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Old 27th Jun 2004, 6:34 pm   #1
fableglade
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Default Murphy A24 (1934) AC Mains Radio

I recently purchased a Murphy A24 from Philip Knighton in Wellington. The set was in very good external condition and looked to have been pretty much left alone.

I removed the back and the nuts securing the knobs in to position, then the 3 chassis bolts and withdrew the chassis. The three electrolytic condensers looked to have been replaced in the 50's, otherwise it had not been touched.

After some preliminary checks on the various windings I replaced all of the condensers (except for the condenser block) and fitted a new mains lead and earthed the chassis. After further checkes for safety power was applied, the set worked reasonably well but I knew the condenser block would need replacing.

I spent a while labelling the de-soldering the leads to the block, 13 in all, and then examined the block carefully after unscrewing it from the chassis.

According to the Trader sheet the block contained resistors and condensers. I carefully prized the lugs open on top of the unit and, after some firm but careful tapping with a small hammer I was able to free the top layer of terminals and resistors, these were kept together but the leads connected to the condensers in the can were snipped off.

After reading in Gerry Wells autiobiography his method of geting the condensers out of the can I decided to try it for myself. I pre-heated the over to 400F (200C) and then put the can containing the condensers only in the over, the tin was upturned and stood on tin foil which, in turn, stood in an old baking tin. I left this in the over for about 20 minutes, by this time a small amount of smoke was coming out of the oven, so all windows were kept open.

The can was removed from the over and was carefully held with a thick cloth whilst a pair of pliers were used to pull on the condenser leads, they came out of the can easily but left the insulation in place around the edge of the can. The can was cleaned out with a blunt knife and left to try, the old condensers were safely disposed of.

I then went back to the terminal block with the resistors still in situ. Using the circuit diagram on the trader sheet I soldered new condensers to the correct tags on the underside of the terminal block, I remembered to take the condensers which went to chassis to terminal 'E' using a common earthing lead, however at least 2 of the condensers do not go to chassis in this block, so care needs to be taken.

Once this was completed and the can was cool the terminals, together with their insulation and spacers were offered back in to the can, they dropped in perfectly and the metal lugs were pushed back in to place.

The entire unit was bolted back in to place and the leads reconnected to the correct terminals, at least 3 terminals have 2 leads going to them, so it's important to label them well. Final checks were made and the set was powered up, then it sounded fantastic.

All in all this took 2 evenings and was well worth the effort. In the past I have shied away from these condenser blocks, but having tacked one it was in fact far easier than I thought and I wouldn;t hesitate to do another one, the results were very rewarding.

The set now works beautifully and has pride of place in the front room.

I obviously can't vouce for the safety of the methods I have used, but personally I have found this to be a very successful restoration project which has also made condenser blocks seem far less daunting, however I would stress that a circuit disgram of the block is essential as it would be virtually impossible to tackle this job without a full circuit diagram.
 
Old 12th Mar 2005, 8:43 pm   #2
JHGibson
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Join Date: Apr 2003
Location: Salt Spring Island, BC, Canada
Posts: 368
Default Re: Murphy A24 (1934) AC Mains Radio

I restored this model radio some years back and in the process replaced all resistors and capacitors. I had the same problem opening up the metal can which had many components all buried in tar. My wife had forbidden all access to "her" kitchen stove so melting the tar out was not an option. I ended up putting the metal can in a sealed container of paint thinner for a month. At the end of that time the solid tar had become a liquid and could be poured out.
The Murphy A24 is a fine looking radio and ranks high in my collection. The circuit is rather unique in that the IF is 117 kcs. With such a low IF, it might be expected that there would be a problem with images (second spot), but there is a clever arrangement of the antenna input winding to neutralize or antiphase the images so they are reduced to a low level, some 50-60 db down.

Last edited by JHGibson; 12th Mar 2005 at 8:56 pm.
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