Murphy A4

[vintage_8bit]

Hi again, still progressing.

Electrolytics now re-fitted and of the correct value.

Removed C17 in the metal box for re-stuffing. Next to this I found an extra .01uF cap. This has been fitted between the wiper of the gain control R7 (Chassis) and a 4th terminal on pot, which is a taping off the track. (See photo, pot released and turned) The value of R7 is correct (8K) I assume this is an unaficial mod. I intend to remove cap to keep to original circuit. Anyone know why this was done? The pot shaft fits the Murphy knobs and nut perfectly, but the rear casing is tight against L20 so R7 may be a replacement?

All circuit references from Trader 544.

[PJL]

I would check the value of the pot which should be 8Kohms. The gain control has two actions: shunt the aerial to reduce the input signal; decrease V2 cathode resistor to increase the anode current which will increase the gain as it is a vari-u valve.

V2 cathode is already decoupled by C7 so the tapped capacitor would have no effect on V2 gain, however, it would cause the aerial shunt impedance to stop changing at the high volume position and that sounds wrong.

Another possibility is the design has been substantially altered to accommodate this volume control arrangement that is generally used to change the frequency response as the volume is changed.

R7 should be an 8K wirewound pot as it is carrying current.

[vintage_8bit]

Good point, I will check for other mods in that area, bearing in mind the output stage had allready been modified, see earlier in post. R7 is the correct value though.

[vintage_8bit]

Work slowly continues.

I have just removed the 3 gang condenser to change the rubber mounting bushes.

My question today is, what's the wire for on C23 to an insulated mounting on C26? There are no other connections to this wire.
The connections to all 3 variable capacitors are made on the other side. The common chassis earth is made through the scale lamp bracket. See attached image.

Component references to Trader 544.

[G6Tanuki]

I don't know the circuit of the radio in question, but could that wire be a "gimmick" capacitor providing top-capacity coupling between two tuned-circuits?

Such fudges (often involving two short lengths of wire twisted together) were quite common!

[vintage_8bit]

Firstly, in a late reply to PJL post 22. The marked value of the pot was 7k but fortunately it actually measures nearer 9k, it's not wire-wound though. Anyway I've not got a replacement, specially with the Murphy threaded end so I will try it for now. I checked for any further modifications and couldn't find any.

G6TANUKI.
I had a second look at the circuit and there's no capacitor shown. I will leave it there for now and see what happens. The round insulated cylinder looks original.

[vintage_8bit]

The chassis has now everything back in place.

Although the wave change switch still needs attention, I have now carefully powered set. To my surprise it pulls in many stations on medium wave and with some volume. I have not yet checked alignment. I am really quite pleased with the set as this in my first venture into the 30's.

With the exception of the output valve the rest of the valves are original to the set when I got it.

[vintage_8bit]

I have now repaired the wave change wafer assembly, see pic using .3mm phosphor bronze as suggested by "Herald1360" it was the perfect material.

I am now looking at the spring on the wave change locating cam. Mine looks home made, see pic and hasn't got quite enough spring. Anyone got a picture of an original?. I was thinking of using stainless wire as used by the dental industry, as I can get hold of this in different gauges.

[Herald1360]

Complete with soldered contact, too! Nice job there.

[vintage_8bit]

Thank you. I was able to use the original contact and cut down the leaf. The contact I originally thought was too big, once cleaned of excess solder was correct. Colin