Circuit diagram of the line timbase.
The circuit diagram of the frame timebase. Note the direct coupling between the anode of the oscillator valve and control grid of the output valve. Frame linearity is effected by varying the bias on the output valve. In the set under discussion a 10Kohm resistor is inserted in series with the ramp forming capacitor C129. This resistor will give a degree of "sit up" on the sawtooth waveform. Official Murphy modification? I don't know.
Till Eulenspiegel.
Hooked up a BRC tripler between the anode cap of the 30P4 and the final anode connection of the CRT. The attached picture shows that at least the CRT is useable. The Cyldon type C turret tuner is loaded with coils for 1 to 5 and 8 to 10. No signals coming through yet.
Till Eulenspiegel.
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.
Till Eulenspiegel.
Sounds familiar.
In my case, it was a small coil of wire in series with the heater of the U25 that was open.
To understand the black art of electronics is to understand witchcraft. Andrew.
I don't know if this will be of any help, but here are the crude wiring sketches I made when doing my LOPT can. First is the view from the top before removal from the chassis, and secondly the wiring positions on the actual LOPT when removed from said can.
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Andrew, useful information. much appreciated.
21" models: V290CA and V300C. The V290CA employs a plug-in EHT rectifier valve, a Mazda U26. According to the R & T servicing book the V300C is electrically similar to the V280 series so it can be assumed the EHT rectifier is an U25. 15KV EHT. Both models have the 70* Mazda CRM211 CRT.
The 14" model V270 employs a Marconi-Osram U45 as the EHT rectifier.
Till Eulenspiegel.
The attached picture shows a line output transformer removed from a Murphy Astra V659. Trevor (MurphyV310) passed this transformer over to me to try out in my own Astra TV. The plan was to use Dow Corning potting compound, the same as used in the BRC jellypot transformers.
We're pretty certain the U25 heater resistor in the V280's can is open circuit. Here's an idea to consider, move the EHT rectifier diode outside the can, this will give us an opportunity to employ a plug-in valve like the U26. Or, as an alternative a solid state rectifier of the TV20 type. Does anyone really wish to go into the performance of opening up the LOPTx can when the EHT rectifier fails again?
Till Eulenspiegel.
In the case of the V280 lopt, a solid state recifier out of the can would be easier and make sense. Otherwise, you would need two additional holes in the top for the heater winding wires to come out for a valve rectifier.
With mine, I did consider fitting a solid state rectifier inside the can to replace the U25. But I opted to replace the valve like for like in the end and repair the broken heater series coil.
Would the Dow Corning potting compound have the same thermal conduction as the oil? Just thinking of the heat generated by the lopt inside.
To understand the black art of electronics is to understand witchcraft. Andrew.
Would the Dow Corning potting compound have the same thermal conduction as the oil? Just thinking of the heat generated by the lopt inside.
Well if one considers how reliable the Thorn jellypot transformers were I'm sure the Dow Corning potting compound might well be the answer to all our Murphy oil can transformer problems. But there are other considerations such as how does the dielectric constant of the jelly compound compare with oil?
Link to Farnell potting compounds:
Till Eulenspiegel.
Another thing to possibly consider is the penetration ability of the jelly into the windings of the lopt. I know that the oil soaks right in between the layers of windings. If there were voids, could the insulation be compromised?
To understand the black art of electronics is to understand witchcraft. Andrew.
I was informed that oil is still the best substance to fill the can, but which type of oil?
Link to a discussion about the electrical properties of vegetable oils:
https://www.sciencedirect.com/science/article/abs/pii/S0360319913029704
300K = 26.85C
How about DOT5 silicone oil?
Here is a quote from Trevor's excellent information on servicing the Murphy's....
"The selection of oil is another important thing, the oil needs to have a low thermal coefficient, you don’t want the oil to expand too much when warm, you also need a low hygroscopic oil so it doesnt absorb moisture and it MUSTN’T be mineral based. Olive oil is pretty good and the more refined the better, recently though I have tried DOT5 Brake fluid, it is silicone based, low expansion, and non hygroscopic, it doesn’t attack the LOPT in anyway and is by far the best oil to use. it is also not that much more expensive than premium olive oil. DO NOT use synthetic brake fluid (DOT 3, 4) ONLY DOT 5, use of the older types will ruin the LOPT as does Mineral based oils so no Duckhams Q20-50 please."
To understand the black art of electronics is to understand witchcraft. Andrew.
DOT5: As long as it is good for >15KV pulse operation @ 10Khz and harmonics. I'll try it in the V659 transformer can. That'll be a good test because of the extra power requirements compared with 70° deflection angle V280.
Till Eulenspiegel.
There is a difference between DOT5 and DOT5.1 brake fluid. DOT5 is silicone based and DOT5.1 is glycol based.
Till Eulenspiegel.
Supposedly, the large-scale development and usage of DOT5 silicone-based fluids was accelerated after the Yom Kippur war when many severe armoured vehicle fires were ascribed to the vulnerabilty of complex hydraulics containing large amounts of pressurised hydraulic oil- previous conflicts had emphasised the need to protect fuel and ammunition stowage. There was also some talk of car fires often being started by the plastic reservoir popping off its grommets on the master cylinder in accident impacts and spilling fluid on hot manifolding (conventional fluids have quite a low flashpoint), but I'm not sure how much formal research has been done on this. They are completely incompatible with other fluids (including DOT5.1) and are best used in systems from new. They have much lower hygroscopy than other oils, whose water miscibility can actually help in reducing corrosion within systems up to a point (as it doesn't get shrugged off into a surface film on pistons and bores) but it also reduces their boiling point.
The lack of flammability is the main plus point and the low water affinity means that they can be left in systems for longer- useful for users like the military who might use vehicles infrequently and lay them up for long periods, but they are more prone to foaming and absorbing air, resulting in higher compressibilty and poor pedal feel/loss of effectiveness. The lack of compatibility with other fluids also favours users like the military, who can be sure of rigorous servicing and maintenance regimes, whereas privately-owned cars can be a bit of a jungle in this respect!
In other words, DOT5 sounds promising for the application in mind, but it would seem wise to clean and dry everything involved thoroughly before introducing it. I agree with the point about absolutely not using other types of fluid- apart from the flammability aspect, they were originally formulated to not attack the natural rubber seals used in the earlier days of motoring but the cussedness of nature means that they do attack all sorts of other things instead.
Being unable to buy DOT5 silicone brake fluid from local motor factors and service stations I will order the special fluid on-line from Moss-Europe.
From the website:
Silicone brake fluid offers a variety of benefits over traditional glycol brake fluids, making it ideal for Historic vehicles and vehicles that are unused for long periods of time.
SBF (silicone brake fluid) is non-hygroscopic, meaning it does not absorb water. Glycol based fluids are hygroscopic and absorb water which can lead to corrosion of metal components within the hydraulic system - this is why Glycol based fluids are recommended to be changed on a regular basis. SBF is also non-corrosive, so it will not degrade the metal or rubber components in the hydraulic system. It is compatible clutch and brake systems traditionally using glycol fluids. It can be used to replace glycol fluid in an existing system; ensure the glycol fluid is thoroughly drained, then refill and bleed the system with silicone fluid.
Unlike glycol based fluids SBF does not need to be replaced. If new a hydraulic system is filled purely with SBF and no glycol fluid is present, it can last the lifetime of a vehicle.Also, it has a wide operating temperature range from -50° to 260°C, and its boiling point (260°C) is maintained throughout the life of the fluid. It is manufactured to DOT 5 specifications and used by the US military for their light vehicles.
SBF will not damage paintwork if spilt, making it ideal for show cars. It resists the formation of mould and bacterial degrading.
I believe it can be obtained via ebay, about £15 for 500ml.
To understand the black art of electronics is to understand witchcraft. Andrew.
Ordered 1litre of DOT5 brake fluid today from Moss Europe.
£22.50p + postage.
When the oil arrives the plan is to fill the Murphy V659 transformer can first, test the receiver over several days to determine how well the oil and transformer performs.
Then if everything proves to be satisfactory give another V659 transformer an oil change. After that, if all goes well with the 110° CRT transformers we'll attend to the V280 line output transformer.
Till Eulenspiegel.
Dot 5 is silicone based and the innards of the transformer must be totally drained of any mineral or vegetable oils prior to using silicone oil.
I'd leave the old transformer out of the can for a week in a warm room to let it drip dry.
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
