Sobell 636WF no FM

[PaulR]

I am in the process of restoring the above set.

So far I have replaced the audio coupling cap, the smoothing electrolytics and the discriminator electrolytic (the right way round!). The Hunts caps actually look to be in good condition with no splits but I will be replacing them in due course.

It works well on AM but there is nothing on FM. The anode voltages are all reasonably within those shown on the sheet (the Trader sheet for the similar 626A is actually easier to follow, but I have both) except for the EABC80 anode pin9 which is low. If I inject an AM signal of a few mV into the aerial socket I can hear the tone and this tunes in at about the right place on the dial. V1 is an ECF80 and I don't have a replacement for this to try, but I notice that the HT to this is switched off when the set is switched to AM which makes me wonder about cathode poisoning.

Is a new ECF80 the next thing to try or can anyone suggest anything else?

Thank you

Paul

[Nuvistor]

Have you tested the IF and detector stages with a 10.7Mhz signal?

Frank

[PaulR]

I haven't tried that yet. Would the fact that an AM signal at VHF frequencies makes it through at about the right place on the dial indicate that those stages are working?

Thanks

Paul

[Nuvistor]

I have not studied the circuit closely but the IF secondaries appear to be switched between the two frequencies. It's also possible that a fault in a 10.7Mhz IF TX would not affect MW AM. Test the IF from the VHF Mixer Grid obtaining an audio signal out of the detector. Starting with the grid of the last IF amp and moving towards the front end.

The ECF80 could well be faulty but there are many other causes, may as well test the IF/det section.
It is possible that an VHF AM signal could get through the VHF section, that would tend to suggest the VHF osc is working.

Frank

[PaulR]

Thanks Frank. I will have a look at that tomorrow.

Paul

[PaulR]

I have put a 10.7 signal through the set from the valve control grids and get a voltage on the discriminator capacitor. If I switch on the AM modulation I get the sound out of the speaker. I am not sure whether that indicates a fault or not!

I have sent off for a replacement ECF80 to eliminate that possibility and will post the results.

Thanks

Pau

[Nuvistor]

The IF/det seem ok, put enough AM through and you will get some output, the ratio detector and last IF limiter remove much but if there is a lot some will get through.
See what the ECF80 does for it.

Frank

[PaulR]

The ECF80 arrived today and has improved things slightly. I can now hear strong FM stations very faintly but it is still very deaf. Might the best way forward to be making a start on replacing those Hunts caps?

Thanks

Paul

[Nuvistor]

Hope replacing the capacitors improves the performance.

[PaulR]

I will make a start from the front end and see what happens

[Boater Sam]

Check trimmer C10 (trader) for shorts, its happened before.

[PaulR]

Ok Sam I will.


Thanks

[PaulR]

C10 isn't exactly shorted but it does have a measurable resistance of around 1M. Moving it very slightly has brought in some other stations very weakly. They are quite distorted with a background tunable hum if that helps the diagnosis.

[Boater Sam]

I think that C10 is part of the problem then. It is part of the circuit to prevent RF radiation from the aerial.
Incidentally, Radiomuseum shows 636WF with either ECF80 or ECF82. Whether it makes any difference I don't know.

[PaulR]

I have sent off for a new trimmer cap and will report back when I have fitted it.

Thank you

Paul

[Boater Sam]

Paul, did you disconnect one end of C10 before you measured it? I ask because R4 & R5 are 470K each in parallel which gives you about 1M.
I may have led you on a red herring search, sorry.
Sam.

[1955APREN]

Sam Did you mean IN Series Not parallel regarding R4/R5
regards Derrick

[Boater Sam]

Sorry, not too clear, the resistors are in series with each other, but in parallel with the trimmer. So the trimmer is probably fine, the meter would read the sum of the resistances across the trimmer.

[PaulR]

That's fine. I have sent for replacements but £2 won't exactly break the bank! Any help and suggestions are always welcome.

[stitch1]

Originally Posted by PaulR

... Might the best way forward to be making a start on replacing those Hunts caps?

I don't know this model but I have a Rolland 445 which has a Sobell 515 chassis similar to this but no FM ... it was also full of brown Hunts and as with other radios of this era benefited from replacing them all. Even if not physically broken I'd replace them all (one by one) before chasing any other faults.

John