I passed the 310's onto a friend, so no longer have them as spares. The only way to repair the LOPT assembly is to open it up. I think with care and patience, a reasonable job can be made.
The frame stage sprung to life after gently tapping the 30p12 output valve, so all in all it would appear to be a set well worth repairing. It is unfortunate about the LOPT, but I won't be giving up too easily.
To understand the black art of electronics is to understand witchcraft. Andrew.
What I will begin to do for now is to re-stuff the wax capacitors, as already some have begun to overheat and leak warm molten wax even from this short period of having the set powered. It will be easier to do this with the CRT removed and the chassis inverted. Furthermore, I will probably remove the LOPT so to avoid the risk of any oil leaking. It is going to have to come out anyway.
To understand the black art of electronics is to understand witchcraft. Andrew.
This morning I removed the CRT. This will make it easier to work on the chassis, re the wax capacitors. At the same time the LOPT can was also removed, making careful notes on where the various connections all go. The CRT and LOPT will pe put out of the way for the time being. The rubber seal of the LOPT is in excellent condition, shame the EHT stage within is not in such a healthy condition.
To understand the black art of electronics is to understand witchcraft. Andrew.
Lots of excellent advice from Trevor.
The Murphy sets can give excellent pictures, by 1959/60 many of the makers were producing fine 405 line sets.
Frank
The Mazda U25 has a 2V filament so a 6.3V EY51 is not a substitute.
I didn't say it was - I was merely musing aloud about wire-ended EHT rectifiers.
I didn't say you did but thought it would be useful to emphasise it in case anybody else jumped to the wrong conclusion. ?
When all else fails, read the instructions
One thing that I have found with the LOPT is that there is approx. 10 meg resistance between ground and all the other terminals, except the low resistance feedback winding which has one end at ground, and the EHT connection which is of course infinate resistance.
This can only mean that there is internal leakage to possibly the grounded core of the LOPT, or the earthy feedback winding. The individual resistances of the various tappings of the winding are normal, but of course I can't check the resistance of the overwind until I open the can.
Ths could explain why the boost voltage was down to about 275 volts instead of 500. According to the circuit, there should be nothing to cause a 10 meg resistance to appear between ground and the other high voltage windings.
To understand the black art of electronics is to understand witchcraft. Andrew.
Late last night I took the plunge and opened up the LOPT can. After gently lifting up the lip around the top of the can, then removing the little drain plug, I was able to dispense most of the oil into a jug. I then carefully lifted out the actual LOPT and poured out the remaining oil. The oil looks nice and clean, in the jug there are the little sleeves that go over the 8 corner lugs of the frame of the transformer.
My first check was to see if there were any burn marks as would be left by arcing. Nothing bad was found. I then lifted a connection to the U25 heater and found that the heater was intact. The resistor in the heater circuit is in fact a small coil of wire. Nothing would appear very much wrong here either. The overwind is 50 ohms as it should be, according to the circuit.
What is interesting though is that after last night when I measured 10 meg or so between ground and top cap connections, today it is in the order of 50 meg ohms. Now, it has been very hot today and low humidity in the workshop. I am wondering if there is a case of moisture ingress in the oil. Hard to imagine as the seals were very good. But even so, there must be something to cause that resistance between the windings and grounded LOPT core.
To understand the black art of electronics is to understand witchcraft. Andrew.
It looks brand new.
Frank
My plan now is to warm the LOPT for a few days to allow any moisture to escape. I can then test again and if ok, the LOPT may be re-installed in it's can and oil for testing. However, before that, I might try an insulation test to be sure of no leakage. If this proves no good, then I might have to dismantle the LOPT and examine the coil to bobbin insulation material.
To understand the black art of electronics is to understand witchcraft. Andrew.
You know what will really bug me is that if it was only moisture inside the can, could I simply just have removed the vent plug and heated the whole thing without needing to open the can? Would that be sufficient to drive off any internal moisture I wonder? Hmmm.
To understand the black art of electronics is to understand witchcraft. Andrew.
The issue of leakage may not be within the windings to bobbin insulation. I can measure a resistance of about 75 megs between just ONE unused brass eyelet to the bobbin (by my thumb in the second photo). It looks like the fibre material itself that is the problem. I will bake the assembly in the oven at about 60 degrees overnight and see if any improvement can be made.
To understand the black art of electronics is to understand witchcraft. Andrew.
The results of leaving the LOPT overnight in the oven at 60 degrees would seem to be very good. I can measure no resistance between all of the previously mentioned areas and a 500v insulation test indicates no leakage either. I have replaced the U25, but will be baking the LOPT again at 60 degrees for several days to be sure of no moisture. I plan also to put the oil in the oven along side the LOPT, to be sure of reducing any moisture content within the oil. Whether the oil is holding moisture I don't know, but it won't do any harm to be sure.
To understand the black art of electronics is to understand witchcraft. Andrew.
Obviously the oil must have a moisture content otherwise it surely would have not made it into the fibre material of the LOPT support. Now, the LOPT can was sealed.....but how well? Unless the moisture was already in there, the only way for it to get in is through the top. Clearly the can was sealed well enough to prevent oil from leaking, but of course it surely is not totally air-tight.
To understand the black art of electronics is to understand witchcraft. Andrew.
You know what will really bug me is that if it was only moisture inside the can, could I simply just have removed the vent plug and heated the whole thing without needing to open the can? Would that be sufficient to drive off any internal moisture I wonder? Hmmm.
I think I can answer "no" to my own question. The problem I see is that the base of the small vent tube sits below the level of the oil. Any moisture driven off would likely collect in the small void between the oil and the rubber top. In time, the same problem of leakage would occur.
To understand the black art of electronics is to understand witchcraft. Andrew.
The resistor in the heater circuit is in fact a small coil of wire. Nothing would appear very much wrong here either.
Well now, that is not strictly true. Even though the small coil had a resistance of around 26 ohms, this is not correct. I assumed it was ok, but as a precautionary check, I examined the coil more carefully and as soon as I touched it, the strand you see extreme right of below photo broke away at the coil. I unwound the coil and re-made it. A simple task as there were just a few turns and apart from the break, the wire is in good condition. This time, there is about 5 ohms resistance of the coil.
This could have been the real reason for no EHT as of course the U25 heater probably was not lit.
To understand the black art of electronics is to understand witchcraft. Andrew.
I will pick this up again in a week's time as now off on holiday to the Suffolk coast. ?
To understand the black art of electronics is to understand witchcraft. Andrew.
Just before I went on holiday, I did briefly put the LOPT back into the oil and connect it to the set. I had good EHT and a raster on the screen. I did not test the LOPT dry as I thought that it could arc due to not being immersed in the oil. Some high voltage connections are quite physically close to grounded areas.
This afternoon I spent time properly re-installing the LOPT into it's can. I poured in most of the oil and added the remainder gradually to top it off after I sealed the edge with a silicone compound that is used for making gaskets in engines. I then put a U shaped rubber edge protector around the outside of the top can edge. It looks good and I am pleased with the result.
I used an improvised funnel with a little bit of tubing to add the last of the oil and squeezed the rubber top of the LOPT to expel air bubbles. It would seem to be well sealed.
To understand the black art of electronics is to understand witchcraft. Andrew.
Looks very neat! I'll probably have this to look forward to when I finally have a go at my similar Murphy.
Regards,
Lloyd
Thanks Lloyd ?
It has not been easy and time will tell of course as to how successful it has been.
So....time to fit back into the set. I found this easier with the chassis overhanging the bench so the LOPT could be offered up from below. It was straight forward enough to wiggle all the wires through and secure the can to the chassis. I re-fitted the anode cap as this had to be removed to allow the EHT lead to pass through a hole in the line stage sheild. I made sure all the leads were soldered in their correct positions and powered up the set. A raster came up together with a nice bit of static on the screen face. Approx. 15 KV of EHT is now available ! A flick over to CH9 and we have an image. I needed to adjust the ion-trap magnet and picture centering contol plus height, contrast etc, but the results seem pretty good.
To understand the black art of electronics is to understand witchcraft. Andrew.
