A regular client brought this Murphy TV in for repair and restoration. As the attachments show this is the full fringe area specification model with flywheel line sync and a gated vision AGC system. An additional common vision and sound IF amplifier was added for extra gain so that the set will receive signals in difficult reception locations. The CRT appears to be the original Mazda CRM172 so we won't know how good it is until the timebases are working and there is that oil filled line output transformer to consider.
The sixty-three year old set was bought from it's one and only owner who lives in Stamfordham, Northumberland.
It's possible that the valves and CRT have had an easy life because the mains voltage carousel was set to 250volts.
Till Eulenspiegel.
Now, I really am looking forward to reading all about this one Till. ?
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Andrew, the Murphy is already on the bench for initial tests. Set is full of TCC waxies.
Till Eulenspiegel.
Once I knew my V280C was going to be serviceable, I eventually got round to re-stuffing all of the wax capacitors. A lengthy task and not everyone's cup of tea, but as the chassis was very clean to begin with, I wanted to preserve the original look.
I hope the LOPT will be ok for you.....fingers crossed eh?
To understand the black art of electronics is to understand witchcraft. Andrew.
Although the line output transformer is immersed in an oil filled can it might still be a good idea to warm up the transformer by passing 20 to 30mA through the EHT overwind and primary.
As I have have mentioned many times before those yellow Hi-Viz capacitors don't look good in vintage equipment.
Till Eulenspiegel.
Hi guys,
That chassis looks very busy! And not unlike my V216C, other than mine being originally band 1 only. I always felt that the round CRT of the 216 could have been replaced with a suitable rectangular version, and this V280 only serves to strengthen that belief.
These chassis are a work of art, in my eyes. . . . Buggers to work on as I recall, but a real work of art. At least this chassis would not seem to need the Visconol EHT smoothing cap - assuming that this is the original CRT?
I'll bet it will give a good account of itself, once restored.
I think this set has the additional 500pf capacitor and then a 470k resistor in series with the EHT output to the CRT.
As the output from the overwind is inside the can connected to the U25, it would be impossible to connect a supply to heat the overwind, although of course the primary can be connected.
It might be better to completely remove the can from the chassis and thoroughly clean the exterior, tilt the can so the dip-stick is uppermost, remove the dip-stick to allow any moisture vapour to escape, then gently warm the whole thing at about 60-70 degrees for a couple of days or so in an oven.
There is some good advice in "Servicing the Murphy's" within the Service Dept link at the top.
To understand the black art of electronics is to understand witchcraft. Andrew.
As I have have mentioned many times before those yellow Hi-Viz capacitors don't look good in vintage equipment.
Till Eulenspiegel.
Whilst I'd agree with that sentiment, I reckon 'Hi-Viz' would look more at home than the 'tropical fish' I've seen in some restorations.
I like the idea of restuffing the old caps, but it's got to be a dirty and time consuming job, and has to be a work of dedication to ultimate cosmetic restoration, given that hardly anyone else ever see the result. ? ?
It might be better to completely remove the can from the chassis, tilt the can so the dip-stick is uppermost, remove the dip-stick to allow any moisture vapour to escape, then gently warm the whole thing at about 60-70 degrees for a couple of days or so.
There is some good advice in "Servicing the Murphy's" within the Service Dept link at the top.
That's where one those 'mini-fridge' cooler/heater boxes can come in handy - you can put things in there, switch on and leave as long as you like. They reach around that sort of temperature, or at least, I used to use one to heat up pastries, pies and pasties, and they were always nicely warmed up without roasting them. ✔
If I were forced to go through the whole procedure again as I did with the LOPT in my V280C, the only thing I would do differently would not be to re-use the original Murphy oil. I would measure carefully the quantity though and obtain a supply of DOT5 silicone based oil.
DOT5 is quite different to the normal brake fluid, so don't be alarmed at the suggestion.
http://www.clearcoproducts.com/dot5-brake-fluid.html
I would then bake the internal naked LOPT for a few days to really dry it out, immerse the LOPT in the new oil to allow it to soak in, then re-install the LOPT into the can. In the can would already be a measured amount of new oil, perhaps with a very small extra amount to allow for any that was spilled or soaked into the LOPT itself.
To understand the black art of electronics is to understand witchcraft. Andrew.
I sincerely hope that the above scenario will not be met in your case with this set Till.
I wouldn't wish it on anybody, as it is a real pain in the you-know-what.
To understand the black art of electronics is to understand witchcraft. Andrew.
I believe British Relay Wireless had "dry" line output transformers specially made for certain Murphy TV receivers.
Till Eulenspiegel.
The V350 "transportable" has a dry LOPT, or at least, mine does.
So does the 12" V230 portable TV.
The attachments show the busy underside of the chassis - lots of waxies to replace.
Checked the 0.01mfd coupling capacitor between the line oscillator and the grid of the output valve and although it doesn't show any leakage on the Simpson multimeter it does show an incorrect reading on the capacitance tester. Better to replace it. C155 in the Murphy service manual.
A miniature thermistor is broken. See attachment below. I'll have to find out what it's function is in the circuit.
Till Eulenspiegel.
A miniature thermistor is broken..... I'll have to find out what it's function is in the circuit.
The component is a voltage dependent resistor and is R196 in the Murphy service manual. Forms part of the anode load resistor of the flywheel sync DC amplifier.
Later Murphy sets employ a much simpler circuit. No DC amplifier and the control voltage from the discriminator is supplied via the filter network direct to the line oscillator. See the circuit diagrams of models V310A and V410A.
Till Eulenspiegel.
If my understanding is correct the VDR action would tend to give a stabilised voltage at the junction of the VDR and R197 so the the DC amp would be fed from a stabilised supply.
Frank
Hi Frank, what I really need is sight of the manufactures service manual, this would speed up the restoration of the set.
I'd say your suggestion how the VDR functions in the circuit holds up well.
Till Eulenspiegel.
The Mullard Maintenance Manual informs us that the Murphy V280A employs two thermistors which are types VA1015 and VA1008. The VA1015 is in the heater chain and I'm now sure that R196 in the anode load of the line flywheel sync DC amplifier is the VA1008.
Till Eulenspiegel.
Working my way through the Murphy. The two large TCC waxies are in the vision AGC line. Both check OK for leaks, let's see how these capacitors perform when the set is restored to working order. The 0.01mfd coupling capacitor between the line oscillator and the grid of the output valve has been replaced, as has the 0.5mfd filter capacitor C148 in the flywheel sync control circuit. Resistance reading between the HT line and chassis is 20Kohms, should be a bit higher than that.
Till Eulenspiegel
Last night I made some progress with the Murphy V280A. First, disconnect the metal HT rectifier and get the heater chain working which turned out to be not an easy task. Lots of valveholder problems but in due course all sorted out.
Next, supply the set from the variac and slowly increase the mains voltage in order to give the HT smoothing capacitor a chance to recover from it's long rest. By the time the applied voltage was 180V the line oscillator sprang into life, at first the sawtooth waveform at the grid pin of the 30P4 line output valve was not clamped to ground potential but when the supply voltage was increased to 200V the 30P4 commenced to conduct and the negative going waveform was secured. No bangs and flashes and eventually full mains voltage was supplied, and again no drama. The HT voltage on the smoothing capacitor is thirty volts down at 163V so it's likely the LW7 finned metal rectifier has high forward resistance. Nevertheless, a useful 400volts is available on the boost HT rail. A silicon diode and series resistor will be used to replace the failing rectifier. So if boost HT is present we must have EHT, 'fraid not, just a tiny spark. I remember these sets, back in the day - if one wishes to use that awful term, a common fault was the resistor in series with the U25 EHT diode having gone open circuit. Looks like I'm having to open up the oil can.
Some good news, the frame output transformer can be heard to be purring.
Till Eulenspiegel.
