Murphy A104

[60 oldjohn]

I have a Murphy A104 on the bench, Got it partially working but before I go any further I would like to change C64 the AF coupling capacitor. This Cap is 0.02uF 750V DC I only have caps rated 630V. I just wondered if these were over rated for reliability? or cheap war surplus? Also in photos is another rated at 1000V 0.01uF C29 this is not a Tone Correction cap. Any thoughts, replacement 630V caps ok? They are American made Sprague metal clad capacitors.

TIA John.

[marconi_pete]

Yes, looking at the circuit diagram for the set 630v caps should be fine in those positions. Not sure why the originals were rated so high. I imagine they're war surplus. A lovely set once restored.

Thanks,
Peter

Indeed a very good set, the projector dial is a sight to behold. I did a bit on these here https://www.vintage-radio.net/forum/...ad.php?t=62003 So good in fact that when "Glowing Anode" came round he said "is that off air?", it was.

[crackle]

Thats a lovely clean looking chassis.

Mike

[60 oldjohn]

Thank you for all your replies, I will bung in a 630v cap. Yes nice set, I have been reading up on the care of the optical system, the diffuser has a slight mark on it and it looks like the mirror will need replacing as there is a lot of the backing is missing. There is a lot of perished wire that will need replacing, lucky I have plenty of suitable silicon wire.


John

[Boater Sam]

I restored one of these this summer and used 650v caps throughout without any problems, I suspect that they had a lot of caps in stock and just used them up. The tone correction caps across the output transformer should be 1000v but I prefer to take them from anode to ground, then their life is less onerous, and 650v seem to be fine then.
The optical system is a fine piece of work when set up properly, for a 2w bulb its surprisingly bright. If the silvering on the mirror has gone, try a piece of aluminum foil wrapped round it, shiny side out.

[ms660]

Why would the capacitors life be less onerous?.....DC wise it would be subjected to a higher voltage.

If it failed short circuit across the transformer then no damage done to the transformer, if it failed in series with the transformer then it could be curtains for the transformer.

Lawrence.

[crackle]

I restored one of these a few years ago, and was never able to stop all the rattles from the chassis and other bits of metal work when the volume was turned up.
But the optical SW tuning was great.

Mike

[Radio Tech]

Don't forget though John, you can obtain those nice yellow polyester 630V caps from Jellyfish Audio, and they do some nice dual cap can electrolytic ones. Nice set though you have.

Ken

[60 oldjohn]

Thanks Ken, I am well stocked up with 630v caps and all waxes changed. I am in the process of replacing wiring and the three 450v electrolytics two which connect onto the choke L19 which has a third "spare" terminal which is an anchor for a resistor. Anyone know what value this resistor should be? I presume this 2 watt resistor should be high voltage.


John.

[ms660]

A104 schematic shows 10k.

That's the HT feed resistor for V3 anode supply (R13)

It won't be dropping a lot of voltage.

Lawrence.

[60 oldjohn]

Thank you Lawrence, R13 is shown mounted on the upper tag strip, I presume mounting it on the choke was a production modification, just to confuse me. I will check resistor value when I renew the wire and other 8uf & 16uf 450v caps.
Anyone know of any ill effect by changing the two 50uf 12v caps for two 100uf actually measure 98uf, they are C33 C36 ? set works ok with them but will it stress other parts?


John.

[ms660]

Ok, the confusion might be that the schematic in the A104 manual shows L19 to be the loudspeaker field coil and R13 is on a dedicated tag strip.

The A104R the r/gram version) shows L19 to be a stand alone choke (not a field coil) and that R13 is fitted between two tags on that choke.

I've never worked on either so far as I can remember, but from the looks of it the A104 went through a production change from field coil loudspeaker to permanent magnet loudspeaker, the manual gives the latter's model number as a A104PM and it also confirms the circuit changes, in that one R13 is shown as 2.7K, that resistor also feeds the screen grid, like it does in the A104R. So for the moment ignore the 10k value that I stated earlier for R13, it's most likely to be 2.7k, the screen connection arrangement will confirm.

Lawrence.

[60 oldjohn]

As one side of this resistor connects to L19 I would assume it could be R13 as it is missing from my upper tag strip. but shown on schematics. According to circuit diagram only other possibilities are R7 another 10k and R3, 53k.


John.

[60 oldjohn]

I have removed the resistor attached to the Field coil L19 1800r, the resistor measures 1.75k and L19 is open circuit. The set work with the resistor in place of the coil without any humming, is it ok to use like this?


John.

[ms660]

I thought your receiver was a permanent magnet job?

Lawrence.

[60 oldjohn]

Hi Lawrence, Yes it is PM speaker, the confusion starts at #13 I would call L19 1800 ohms on the schematic above a choke. Would a resistor in place of the choke be harmful to the set ?


John.

[ms660]

I suspect the choke in your receiver is the same as the one used in the radiogram version, that one has a resistance of 265 ohms according to the A104R manual, I have never replaced an HT choke with a resistor but plenty on here have, just remember that the higher the value of the resistor used in place of the choke the lower the HT will be, and the lower the value the greater the ripple voltage will be.

Looking at the A104 and the A104R manuals there's also a difference in the HT winding resistance between the two, the resistances given suggest that the HT voltage at the reservoir would be lower in your receiver than that of the receiver fitted with a field coil loudspeaker.

Lawrence.