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Making a Murphy SAD94L happy again!

 
Lloyd
(@lloyd)
Noble V-Ratter Registered
IMG 7530

Can it be done? I picked up a very sad 5 quid Murphy at the NVCF a couple of years back (2018 in fact), as found it had no knobs, a massive crack running back to front, and no back. As usual, I took pity on it, I always do when I see something like this, I just can't resist the temptation to make it look nice again!

Before long I'd raided most of the boxes of knobs on various stalls, whilst looking at photo's of the set online (these iPhone things are good for something!), and come up with a set of knobs for it, they are not quite right, but are at least the right shape, just missing the correct legends. On getting it home and trying to see if it'd work I discovered the TH233 valve heater to be open circuit, so that would need to be replaced, along with all the rubber wiring and a good few wax caps, none of the major components were duff, so that at least was a good start. I got some power into it with the TH233 heater linked out, and had the audio stages working.

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Fast forward into a new decade, and I was messing about with other damaged Bakelite projects, so pulled this one from the depths and decided to have a go at fixing the cracked cabinet. To begin with, the crack was cleaned out with IPA and a brush, as it's an old crack it has accumulated dust and dirt from storage, if this is going to be successfully repaired we don't want any foreign bits getting in the way.

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Then many attempts at closing the crack were trialled, and it was found that it wouldn't close up nicely on the front of the cabinet, so to get this to close up nicely I cut away at the Bakelite from inside the cabinet, just behind the crack, so that whatever was stopping it closing up was removed, but not so much that it'd leave a hole in the cabinet front.

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Once this was done the crack could then be closed and you could feel the surface was flat. I found a suitable clamp to keep the crack closed during gluing, tested it to make sure it would do the job, then opened the crack and used cyanoacrylate based 'mitre bond' (branded Mitre Fast), which is basically fairly thick superglue, with a spray activator to harden it quickly. Once I was happy with the glue, I then clamped up the cabinet, made sure the crack had closed correctly, then gave it a quick blast of the activator and left it to do it's thing overnight, it actually sets in about 10 seconds, but I always give it plenty of time anyway.

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Once happy that it had set, I then carefully sanded the excess away on the outside of the cabinet with P600 wet & dry, taking care not to fling the dust everywhere, and also using an extractor, a dust-mask and cleaning the dust away regularly with a damp cloth. I did the front of the cabinet first, and probably went a bit too far in one point as there ended up being a bit of a dip in the Bakelite! I finished by polishing it with Greygate paste polishing no.5 until the shine came back, It looked quite good, the crack is still visible up close, but otherwise it's a nice surface finish.

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For the crack running down the side of the cabinet, I added some extra support in the form of fibreglass matting, soaked in some leftover black fire-retardant potting compound that I'd mixed up for making new knobs for the Ekco A22 (which went very well!), I didn't quite have enough mixed, so I only got some of the matting soaked well in it, but enough to give it strength.

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With the crack all stuck down and strengthened, the rest of the cabinet needed a damn good polish, it was treated to a good going over with no.5, but the bits on the top and front of the cabinet were a right pain to get into! So out came the Dremel (well, black and decker's version of one), and I chopped a cotton bud in half and stuck it in the chuck with some polish on it! Got into the corners really well, and wasn't destroying the ends of my fingers anymore. After the No.5 had done its work I also gave the cabinet a rub with some Briwax to really make it shine.

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The result of all this messing about is that I now have a good solid cabinet once again, and it looks good too, whilst the crack is still visible when viewed at close quarters, the set still displays well, which is the aim of this repair after all.

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All that is left to do now is to restore the chassis to working order, and either make or find an original back panel. Also the tuning scale has lost it's legends, I guess damp storage has finished those off (anyone got a good scan of one?), it looks like it was printed onto the surface of the glass, I'm not sure why Murphy made it like this? and another weird thing they did was hide the scale lamp behind a bracket with a hole in it, which must have cast some strange shadows on the scale!

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The design of the cabinet is certainly striking, and for some reason makes me think of the gates of Mordor from The Lord Of The Rings films.

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Regards,
Lloyd.

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Topic starter Posted : 02/02/2020 7:25 pm
Cathovisor
(@cathovisor)
Illustrious V-Ratter Registered

There is a post-war Marconi - the model number of which escapes me at the moment - that has a very odd habit of the letters and numbers falling off the tuning scale onto the backing plate below, so it looks like someone has dropped Alphabetti Spaghetti everywhere!

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Posted : 02/02/2020 9:13 pm
Lloyd liked
Lloyd
(@lloyd)
Noble V-Ratter Registered

Right, where did I get to?

The cabinet was fixed, but the tuning scale looked awful, luckily Robert Darwent over on UKVRRR came to the rescue with a nice cleaned up scan of an original scale, all I needed to do was print it! As expected, the printer decided it wasn’t going to play ball, the first printout came out with thick bands over it,

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I guess it’s used up all the ink for cleaning itself, or the head is on its last legs, so I took the original file and used Photoshop to change it to greyscale, then adjust the contrast to lighten the background. Then I raided my Mum’s crafting stuff, and found some ivory paper, which also has a slight sheen to it, and printed the scale onto it.

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There was just enough of the original print left on the old scale to check the new one was the correct size.

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I cut a new piece of glass to stick the scale to, just in case I needed the original for anything in the future, I’m not any good at cutting glass! Made a right mess of it on my first go, but got an acceptable piece second time round. I glued the scale to the glass, and left it on a flat surface with some weight on it. Then someone spilled water near it and all the ink ran ? so I had to print another!

This time I used some double sided adhesive sheet that comes in A4 size, it’s fairly clear too so light can pass through it, this worked out much better than the first attempt, and didn’t need time for any glue to set. I fitted it to the chassis and put it in the cabinet, it looks great! This little Murphy is beginning to look much happier!

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I changed the scale lamp too, since someone had fitted a tubular bulb rather than a round one, and it didn’t fit in place properly. With the round bulb fitted the lamp carrier fits onto its threaded studs properly. I’m still not sure on the scale illumination though, it looks a bit odd to me!

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On to sorting the chassis, I got a replacement TH233 valve, the original valve had an O/C heater, a long wait paid off, there were replacements on eBay, but they were £30! Far too much, eventually someone listed a box of 5 valves for 99p, and I was the only bidder!

So with the heater chain complete, I decided to give it some beans and see what happened. Not much was the answer to that! It all lit up, heaters glowed, and a slight hum from the speaker, but nowt else. A check with the meter revealed positive volts on the grid of the PEN383, C30 was high on the suspect list, with that snipped out the +10V was still present, I thought oh no, not another duff valve? Out it came and a quick test with the Megger showed leakage between the screen and grid, and also between the cathode and grid, then there was a click and the Megger went nuts showing ‘E09’ on the display and making weird noises, which it normally does when you draw an arc between the probes! I must have blasted whatever it was away, because on refitting the valve the positive volts were gone! And the quiet hum had been replaced with a healthy amplifier buzz and interference!

I replaced C30 with a new cap, just tacked in for trial purposes, and had a tune about with the workshop aerial connected, discovered the set was switched to LW, and there isn’t much on there, switching to MW gave nothing, wiggling the wave change switch produced lots of crackling and occasionally some noise, so I gave that a squirt of switch cleaner and exercised it and eventually it started working, I tuned in Absolute radio very loud and clearly! Listened to It for a while then turned off, despite all the wax caps still present it sounded alright, they will be replaced with new ones soon, and I’m sure the radio will sound even better for it.

It was noticed whilst testing the radio that the volume pot had turned into more of a switch, with either quiet or loud settings! I took it out, and opened it, cleaned up the track, luckily there is still enough life left in it, I don't like having to replace volume pots, as trying to find the correct sort is rather difficult these days. Even after cleaning it there still seemed to be the tendency to be a switch...

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I decided I needed to do some more in depth testing, there was only so much I could get from a multimeter, I guess an analogue one would have been better, but the only ones I have don't have batteries fitted... Then I thought, I can use my little scope! It's one of those cheap Chinese ones built from a kit (by me!) Its a Jyetech 'DSO shell' single channel thing with only 200kHz bandwidth, now how to test a pot with it? I connected the bench PSU set to 5V across the pot track, and connected the scope between the centre tag and one end of the pot, set the scope to 0.5S per div, and 2V per div, then wound the pot from end to end slowly, then hit the 'hold' button to stop the trace. I could then use the rotary encoder on the scope to skip back and forth along the waveform to see if there were any dead spots, sod's law, there weren't any visible!

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It was then that I spotted it, when the cables were moved the trace would be disturbed, but only with the pot set mid way?! I found it was the centre tag that was causing it, the rivet holding it on was loose! It only showed up with the pot in the mid position though.. A quick tap with the hammer fixed it, and the pot was functional once again.

The wiring was beginning to show its age, with the insulation crumbling away at any attempt to flex the wires, so I've started on the mammoth task of replacing it all. I'm trying to keep to the original colours where possible, even though all the old wiring looked brown, a quick clean with IPA reveals the original colour.

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I'm also re-stuffing the original wax caps whilst I'm in there, so far done 3! C30 was the easiest, as it was actually covered in a rubber sleeve! It's going to be a while before it's finished, as I started tidying the workshop, it needed it, there was very little room to work!

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Regards,

Lloyd

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Topic starter Posted : 26/04/2020 11:25 am
Cathovisor
(@cathovisor)
Illustrious V-Ratter Registered
Posted by: @lloyd

I’m still not sure on the scale illumination though, it looks a bit odd to me!

It's a bit like the dial 'illumination' on the first generation Bush DAC90. It's more of a pilot light to remind you the set is on than anything else.

The last time I came across a switch-like volume control was in my much-missed Datsun Sunny estate; every now and then the radio would suddenly go full blast! That was the earthy end of the volume pot, but it was easier to buy another radio for £3 from the scrapyard back then.

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Posted : 26/04/2020 1:05 pm
Lloyd
(@lloyd)
Noble V-Ratter Registered

Haha, yes, my car stereo has a habit of random volume adjustments! It’s a 2010 Honda Civic, the volume control is one of those horrible rotary encoder things, and it doesn’t work at all! I have the steering wheel controls luckily so I make do with that. It would be nice if I could replace the head unit easily, but as I found on my dad’s Civic, when we wanted to fit the iPod interface, you have to take apart half the car just to get behind the head unit!

I think the scale lamp on this Murphy is more useless than the DAC90’s, that photo above was taken with the lamp powered by the bench PSU, with it powered by the radio itself, it’s like a candle, after blowing it out and the wick is just still glowing! I might play with the parallel resistor value to try and improve it.

 Regards

Lloyd

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Topic starter Posted : 26/04/2020 6:29 pm
Lloyd
(@lloyd)
Noble V-Ratter Registered

It's been a while since I started this one, all kinds of stuff got in the way, and it was put to one side, mostly lack of parts! Well, today I finally finished gutting and re-stuffing the wax caps, they were all made by Hunts, and were quite easy to do, simple cardboard tube with 2 metal end caps that pull out neatly, then just push out the guts, the new caps were all ones made by TSE and JB, they look to be decent quality caps, and they are not overly expensive either, they fit into the tubes of the old caps with ease, I rolled a bit of paper round them to fill out the tubes, even being daft and signing and dating the bit of paper, you never know, someone might find themselves restoring the set again in 50 years time! I then stuck the end caps back in and soldered the new leadout wires to them, and finally gave them a coat of candle wax to finish them off. I did them one at a time so I couldn't put them back in the wrong place.

I've also replaced a lot of the wiring with silicone covered stuff, bought from the NVCF a good few years ago, some wiring was ok so I've left it for now, if it crumbles in the future then it'll be replaced, but I need to stock up again before that happens! The mains cable was non-existent when I got the set, so I bought some braided stuff off eBay, 0.75mm 2 core, it looks like a piece of old cable, well, more like a bit of old rope! I could probably have got away with the 0.5mm stuff.

One final thing to do is replace the main smoothing and reservoir cap, the one fitted is the wrong diameter can, and just rattles around loose, it's a 16uf + 16uf can, around 33mm diameter, the one fitted is only 25mm! Jellyfish audio have one that will fit nicely, so I'll probably buy one, if I don't spot something suitable at RetroTech next weekend.

The radio is working well now, pulling in plenty of stations, it even picks up the strong ones without any aerial connected.

Regards,
Lloyd

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Topic starter Posted : 19/09/2021 7:57 pm