This is my first project report, please excuse me if I am preaching to the converted etc. but here it is :-
Bought a Murphy A46 from a market trader, full of dirt, dust and spiders, the speaker cloth was ripped and the cabinet scratched, filthy with tea cup ring marks all over the top.
Luckily the innards looked intact with evidence of repairs (electrolytics), done probably in the sixties.
After downloading the Murphy service manual I pleased that it included circuit description, circuit diagram, layout, top and bottom, lists of components and alignment procedures.
This was going to be "repairing by numbers" because the layouts were divided up into squares and each grid square identified with a letter and number like map references. Every component was located in this way, I thought it to be a bit micky-mouse, but in reality it proved very useful. The valves were correct and worked, although I had to re- paint some of the metalisation where it had dropped away.
Several suspect wax caps and one electrolytic were changed together with some replacement wiring and I was left with the radio receiving the most powerful stations but struggling with volume on weak ones.
I had already checked that the agc (avc) line was varying with signal strength and valve voltages seemed to have been there or there about !!
This was a mistake on my part, I then remembered that some voltages need to be reasonably accurate, particularly at the cathode of the first valve, where the agc voltage is applied to the control grid and is in respect to the cathode voltage. Well my cathode voltage was a bit high due to the cathode resistor rising in value from 300ohms up to 385 ohms.
With a new resistance in parallel (since I didn't think 300 was a preferred value) the agc worked much better.
I didn't think the trimmers had been fiddled with but I went through the full alignment anyway and did find it slightly off in both the IF and signal.
The alignment available is not comprehensive, since there are no inductor adjustments only cap trimmers
I stripped the cabinet with B&Q's non toxic paint stripper and finished with my favourite water based wiping polyurethane . I am now pleased with the effort put in and the radio looks good and performs very well. The new speaker cloth was from a discarded pair of my wife's trousers.
HOWEVER there is a problem which must be due to design, unless any of you can advise me.
I have restrung the dial cord but find that the swept distance of the pointer is not enough to fully reach the top/bottom of the dial glass its about 20mm short. with the tuning gang going from closed to fully open. The only solution I can think of is to increase the diameter of the main pulley by about 6mm.
Picture attached
Thanks for staying with it
Jim
Bought a Murphy A46 from a market trader, full of dirt, dust and spiders, the speaker cloth was ripped and the cabinet scratched, filthy with tea cup ring marks all over the top.
Luckily the innards looked intact with evidence of repairs (electrolytics), done probably in the sixties.
After downloading the Murphy service manual I pleased that it included circuit description, circuit diagram, layout, top and bottom, lists of components and alignment procedures.
This was going to be "repairing by numbers" because the layouts were divided up into squares and each grid square identified with a letter and number like map references. Every component was located in this way, I thought it to be a bit micky-mouse, but in reality it proved very useful. The valves were correct and worked, although I had to re- paint some of the metalisation where it had dropped away.
Several suspect wax caps and one electrolytic were changed together with some replacement wiring and I was left with the radio receiving the most powerful stations but struggling with volume on weak ones.
I had already checked that the agc (avc) line was varying with signal strength and valve voltages seemed to have been there or there about !!
This was a mistake on my part, I then remembered that some voltages need to be reasonably accurate, particularly at the cathode of the first valve, where the agc voltage is applied to the control grid and is in respect to the cathode voltage. Well my cathode voltage was a bit high due to the cathode resistor rising in value from 300ohms up to 385 ohms.
With a new resistance in parallel (since I didn't think 300 was a preferred value) the agc worked much better.
I didn't think the trimmers had been fiddled with but I went through the full alignment anyway and did find it slightly off in both the IF and signal.
The alignment available is not comprehensive, since there are no inductor adjustments only cap trimmers
I stripped the cabinet with B&Q's non toxic paint stripper and finished with my favourite water based wiping polyurethane . I am now pleased with the effort put in and the radio looks good and performs very well. The new speaker cloth was from a discarded pair of my wife's trousers.
HOWEVER there is a problem which must be due to design, unless any of you can advise me.
I have restrung the dial cord but find that the swept distance of the pointer is not enough to fully reach the top/bottom of the dial glass its about 20mm short. with the tuning gang going from closed to fully open. The only solution I can think of is to increase the diameter of the main pulley by about 6mm.
Picture attached
Thanks for staying with it
Jim
