MURPHY SAD 94 "In a day " - UK Vintage Radio Repair and Restoration Discussion Forum

yestertech · 16th Oct 2006, 8:47 pm

I like these little sets. This must be about the 3rd one I’ve worked on. As supplied, the lead had been cut-off and one of the knobs had been lost and replaced with a ghastly 1950’s effort, but the dial and cabinet looked OK.
Someone had done some work previously and the volume on/off switch was new, as had been the cable that ironically had been hacked off!
The front chassis mounting screws had gone AWOL, so uncasing was a mater of the rear screws and for once, the knobs came off without a fight ( nearly all of these sets have setscrews rusted into the knobs, I guess that’s why so many you see have knobs butchered or missing ! )
Once inside, some traces of previous capacitor changes were in evidence, but a few waxies remained - the main smoother was still present and after 20 mins on the DC powerpack had reformed nicely. The one large waxie across the mains side of the U403 was still present, and was changed prior to power up on the variac
True to form, at about 150 volts the old girl started to make encouraging noises and once an aerial was connected, a slightly strangulated R4 could be heard. I suspect as last time this will be a shot ‘speaker
So it was time to invert the chassis, temp. unsolder the main electrolytic to make space and fit a new mains lead and replace the usual crumbling cables to the dropper and the speaker baffle, plus one or two rather suspect ones.
Power was reapplied and the IF checks done. All was pretty much spot on but a small tweak on the 1st IFT yielded another 6dB or so – worth having!
The tuning and wavechange switches seemed very stiff, so these were lubricated and the switch contacts cleaned for good measure.
The HT seemed on the low side, and a check was made on the current flowing though the choke – a reading of nearly 90mA showed something amiss – on a hunch the voltage present on g1 of the PEN383 was checked and found to be + 12 volts!
I was about to curse the longevity of the new “mustard” coupling cap someone had fitted, but a test without it showed the voltage coming from within the valve…. This was annoying because it looked like a fairly new replacement. A quick trip into the loft followed and a good used one cured the problem and increased the HT volts a great deal.
As all the testing had been on an external speaker, the original unit was reconnected and sounded grim. A search in the speaker bin produced a ROLA unit, salvaged from a small extension speaker. This all but fitted the mounting studs so the single hole punch was used to increase the clearance around the cork washer at the mounting holes. This just left the choke. In a flash of inspiration (rare!) I found two 4BA-threaded spacers to replace the two RH speaker nuts. Onto these was bolted a piece of brass strip from ‘oddments’, and onto this was bolted the choke, still remaining a reasonable distance from the side of the cabinet when re-fitted (as it carries the hot HT resistor) The finished baffle, looking reasonably vintage was then reunited with the now cleaned and polished cabinet. ( see photo )
Missing knobs on these sets are always a pain, and for the moment, I have fitted a slightly larger Murphy type, which alas is brown, but better than nothing.
The set performs reasonably on a few feet of aerial and more importantly is frugal on space – always a blessing.
Now if only they could have designed that dial light properly…….

ANDY

GJR 11L · 18th Oct 2006, 10:20 pm

Nice job Andy, I've often wondered whether I would get a restoration done within the confines of a day, and I'm not talking about cleaning up a grubby tranny but about all the diagnostic work, problem-solving and alternative part finding that you have done here.

Having read your tale of triumph, I'm absolutely certain that I couldn't!

Then it would spread into the next day, the next week, then I'd have to tidy up and I'd lose fittings and have to search for them, another day, another week etc. ....

No, I'm struggling to sort a DAC90A in one day and the example in question only needs a cleanup and the old de-waxing treatment!

Well done that man.

16th Oct 2006, 8:47 pm	#1
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,130	MURPHY SAD 94 "In a day " I like these little sets. This must be about the 3rd one I’ve worked on. As supplied, the lead had been cut-off and one of the knobs had been lost and replaced with a ghastly 1950’s effort, but the dial and cabinet looked OK. Someone had done some work previously and the volume on/off switch was new, as had been the cable that ironically had been hacked off! The front chassis mounting screws had gone AWOL, so uncasing was a mater of the rear screws and for once, the knobs came off without a fight ( nearly all of these sets have setscrews rusted into the knobs, I guess that’s why so many you see have knobs butchered or missing ! ) Once inside, some traces of previous capacitor changes were in evidence, but a few waxies remained - the main smoother was still present and after 20 mins on the DC powerpack had reformed nicely. The one large waxie across the mains side of the U403 was still present, and was changed prior to power up on the variac True to form, at about 150 volts the old girl started to make encouraging noises and once an aerial was connected, a slightly strangulated R4 could be heard. I suspect as last time this will be a shot ‘speaker So it was time to invert the chassis, temp. unsolder the main electrolytic to make space and fit a new mains lead and replace the usual crumbling cables to the dropper and the speaker baffle, plus one or two rather suspect ones. Power was reapplied and the IF checks done. All was pretty much spot on but a small tweak on the 1st IFT yielded another 6dB or so – worth having! The tuning and wavechange switches seemed very stiff, so these were lubricated and the switch contacts cleaned for good measure. The HT seemed on the low side, and a check was made on the current flowing though the choke – a reading of nearly 90mA showed something amiss – on a hunch the voltage present on g1 of the PEN383 was checked and found to be + 12 volts! I was about to curse the longevity of the new “mustard” coupling cap someone had fitted, but a test without it showed the voltage coming from within the valve…. This was annoying because it looked like a fairly new replacement. A quick trip into the loft followed and a good used one cured the problem and increased the HT volts a great deal. As all the testing had been on an external speaker, the original unit was reconnected and sounded grim. A search in the speaker bin produced a ROLA unit, salvaged from a small extension speaker. This all but fitted the mounting studs so the single hole punch was used to increase the clearance around the cork washer at the mounting holes. This just left the choke. In a flash of inspiration (rare!) I found two 4BA-threaded spacers to replace the two RH speaker nuts. Onto these was bolted a piece of brass strip from ‘oddments’, and onto this was bolted the choke, still remaining a reasonable distance from the side of the cabinet when re-fitted (as it carries the hot HT resistor) The finished baffle, looking reasonably vintage was then reunited with the now cleaned and polished cabinet. ( see photo ) Missing knobs on these sets are always a pain, and for the moment, I have fitted a slightly larger Murphy type, which alas is brown, but better than nothing. The set performs reasonably on a few feet of aerial and more importantly is frugal on space – always a blessing. Now if only they could have designed that dial light properly……. ANDY Attached Thumbnails

18th Oct 2006, 10:20 pm	#2
GJR 11L Heptode Join Date: May 2004 Location: Ellington, Northumberland, UK. Posts: 815	Re: MURPHY SAD 94 "In a day " Nice job Andy, I've often wondered whether I would get a restoration done within the confines of a day, and I'm not talking about cleaning up a grubby tranny but about all the diagnostic work, problem-solving and alternative part finding that you have done here. Having read your tale of triumph, I'm absolutely certain that I couldn't! Then it would spread into the next day, the next week, then I'd have to tidy up and I'd lose fittings and have to search for them, another day, another week etc. .... No, I'm struggling to sort a DAC90A in one day and the example in question only needs a cleanup and the old de-waxing treatment! Well done that man. __________________ John.