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Old 31st Jan 2016, 3:08 pm   #1
Malcolm G6ANZ
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Default Murphy SAD 94

This set was untouched since it was built, as shown by the thick layer of dust covering it. The owner was quite proud of the fact that it still worked. The mains lead rubber insulation was falling apart and it was fused at 13A.

Due to the poor state of the internal wiring and the fragility of the mains lead I didn't test it before I started on it. I replaced all the Hunts wax caps, one of which fell in half when i took it out. The audio coupling cap was leaky. I also replaced the majority of the heater and HT wiring. I left the wave change switch wiring alone as this is not safety critical.
So far so good. Medium wave working correctly and with good volume, however long wave was totally dead. When I replaced the perished wiring and caps I deliberately left the wave change area alone as the wiring was reasonable and the only caps were the silver mica types which are usually OK. The first thought was that one of the coils had become open but a measurement of the showed they were all ok. This set is n AC/DC with a live chassis so I was wary of earthing it even through an isolating transformer so to check on the local oscillator I made a coupling coil to the scope and hung it near the local oscillator area. Sure enough on medium wave there was a large sine wave that changed frequency with the tuning. On long wave there was nothing. So the local oscillator wasn’t running.
The correspondence on the forum is here:
https://www.vintage-radio.net/forum/...d.php?t=122602
I traced the wiring again and found no errors, I then measured the mica capacitors thinking that maybe one had gone open, but again all was well. Following advice I checked the current drawn by each stage by measuring the voltage drop across each feed resistor, all showed to be reasonable. The resistors around the local oscillator were checked and a couple slightly out of spec replaced, but still no sign of life. I then went through the switching circuit again. This is a straightforward set of changeover contacts swopping between the medium and long wave sections, nothing fancy. There was an open circuit on one of the sections, a close inspection showed that the contact had bent back and was no longer connecting. To prove the point I linked the section with croc clips and the oscillator came to life. A gentle push and the contact snapped off. Luckily there was an unused set of contacts on the switch bank so I moved the switching function across to them and success, long and medium wave working. A quick adjustment on the long wave padding cap that I had removed in case it had gone short and Radio4 came up at the correct point on the scale.
Why did the wafer decide to break then? Who knows probably my bad luck? But on reflection if the set was manufactured in 1945 and was in regular use until, say, about 1975 and the switch was moved three times a day that’s 30 years that’s over 30,000 operations. I believe that modern switches are given a working life of 10,000 operations so it’s run well past its intended span.

Malcolm
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Old 1st Feb 2016, 12:30 pm   #2
mole42uk
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Default Re: Murphy SAD 94

Excellent work, sir! I think the vacuum now needs cleaning out?
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Old 1st Feb 2016, 3:32 pm   #3
Jimmyhaflinger
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Default Re: Murphy SAD 94

did you also repair the speaker cone? looks like it's been got at
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Old 1st Feb 2016, 3:51 pm   #4
Malcolm G6ANZ
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Default Re: Murphy SAD 94

Yes I did repair the speaker. I used PVA and kitchen towel. I can't say how much better it sounds repaired as I didn't try the set before I repaired the speaker.
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Old 11th Feb 2016, 2:17 pm   #5
Malcolm G6ANZ
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Default Re: Murphy SAD 94

Just as a follow up.
I gave the case a rub down with T cut which removed 50 years of grime and old polish and restored it to 'as new' condition. I gave it back to the owner who was very pleased and somewhat shocked at the state of the mains cable and the internal wiring, which I had kept and gave to him as a 'present'.
He is now looking forward to listening to the cricket commentary on radio 4 long wave.
So thank to all for suggestions and help in getting this set back to working order.
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Old 11th Feb 2016, 3:11 pm   #6
dave walsh
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Default Re: Murphy SAD 94

Not so sad now eh Malcolm. I had a look at photos of this set esewhere. It's quite stylish so I'm not surprised that the owner hung on to it for so long!
As far as I know there aren't many reported conflagrations in radios [not sure re older Tv's though]. I take it you didn't need to hand it back with a small fire extinguisher
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Old 12th Feb 2016, 11:19 pm   #7
Phil G4SPZ
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Default Re: Murphy SAD 94

Good work Malcolm. A fine example of logical and persistent fault-finding that eventually found the 'culprit', and a neat work-around that fixed the fault for good. Nice write-up too, well done.
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