A dusty relic arrives....

Trader sheet 770 covers the Mullard MAS281, MAS292 and the Philips xxxx set. The data sheet is now in the library under the appropriate section and is titled "Mullard MAS292.pdf"

Happy memories there Andrew, or should I say it's Philips counterpart was as it's one of the first ever valve sets I repaired/restored many moons ago. Sadly I no longer have it but it was nice.

Good luck with it.

I won't give the game away re the Philips model number. 

Marc.

marc said

Happy memories there Andrew, or should I say it's Philips counterpart was as it's one of the first ever valve sets I repaired/restored many moons ago. Sadly I no longer have it but it was nice.

Good luck with it.

I won't give the game away re the Philips model number. 

Marc.  

Thanks Marc.  

I have just been looking at the Trader sheet, some interesting ideas. Comprehensive audio feedback circuit that only provides negative feedback at lower settings of the volume control, at it highest setting there is no neg feedback. I presume this was done that although the set has an RF stage it as no audio voltage amplifier on the radio.

Which brings the Gram circuit into the discussion, this uses the final IF amp as a audio voltage amplifier but with the screen grid, G2, acting as the anode with some interesting switching occurring.

The RF stage is resistor/capacitor coupled to the RF tuned circuits, perhaps switching was easier but I thought they would have used a multi section choke instead of a resistor but that could be down to cost.

The Trader sheet has an excellent circuit description.

I will get back to the circuit, perhaps nothing else interesting but one never knows.

edit, a couple of other items, the voltages were taken with a 20,000 ohms per volt meter, not many service shops would have had one of those, mainly 1000 opv as in the Avo 7.

C47 and C48 are unusual, they are adjusted by taking turns of wire off them, take too many off and you have to replace the capacitor.

The front end used two different types, one with an aluminium gang and one with a copper gang, different coils and alignment procedures due to the slightly different 'Law' used by the gangs.

Thats 3 not a couple of items

Thanks Frank, some excellent observations there  

This evening, I removed the chassis and almost filled a hoover bag with dust from the set. Under all the dust, it appears to be in a good rust-free condition. All nice and original except for a replacement RadioSpares electrolytic.

Now, as can happen, the previous owner had connected it to the mains and said it worked. Probably not very well I would dare say, but at least it proves there is life.

For a 70 year old radio it looks in excellent condition, noticed the Radiospares smoothing cap.

In this picture, you can see a chassis hole where I thought a capacitor would have been located. It is just behind the left control, on top of the chassis. But looking at the circuit, C31 and C32 electrolytics are where they should be.  

 No, it is clearly just a hole and nothing was ever fitted there.

The Philips manual for the Philips set uses what appears to be a slightly smaller chassis and indeed the reservoir capacitor is located where that hole is in your set. The space where your leaking reservoir cap is not there, the gang being up against the trimming capacitors.

Thanks Frank !!  

A mine of information there, young man  

Oh, and the stuff leaking out of the other cap was very tasty on a bit of toast....not !!

Unsurprisingly, the crusty cap has little uf's left and will be opened up and re-stuffed. The RS one seems ok and should re-form. It is a bolt-through type the same as the original, so at least no bodging there has been done in the past.

That ought to be a good performer, and MW/LW radios with an RF stage might become increasingly sought after as the stations get fewer and further away! Good old Philips, adding their own particular take on making radios. To be fair, they often made things that performed well, but leaving a few furrowed brows in their wake. I note their characteristic inductance trimming of the RF coils by running an suitable depth furrow around the screening can over the appropriate wound section.

nuvistor said

C47 and C48 are unusual, they are adjusted by taking turns of wire off them, take too many off and you have to replace the capacitor.

I was always fascinated by those and glad that I never needed to touch them!

It was technique revived in the ELC1043 UHF tuner - there is a little leg that sticks out of one corner of one of the bandpass leca lines that has a few turns of enameled copper wire wound round it with the captive end being soldered to the metal casing.

Although both lines otherwise look identical there must have been some slight difference because the leg only appeared on that one line which had obviously been carefully designed to ensure that only that one would need to be trimmed for them both to track correctly.

nuvistor said

C47 and C48 are unusual, they are adjusted by taking turns of wire off them, take too many off and you have to replace the capacitor.

Actually, if you are careful, the fine wire may be wrapped back again and sealed with a dab of super-glue..... I've been there before   

This evening, I removed the two electrolytic capacitors and begun to re-form the radiospares one. The other original type needed to be opened and re-stuffed. This was simple enough to do by sawing off the top, removing the innards and draining off the remaining electrolyte liquid from inside. The electrode assembly was drilled out and I fitted a bolt to hold the original terminal plus a new terminal inside for the new capacitor to be encased within. I used a 33uf 450v and sealed it up with melted wax. I then pushed the top cap back into place, trapping the ground connection from the new capacitor.                                                                                 

Here is the inner electrode as removed.

Here is the old inner electrode again, next to the base of the finished capacitor.

Top cap pushed back into place.

They didn't have super glue in 1946, anyway I don't think they did. You won't be able to tell the old cap has had anything done to it, it will look original.

Now don't rush with this one Andrew or you will have to buy another next week .

nuvistor said
They didn't have super glue in 1946, anyway I don't think they did. You won't be able to tell the old cap has had anything done to it, it will look original.

Now don't rush with this one Andrew or you will have to buy another next week .  

Hi Frank,

Lol, no I wouldn't think super-glue was available back then, but a dab of varnish or wax would do the trick.

I'll try not to get too carried away and spend only a little time when I can on the set. One thing I am going to do is to only trace and replace components if they give rise to a fault, rather than wholesale replacement of capacitors.

I would like not to have to replace the black tar capacitors as I can't imagine they can be re-stuffed in the same way as the more usual wax type.

The thing that really will take time is the cabinet. This is in a state and will need careful treatment. I won't be rushing this.

Hi Andrew,

Just a thought about replacements for those Philips tar caps, how about covering the normal hi-viz yellows with black shrink wrap ? Should look somewhere near at a guess, they certainly wouldn't stand out. 

Marc.

Hi Marc,

That's a good idea, I certainly don't fancy melting tar and dipping any replacement capacitors in it. Wax is one thing, but molten tar is not very nice stuff to be playing with.

One thing that is quite nice is that the vast majority of the chassis wiring has PVC covering as opposed to the often crumbly perished vulcanised-rubber variety. There are one or two wires that are rubber covered, but it is only the yellow colour that has hardened.

Or... black potting resin?

Cathovisor said
Or... black potting resin?  

An interesting idea... it could be poured into a small cardboard tube around the new capacitor, and then removed when set.

You can in fact get black capacitors....Ansar make them, but they are not cheap and are aimed at the "audio enthusiast". (I'm being polite here  )

I've taped the yellow perils and dipped them in underseal before, they look very authentic.

Yellow rubber cable does seem to degrade worse than any other colour.