Murphy A4 Shunt Condenser

[Tedzed1]

Hello Everyone, first post on the forum, so please be gentle with me

I am in the process of restoring a 1933 Murphy A4 radio, now this radio is a bit of a basket case, and my knowledge is very limited, It has taken me ages to understand how the HT smoothing circuit works, (think I have got it now), but the question is it HT shunt, It states on the trader service 544 which I am working from, it is rated 0.1mf, in the service supplement that I have, it is described as a hum absorption trap with choke, rated at 0.1 for the 50 cycle model and 0.4uf for the 25 cycle model,

my question is, can this be replaced by a normal 0.1 cap, or is there likely to be other components in the block. should it be replaced or should the radio be tested with this original block in circuit,

all suggestions welcome.

[tiv48]

hi paul nice set ,c31 is just a cap but why do you think its faulty ,dont forget the speaker field coil is also part of the smothing circuit
rob t

[Mark R]

On the trader sheet it is C17, it can be replaced by a 0.1uf 450v working cap it is a non electrolytic. I would be inclined to test it for leakage first, but I suspect it will be having full mains.
Looking at the component layout it does not appear to be in a block, it is that big job above the block.

cheers Mark

[Tedzed1]

Hello Mark, yes, that big block is the one, have took more measurements, all the electolytics read low, most of the standard caps are reading high, and some of the resistors are more than 20% out, don't think this has seen mains for many a year.
I think this is a major job, the large component assembly in the middle will need to be removed for further checks, many pictures will need to be taken for reference, as quite a few wires need to be disconnected, will be taking my time on this one.

This is going to be a steep learning curve, I will do some more checks, and order some components, and take it from there.

I think brain surgery would be easier than this, enclosed are some pictures, so you can see the sort of job I have taken on

[Mark R]

Paul
how are you testing the caps? on an esr meter? If you have a variac the mains smoothers may reform on the variac, change C12,13 and 17, and any out of tolerance resistors you see fit, after all it is your set to restore, then plug it in and see what happens on the variac.

cheers Mark

[Mark R]

And C10.

[Mark R]

Ah Rob we are working from the trader sheet, C31 on your sheet......I think

cheers Mark

[Tedzed1]

Thanks Everyone.

Yes, as it states in the supplement, 0.1uf for 50 cycles, and 0.4uf for 25 cycles, so I may try different ratings when up and running,
alas I don't own a variac yet, I think one is going to be needed sooner than later.
will keep everyone informed how this restoration is going, no doubt I will be asking more questions later, so until some parts arrive,
will start on a clean up and take lots of pictures for reference.

[tiv48]

do they go to this trouble because of the low value of the smothing caps? iknow years ago some one sugested puting 0.1mf across the electolytics in a quad 303 power amp. it sounded different but i cant explain why mind you no chokes involved
rob t

[AlanBeckett]

For Murphy sets there's always the Diagram Dungeon.
http://www.murphy-radio.co.uk/dungeon/index.html
Alan