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Old 23rd Jul 2020, 8:02 pm   #1
Gabe001
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Default Sobell 516U

Hello, I have just acquired a lovely Sobell 516 radio and spent a good part of the day working on the case today which is now ready.

It's the 516U version ie the AC/DC version, with a live chassis.

Could I ask if anyone has a good service sheet which shows an under chassis view? I've bought the one from this site and got one from elsewhere, and they all seem truncated around the middle of the third page.

Also, can I confirm whether c41 across the rectifier valve needs to be a type X?

Is there anything specific about this set I need to be aware of, apart from the usual safety considerations?

Thanks
Gabriel
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Old 23rd Jul 2020, 9:06 pm   #2
cathoderay57
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Default Re: Sobell 516U

Hi Gabe, Re: Class X capacitor, I don't have visibility of the circuit so flying blind with this one. Generally Class X is required only if connected directly across mains line-in live and neutral. If C41 is connected between rectifier anode and chassis then, even if there is a dropper resistor between mains live and the anode, use of a Class X is probably good practice. If connected between anode and cathode, then not essential but using a Class X would do no harm. Since this is a live chassis model, watch out for asbestos washers under the mountings for any large chassis-mounted porcelain dropper resistors, and any asbestos heat shield protecting the cabinet. If found, and appear to be intact, don't disturb them and don't hoover up any nearby dust into a domestic vacuum cleaner. There is advice elsewhere on the Forum about safe ways of handling asbestos. Check the valves using an Ohm meter between heater and cathode before powering up because any heater-cathode shorts can result in rupturing of other valve heaters in the series chain. The Ohm meter will not give 100% certainty because h/c leakage sometimes doesn't happen until the valve warms up but it is an easy check to do. Finally, double check the wiring of the mains on-off switch. Sometimes only the neutral line is switched (single pole) and that can lead to the chassis being at mains potential when the set is switched off. Cheers, Jerry
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Old 23rd Jul 2020, 9:15 pm   #3
kestrelmusic
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Default Re: Sobell 516U

This is the underchassis layout diagram for the AC version, but I think the U version should be the same except for the power supply section. Anyway, here it is, for what it's worth.

I'd be interested to hear how you get on with this. I've got one in the "to do" section in the loft!...
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Old 23rd Jul 2020, 9:29 pm   #4
Gabe001
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Default Re: Sobell 516U

Thanks Jerry

Will check the valves as you described. I'm pretty sure the switch is wired as a single pole but looks like it is switched on the "live" side. I don't think it's the original, set looks like it had some work done in the 60s-70s

I didn't notice any asbestos heat shields and the chassis is clean enough, so no hoovering needed.

Gabriel
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Old 23rd Jul 2020, 9:34 pm   #5
Gabe001
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Default Re: Sobell 516U

Quote:
Originally Posted by kestrelmusic View Post
This is the underchassis layout diagram for the AC version, but I think the U version should be the same except for the power supply section. Anyway, here it is, for what it's worth.

I'd be interested to hear how you get on with this. I've got one in the "to do" section in the loft!...
Superb, this is really helpful. Will save me so much time!
Will keep you updated. Need to order some caps so it'll probably be next weekend or so before it's completely ready
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Old 1st Aug 2020, 7:42 pm   #6
Gabe001
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Default Re: Sobell 516U

Dear Jerry/kestrel,

Almost completed the restoration of this Sobell today. The case had quite significant water and sun damage to the veneer, so I had to sand it very gently and gave it a very light stain to compensate for the sun bleaching, and bring it back to the original colour. The knocks to the cabinet were quite superficial and were treated with wax sticks. I polished everything off with Danish oil and once it dries will top this up with beeswax.

The plastic dial cover is very yellowed and had come detached. In the picture I've cut a piece of perspex and put it in its place, but I think I've got 1.5mm acrylic in the garage and I will probably use this, if I can cut it without chipping it. I prefer acrylic as it is rigid and completely clear as opposed to perspex.

Electronically it was quite sound. 2 wires had come off their soldering tabs, and there were quite a number of wax and electrolytic capacitors to replace. I checked the valves as Jerry described and replaced the 2 dial lamps. The main smoothing caps would probably have reformed, but I ended up replacing them with new ones anyway.

Tested medium wave reception so far and playing via pickup and so far so good. I'm calling it a day for today.

Some pictures below. Thank you to everyone who gave me advice.

Gabriel
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Old 1st Aug 2020, 10:33 pm   #7
cathoderay57
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Default Re: Sobell 516U

Hi Gabriel, nice work. I was a bit concerned by your comment about playing via pickup since this is a live chassis radio. How have you connected the earth side of the pickup? Cheers, Jerry
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Old 2nd Aug 2020, 6:23 am   #8
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Default Re: Sobell 516U

Hi Jerry,
Socket to chassis via a 630v 47nf resistor. The pick up socket came with the radio, it's not my addition, although looking at picture 3 I'm now thinking that it may have been added later at some point. I did check it before I plugged my phone into it.

Maybe internal Bluetooth module may be safer? I'll do some research.

Gabriel

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Old 3rd Aug 2020, 1:49 pm   #9
Gabe001
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Default Re: Sobell 516U

Jerry,

I've read a few posts about this and I get why you raised concern. I'll replace the capacitor between the earth side on the pickup and chassis with a class y 10nf cap, which should improve safety somewhat. The original was 20uf which apparently may leak enough mA to make things unpleasant.

I'm unclear about whether the one on the signal end should also be replaced too with a class Y. Most references to this in other threads refer specifically to the connection on the earth side, although some people seem to advocate replacing both. Looking at the circuit diagram, AC from mains, if wired in reverse, would need to flow through the resistor at volume control before hitting the capacitor at the signal end. Will the class y capacitor with a rating of 275v (AC) here be safer than a 630v (DC) one? I've attached a picture of the relevant circuit (from the AC model, unfortunately there's no diagram for the U version except for the 'power' end). What do you think?

Interestingly the capacitor on the earth side of the pickup doesn't appear on the circuit diagram, but there was one.

I understand the above is the bare minimum for safety and that's it's probably best not using the pickup in a live chassis radio without isolation transformers and other electronic wizardry that's beyond me at my current level.

A small Bluetooth module internally is probably within my level of competence, but if I do that I'd be worried about falling foul of the law of I do decide to sell it at some point in the future, due to this being a "modification". I know this is a controversial issue which is beyond the scope of this discussion.

Meanwhile I'll enjoy its curvy aesthetic and listen to radio 4.

Gabriel

PS: just looked up some pictures of the 516u on radio museum, and it seems the pickup sockets were a later addition on my set.
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Old 3rd Aug 2020, 6:32 pm   #10
cathoderay57
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Default Re: Sobell 516U

Hi Gabriel, to avoid confusion over Class Y capacitors, it's worth a recap . Class X and Class Y capacitors are safety-certified and designed for ac mains supply-line filtering. Class X is for use between live and neutral, and Class Y between live and earth and between neutral and earth. The danger comes when these caps fail. If the Class X fails short circuit it will blow a fuse or trip a breaker so no harm to life or limb. However, if the Class Y fails short circuit it can put live onto chassis and be lethal. That's why the Class Y is designed to fail open circuit (in theory, anyway). The coupling capacitors to an AC/DC radio signal input probably fall outside the legal requirements (although I stand corrected if someone knows better, and you could also argue that if the pickup deck is earthed, then you would in fact be connecting the cap between neutral and earth). So you should be tolerably safe using a 630v poly film capacitor in the earth line from the pickup although a Class Y would work just as well. If you literally mean to use a record player pickup then you would need to use one with a crystal cartridge since a moving magnet type will not produce enough output for sensible listening levels.
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Old 4th Aug 2020, 8:11 am   #11
Gabe001
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Default Re: Sobell 516U

Thanks Jerry. I'll use a class y, just in case. The 45nf I have in place is too high anyway for safety so it needs replacing anyway, will replace with 10nf
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Old 5th Aug 2020, 12:24 pm   #12
Gabe001
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Default Re: Sobell 516U

Am I right in assuming that the safety element provided by a capacitor on an AC circuit comes from capacitor reactance?

For a 10nf capacitor this would be the equivalent of a 318,000 ohm resistor in series ie maximum allowable current of 0.75mA?

Does this make sense?

I use term 'safety' loosely, I know capacitors can leak and I'm not going to do anything stupid

Gabriel
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Old 5th Aug 2020, 5:18 pm   #13
cathoderay57
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Default Re: Sobell 516U

At an AC frequency of 50Hz that would be correct. Cheers, J
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Old 5th Aug 2020, 6:29 pm   #14
Gabe001
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Default Re: Sobell 516U

Thanks Jerry
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Old 5th Aug 2020, 9:06 pm   #15
Ed_Dinning
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Default Re: Sobell 516U

Hi Gabe, the reactance of the capacitor is calculated so it will not trip the RCD (30mA), sadly that still happens these days due to the multiplicity of Class Y devices in a normal house

Ed
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Old 6th Aug 2020, 3:31 pm   #16
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Default Re: Sobell 516U

Thanks Ed.
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