Sobell 636. Limited Station Reception.

[oldnotbold]

Hello all.

Not just a newbie (joined today) but a complete vintage radio virgin. Having just acquired my first one a Sobell 636 with a 4 position wave change switch indicating FM, MW, LW and Gram. However it will only pick up one FM station on MW setting, a few MW stations on LW setting, and the Gram setting performance is unknown.

It does this with only the ferrite rod aerial in situ. There is evidence of much underside work carried out in the past. Seeking wise counsel; is it a switch issue, switch wafer wiring, alignment problem or.....?

Many thanks, and a Happy New Year to all my readers.

[Station X]

Hello and welcome to the forums.

The fact that you receive stations on the wrong waveband could point to a problem with the wave change switch.

I would suggest that as a first step you clean its contacts with Servisol. Ther are plenty of posts describing how to do this.

[oldnotbold]

Graham!
Thank you for the welcome. A senior moment made me forget to mention that I had liberally (though maybe not accurately) dosed the switch wafers with Iso Alc PLUS that ludicrously expensive American De-Oxit stuff, but I will try to do it again using brain rather than brawn.
Regards...Mark

[Herald1360]

Check the wavechange switch knob is correctly aligned with the switch!

See if a couple of feet of wire for an FM aerial improves things- it should get you at least your local 1 2 3 4 Classic FM stations.

[mole42uk]

Liberal use of any switch cleaning chemicals is not really a good idea. It's best to use a few cotton buds to dry off any surplus on your wavechange switch.
It's usually best to spray a little onto a cotton bud and use that to wipe the contacts. De-Oxit is pretty good.

[Station X]

This is another one of those receivers where the HT supply to the FM tuner is cut off when FM is not selected, leading to cathode poisoning of the valve in the tuner.

Changing V1 might help.

[oldnotbold]

Grateful for the interest and delighted with the response.
When re-cleaning the band switch contacts, noticed that one of the greasy cap bodies was fouling the actuating shaft, so removed it prior to replacement. Checked for power-up, but now nothing. Is this normal?
The cap was 50pF and went from one contact to another on the wafer.
Time to open a bottle of Festive cheer, I think....

[ms660]

So far as I can make out from the schematic I have, C22 is the only 50pf capacitor connected between two contacts on the wave change switch, it's part of the AM local oscillator, if wrongly connected or damaged then the local oscillator might not function as it should on AM, that would only affect the functioning MW and LW, it shouldn't affect the functioning of FM.

Lawrence.

[oldnotbold]

Thanks Lawrence & co -
Well, you are quite right about the cap rating - I replaced it (with difficulty) and turned the machine on (with a Variac) in the upside-down position to check for faults. Fabulous!! All three wavelengths working!
Left it on, went to find my wife for some much-needed praise and returned to find a lump of melted transformer gunge all over the table.
When I switch on now there is absolutely nothing except a very hot pilot light wire and a that-doesn't-smell-right odour.
So, I am guessing that turning the radio upside down was not a good idea, the transformer is now u/s, and my first attempt at a repair has not been a resounding success.
I cannot find any specification for the transformer other than 'T2' and so the machine is 'On the back burner' at the moment, and I am a chastened soul.

Mark

[AC/HL]

Hi Mark,
The hot wire to the pilot light suggests a short circuit at or near the light itself. There's a chance that the transformer survived this, and is just understandably overloaded.
Circuit details here if you haven't got them already: http://www.service-data.com/section.php/7538/1/636wf

[oldnotbold]

Bill!
GENIUS!! You were absolutely correct; my fumbling must have pushed the pilot light cage against the background dial plate, creating a short.
All up and running again, waiting to be re-fitted.
Very many thanks..........Mark

[Station X]

I would suggest changing C65 (R&TVS sheet), the audio coupling capacitor. This may save you a lot of grief later.

[G8HQP Dave]

Never leave a vintage wireless switched on and unattended until you are sure it is in full working order.

Make and use a lamp limiter.

Always carefully carry out a visual check before switching on. Listen carefully after switching on, as old circuits often announce problems by making strange noises. Use your nose too!

[Sideband]

Originally Posted by G8HQP Dave

Never leave a vintage wireless switched on and unattended until you are sure it is in full working order.

....and even then think twice about leaving it unattended for any length of time. I was caught out by a hitherto reliable little set that just got switched on and used more-or-less daily. It was running as normal and I got called away and left it for about half an hour.....

I came back to the smell of well-roasted mains transformer and medium-rare smoothing caps that hadn't actually exploded (I'd have heard that) but had exuded most of their innards over the bottom of the cabinet and table-top (through the bottom vents). To say I was a little upset was an understatement....

All ended well though as I was able to source a good transformer and so began its repair. The cause of the problem was the rectifier valve that had decided to go heater-cathode short. Whilst doing the repair I noticed that the old valve looked a little black and when removed, bits of the cathode and heater were rattling about inside and you could actually see where the cathode was missing. It must have been quite a spectacular firework display inside but I wasn't there to see it....!

Anyway after a few hours work and a major clean-up of the chassis and cabinet, the old girl was running again and continues to do so even now (some 10 years later).

[oldnotbold]

Graham -
Done. Located and replaced cap - relatively easy, compared with some that I have re-soldered.
G8HQP Dave -
Mea Culpa; shan't make that mistake again. You frightened me to death. Weren't we in the same Staff Room together?
Sideband -
Grateful for your input. Will now gently agitate all valves to ensure no hibernating gremlins inside, and put a cover over the top of the cabinet.

Many thanks.............Mark

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