to order a suitable 16 x 16uF cap for smoothing and an 8 x 8 for the HT decoupling. I could use separates under the chassis of course and leave the old ones in place but I'm running short of high-voltage electrolytics so I need to restock anyway.  I may cobble together some capacitors just to be able to give it a test.

 Waxies evicted (all checked as leaky anyway, in particular the audio coupling which would probably account for the frazzled cathode resistor on the output valve).

Looks like it will soon be ready to tune into the shipping forcast. 👍 

Well, unless there is some nasty hidden problem like green spot in an IF transformer, I have every confidence that it will burst into life and work well.

I am going to order a 16+16uF from Cricklewood Electronics and I'll use a couple of 10uF 400v for the 8 + 8. The 16+16 will fit directly in place of the original and use the same clamp. I'll disconnect the 8+8 and hide the two 10uF caps underneath.

In the meantime I'll cobble some caps together to make the values up and give the radio some juice to see what happens when I next get into the workshop

Well that wasn't too difficult! I found four 15uF 450V capacitors today that I rescued from a switch mode PSU a few years ago. They had very short leads as originally they were PCB mounted. However I was able to cobble these into the chassis in place of the original smoothing and decoupling caps and then fitted a mains lead. The moment of truth.....as it had good HT and decoupling caps, albeit temporary ones, I didn't see much point in using a variac to bring the mains up slowly so after some basic checks between HT and chassis, (no shorts), fitted all but the rectifier valve and switched on. Valves lit and both pilot bulbs glowed brightly. Perfect. Now fit the rectifier, clip the meter probes between HT and chassis, connect aerial and temporary bench speaker and switch on....keeping a hand on the on/off just in case....No problem, HT came up and settled at 280V and there were encouraging noises from the speaker. Tuned around and R4 came in clearly. Loads of noise on MW here so wasn't expecting floods of stations. LW was also working but nothing on SW. Volume and tone controls crackly. Switch off, give wavechange switch a squirt of cleaner and work it a few times. Same with volume and tone controls. Switch on again. SW now working and the Voice Of China (as usual) came crashing in after a quick tune around along with a Spanish station further along. 

So there we are. Time to order replacement smoothing and decoupling caps and generally check the set over. 

Oh yes since Turretslug asked, the replacement 6K7 is a VR56.