Servicing The Murphys
By Trevor Goodenough
Forum Member: MurphyV310
Retired Radio & Television Engineer
This article will give some additional tips and perhaps help for those wanting to fix up any Murphy from the V310 to the V759.
The V300 series was a radical design, it moved away from the “box” we all knew for so long, the shaped cabinet, the “sound Mirror” lid housing the controls and the lid itself being the switch, the presentation of the screen made the set compact and well received. For the engineer it was a two minute job to access the full chassis with the whole cabinet being able to be lifted off the chassis after removing two bolts.
The V310 electrically was conventional bar two items, the tuner was made to a very high standard, it is virtually drift free and has no fine tuner, the base V310 has no AGC, it is direct line sync and initially had a self oscillating line output stage. The self oscillating line stage was trouble from day one, some mods were done initially with resistor value changes and tighter tolerance, the addition of specially selected 30P4’s did make a marginal improvement to the line drift it had, in fact Mazda marked these specially selected valves 30P4MR (MR= Murphy Radio) they were available for many years even after the improved 30P19 was available.
Murphy quickly got rid of the single valve line oscillator/output and well within 6 months the V310 had a 6/30L2 multi-vibrator fitted just behind the mains socket, this cured all the line drift issues. A kit was available from Murphy to modify the earlier sets, I very much doubt that the early sets survive now less the modification. It is a pig to do as there is very little access. During the V310 service life the set was reliable with many lasting longer than the 10 years expected, faults were decoupling caps in the IF stages, tubes (CRM172) and line transformers.
Line Output Transformers
The oil filled transformers on the V310 right through to the V759 all have similar issues. Firstly I’ll say categorically they should not leak, my V320A is totally dry and its the original transformer, if it leaks it is usually caused either by a faulty transformer, excessive oil in manufacture or contamination by coal fires or fags, this goes for the rubber seal. Faults on the transformer are, no EHT, if you have a healthy spark on the U26 top cap and no heaters it is caused by either the heater winding choke or resistor in the LOPT can going O/C, its as common as muck and was one of the biggest failures of the time, today its a 10 minute fix, just fit a TV20 EHT stick. Carefully solder a stout lead to the + end (red Band) of the stick and push the lead into the correct pin of the U26 base and put the top cap from the LOPT onto the the other end of the stick and off you go… fixed.
If you have a good picture when the set is cold but it gradually looses focus and the EHT & Boost drop after half an hour, the LOPT casing gets quite warm and begins to leak it is suffering with moisture contamination, I’ve opened them up before with quite a bit of water inside, this is usually caused by a bad seal on the transformer and after 50 odd years a lot of water can accumulate. These transformers come in two sizes one is the size of a Campbells Condensed soup tin the other a Heinz beans tin size, yes Murphy must have used tins for trials back then. Open the can either by unpeening the can or cutting it at the lip. Clean it properly, get rid of the swarf and wash it in solvent, not too strong to to damage it, IPA can be used but remember it is hygroscopic and retains moisture, then let it dry, keep it at 60-70c for a couple of days or pass some current through the overwind so it gets warm 40c for a good few days. Rebuild the transformer in the correct tin for the LOPT diameter minding to put the poly sheeting from the old can in, experiment with the oil lever to just cover the top of the actual transformer and no more, use quality veg oil or transformer oil if you can get it not mineral oil as it goes for rubber.
Line Stage Tips
Building upon the above, here is some more info on the Line stage and what to do if the LOPT is truly duff.
The multi-vibrator line oscillator is reliable and drift free but after 50 odd years it is prudent to replace out of spec resistors, the cathode resistor is one to check and replace if out, it can cause the flyback time to be incorrect and will give a left hand foldover if well out, I always change this resistor with a quality modern component irrespective of its value. Also replace the screen feed resistors on the line output as a matter of course, this applies to all models from the 310 to 759 and the large screen variants. Ensure that the line drive coupling cap is replaced and the line output valve grid leak replaced too, a 470k risen to even 560k or more effects the shape of the drive waveform, its important to make sure the line output is properly driven with the correct amplitude and shape of the waveform.
If the LOPT has died whether due to damage, burn up etc there is two ways to go. You can if the overwind is the problem remove it and fit an EHT Multiplier to the 30P4 anode, it can be done in the LOPT can as well if you construct your own, it’s not my preferred method though. I have successfully fitted a Thorn Jellypot to a V659 using in the main the 1400 circuit with stabilised width and EHT, it works very well and even better with the Thorn scan coils, the advantage is these LOPT last and last, just be careful soldering to them. I see no reason why you cannot fit a Jellypot to a 310 or 410, on these lower defection power sets it is an idea to run the line output stage at a lower HT, this will reduce the scan power and also bring down the EHT to a level required for the smaller CRT’s.
Note: “I do have a V410 with a faulty LOPT here and I hope to fit a 1400 Jellypot in the near future and will by all means report back with my findings”.
Regarding the the tuner unit, biscuits are hard to get hold of these days, after many years the oscillator core will be stuck fast so don’t be tempted to adjust the core until you’ve removed it from the turret and checked it is free to turn, if not a few drops of WD40 will soften up the hardened wax and you’ll be able to do any adjustments OK, the V310 needs a long plastic trimmer to do the adjustments and at one time a plastic knitting needle did the trick, now it seems you cant buy plastic needles so if you have one keep it and don’t loose it.
The V659 & V759 can suffer with height drift, in most cases this is a pain to sort out as it is caused by leakage in the paxolin board above the frame output transformer. It is prevalent on sets that have been exposed to damp conditions, the board contains the focus and linearity sliders and it can be clearly seen on a meter there is voltage on the actual paxolin, move the prof nearer the focus slider and the reading rises, yes it is this bad, the leakage gets worse the longer the set is on, on mine I made a new board out of one glass fibre laminate, drilled and pop riveted on solder tags, this effected a cure and I no longer had height drift.
The rubber CRT surround in the barrel sets can disintegrate, it could be due to dampness or the suns rays, I’m not sure but I have been able to halt the issue by removing it and letting it soak in a hot bath of detergent and water for an hour or so, this completely stopped the issue worsening on a V530 and although my V320A was OK in this department it got the treatment 12 years ago and is still perfect. I have one less a mask due to this and would appreciate any comment on how to make a new mask or if anyone has one going spare, its for a V330 (same mask as a V310)
One good mod to do on the V430 or V530 is to remove the 441 ohm resistor for the radio heater dropper and fit a 4.5uf Motor run cap (Bypassed with a 100K) as the heat on the original resistor dropper damages the the EHT rectifier valve base.