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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 18th Dec 2021, 9:25 am   #1
kestrelmusic
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Default Murphy A26

I was given this set as a “derelict” to strip for parts, and indeed I nearly did just that. But, looking at it, it appeared to be pretty well complete, so I checked the coils and IFTs for continuity. All good! So I thought, let’s have a stab at restoration.

So the chassis came out of the case (it needed some penetrating oil and firm persuasion to shift the bolts) and large quantities of grime and livestock were removed. I then fitted a temporary mains lead, replaced the missing output valve and powered up – through a lamp limiter, of course - and with a 2k 10 watt resistor in place of the field coil.

Monitoring the voltages, the HT was much too low. This was due to the reservoir capacitor which had become a low value resistor. However, easily enough replaced. HT was now much too high, due to the output transformer primary being open circuit. I had a suitable transformer to hand, so I stuck it in and the voltages settled to around the right level. I got hum and rustling noises from the speaker, but nothing else.

A check with the ‘scope showed that the oscillator was running, though intermittently; some of the positions of the tuning gang seemed to stop it. So – lift the lid off the gang and found much iron oxide. A good blow through with an aero duster, and the oscillator ran happily, at least in the LW range.
The frequency changer, a Mazda AC/TP, didn’t seem to be mixing the signal. This turned out to be the top cap grid lead: the rubber insulation within had perished and the signal lead was shorting against the earth braiding.

This done – hooray! – a 200kc/s signal produced a sound in the speaker. I managed to get it to tune BBC R4 with a good strong response. But turning the wavechange to MW stopped the oscillator dead. Much fiddling with contact cleaner and fine emery paper failed to produce more than a very uncertain and intermittent response. The problem proved to be that two of the brass tongues had split and were only held in place by a layer of muck and oxide. I didn’t want to try stripping the switch and replacing the tongues, unless it was an absolute last resort, so I soldered the tongues back in place with tiny slips of brass shim. The set was now working. But then my nostrils were assailed with the smell which strikes fear into the heart of any radio engineer: burning shellac. The mains transformer was almost too hot to touch. The fault proved to be one of the HT secondaries which had shorted internally.

So, I rewound the secondary, a job I had never tackled before but was surprised to find not too difficult. The original wire was so badly fused and burned that it was impossible to establish how many turns it had had, but one of the LT windings was on the same bobbin, so I counted the turns of that and calculated the number of turns for the HT winding – it turned out to be 1400. I’d had to use a cold chisel to start shifting the laminations, and I never did get them all back, but the transformer was working, and the voltages of the secondaries were very close, so that went back in place.

Next: the case. The original speaker was past repair, so I replaced it with one from stock. However, the original field coil was still in place and still intact, so I fixed that into a corner of the case as it formed part of the original HT smoothing. I was surprised to find that with only 8uF reservoir and 8uF smoothing, with the field coil in place there was no appreciable hum.
The case was very battered. There was no point in trying to patch the veneer which had come away and was in fragments, so I re-did the whole job, using iron-on veneer. It’s much easier to apply than the conventional veneer using glue, but you do have to make sure that it really has been heated through and pressed flat, otherwise it will tend to “bubble”.

Knobs were cleaned up with No 2 polishing paste. The original speaker cloth had disintegrated, but I have a large piece of herringbone trouser fabric which is a pretty good imitation of the real thing, so a square of that was pressed (literally!) into service. The mains switch, ganged with the volume control, was knackered. It was not practical to replace the pot as the shafts of all of the controls are non-standard, so I just fitted a Bulgin switch next to the cable inlet at the back of the chassis.

The circuit is pretty conventional for the time. There are two tuned circuits on the aerial side of the frequency changer, which is a cathode coupled triode pentode. Variable-mu pentode IF amplifier (IF is 117kc/s) followed by double diode detector/AVC feeding a high slope output pentode. The mains transformer has three LT secondaries, one for the rectifier, one for the AF and output stages and one for the FC and IF valves. Apparently this eliminates modulation hum!

So there it is. Job done! I don’t mind admitting I am quite chuffed with the result.
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Old 18th Dec 2021, 3:43 pm   #2
dave walsh
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Default Re: Murphy A26

That's impressive Kestrell. You've rescued it from death's door and taken a suitably pragmatic approach with your restoration, given the overall condition It's a working Wireless again which is the important thing really.

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Old 19th Dec 2021, 2:59 pm   #3
Tractionist
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Default Re: Murphy A26

Dogged determination - well done! I think a mate of mine has a good spare cabinet for one of these - PM me if you're interested.
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Old 20th Dec 2021, 4:36 am   #4
unitaudio
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Default Re: Murphy A26

This rather puts my own projects into perspective. Fantastic job you've done there!

Regards,
Paul
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Old 20th Dec 2021, 9:40 am   #5
AdrianH
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Default Re: Murphy A26

Congratulations on the radio's recovery, I must ask about the main capacitor block, did you re-stuff it or just replace using new caps. if you re-stuffed it, how did you get the old caps out?


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Old 20th Dec 2021, 3:06 pm   #6
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Default Re: Murphy A26

Well done sir! Talk about pulling victory from the jaws of defeat! Having had similar with a Murphy A98 as far as the speaker went (totally U/S except the field coil) I can sympathise. Congratulations again.
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Old 21st Dec 2021, 6:55 pm   #7
kestrelmusic
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Default Re: Murphy A26

Quote:
Originally Posted by AdrianH View Post
Congratulations on the radio's recovery, I must ask about the main capacitor block, did you re-stuff it or just replace using new caps. if you re-stuffed it, how did you get the old caps out?


Adrian
The original block had been removed and replaced with individual electrolytics. I replaced them as found, but if I can find a picture and measurements for the original block (the pic in the Trader sheet is too vague to be any help) I will make up a new one. Do you know, was it metal or card cased?
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Old 21st Dec 2021, 8:20 pm   #8
AdrianH
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Default Re: Murphy A26

The block is metal and has a paxolin end, held in with tabs and the block screws to the underside there should be some holes for it, not sure if 4 or 2. It is in the garage and if you wish I can take some measurements over the next few days and pass them to you.

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Old 21st Dec 2021, 8:33 pm   #9
Lloyd 1985
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Default Re: Murphy A26

Fantastic result, very well done saving that set! Restoring basket cases like this is something I really enjoy, somehow makes it more satisfying when you get the end result, and look back at how it was to begin with.

Regards,
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Old 21st Dec 2021, 10:10 pm   #10
unitelex
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Default Re: Murphy A26

Nice work
Impressed you tacked the mains transformer yourself
Well done!
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Old 31st Jan 2022, 2:59 pm   #11
MurphyNut
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Default Re: Murphy A26

Well done, these sets can sound really good, one a factor is the heavy and remarkably ridged cabinets.
I was lucky with mine only the capacitors needed doing to get up and running really well.
With unrestored cabinet's they can look rather uninspiring, but with a bit of time spent here they can really look quite classy!
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