We were still fitting LOPTs in these into the early 70s and even the earlier V310s as they were still available from RBM. The types with the submerged U25 used in the V250 series was obsolete and these were scrapped on sight. Never opened one up but we did puncture the seal and pour out the old oil into a container, this was used on the cabinets when refurbishing them. 😱
John.
It is such a shame, once again, that something previously so readily available is no longer. But alas, there are so many things that statement can apply to.
I will eventually test the LOPT in the set first before I reach for the can-opener, regardless of if there is any measured leakage still.
To understand the black art of electronics is to understand witchcraft. Andrew.
There is also the option to substitute the LOPT if all else fails. To that end, I wonder if the LOPT from a V350 would fit the bill. The V350 uses a "dry" LOPT, which has certain advantages.
However, before exploring that route, how about draining the oil from this LOPT? It could be done by inverting the LOPT over an open container, canted slightly to place the vent hole as low as possible, then cyclically heating and cooling the LOPT canister to expand and contract the oil and/or air inside to force the oil out. Left long enough to allow the oil to drip dry from the Tx, then apply a small current through the windings to drive off any remaining moisture. Afterwards, you can refill with a new dose of water resistant oil. A hypodermic syringe can be used for that task. Some folk use veggie oil, but beware, it is water miscible. I'd favour 'transformer' dielectric oil, if you can get in small enough quantities. My own thought was to salvage the oil from an electric radiator.
Once out of the can, I wonder if the old oil could be heated sufficiently to drive off the moisture, and then be reused - I'm not sure if that's possible, but it would be interesting to find out.
Hi Marion, that is very interesting and well worth thinking about. 👍
Aside from the fault of the open heater circuit to the U25, the thing I found with the LOPT in my V280C was that the actual fibre material supporting the various internal connections to the coils was a source of voltage leakage. The bobbin with the windings were perfectly ok. Once that fibre material was warmed by around 60 degrees in the open air for a period of time, all traces of leakage vanished.
Like this LOPT, the seal was good. Even so, it seems logical to me that some moisture must have already been inside or had found it's way in over the years. I also think that with this present leakage, there is a strong possibility that EHT tracking will occur. This may of course already happened and damaged the fibre material, but I really want to try at least to remove any moisture without opening the can. The idea of draining the oil and then perhaps baking the entire assembly at 60 degrees in the oven seems a possible next step, if it will not respond to the application of current to warm it inside the oil.
As I discovered the other day, when cold the measured leakage is MUCH less than when the can is warm. I am wondering that if the material was already carbonised due to arcing internally, then the leakage would remain the same or indeed be much more severe regardless of temperature.... Your thoughts on this are most welcome.
To understand the black art of electronics is to understand witchcraft. Andrew.
On the other hand, could I be unduly concerned about this leakage and by simply connecting the LOPT back into the set and operating it, things will be perfectly fine and dandy?
Again, your thoughts will be most welcome. 😊
To understand the black art of electronics is to understand witchcraft. Andrew.
As a further matter of interest, at work I have access to a powerful X-RAY machine which is used for detecting defects in the internal tracks and surface mount components of printed circuit boards.
I am wondering if it could be used to examine this LOPT in order to detect any signs of arcing or carbonisation of the internals. I have a picture of a bad one for reference, namely that of Nick's LOPT on UKVRRR (See earlier link as I am not sure I should reproduce his photo here).
To understand the black art of electronics is to understand witchcraft. Andrew.
I took the LOPT to work today and we did x-ray it. I was able to see all of the internals, windings, wires, fixtures, fittings, eyelets etc etc but could not really make out the very material that I wanted to see if any tracking had occured on. From what was visible, there were no signs of damage. This of course does not mean there is no tracking, simply that it was not visible. So all in all, this excercise has been inconclusive.
To understand the black art of electronics is to understand witchcraft. Andrew.
One thing I did not do previously but am doing now, is to make sure that the can is tilted so that the internal base of the little filler hole is not sitting below the level of the oil. There is a short extension of this pipe into the can so when upright, it sits below the oil level. At least this way, any moisture can more easily escape. The can has been like this now for a couple of hours with current flowing through the windings to warm it. So far, no oil has leaked elsewhere around the seal which of course is now partly sitting below the oil due to the tilted can.
To understand the black art of electronics is to understand witchcraft. Andrew.
I took the LOPT to work today and we did x-ray it. I was able to see all of the internals, windings, wires, fixtures, fittings, eyelets etc etc but could not really make out the very material that I wanted to see if any tracking had occurred on.
One interesting thought comes to mind - If there had previously been any leakage, the oil would have quenched the arc, much as does the oil immersed switchgear in HV power transmission, as such, without an arc and/or the absence of oxygen, there should be no carbonisation of the insulating materials, or would there? - I would personally think not, but I'm not 100% sure, though I wonder if there might be erosion instead?
It's also quite handy now you know what lies where inside the can, and if nothing else, it helps you to determine which way to tilt and turn the can in order to drain and refill, if that becomes necessary.
Did you get any pictures of what you saw?
No pictures of it i'm afraid, there was nothing with any extra information worth noting really. There are several good pictures revealing the internals of other LOPT's already, plus I know by DC measurement (and ringing test) there are no direct shorts or opens with this one.
I am no chemist, but I would not be surprised if the oil itself has a moisture content. In fact, I would be surprised if it didn't to a certain degree. The question is, how much is tolerable? None in an ideal world of course though.
If no signs of a reduction in DC leakage will occur by heating the windings with a voltage, then I might empty a small quantity of the oil and attempt a DC leakage test on the oil itself.
If the oil alone is ok, then it may be neccessary to drain the oil completely and heat the can by external means to dry out the internals. For example, about 60 degrees in an oven. I can see no reason why this cannot be done, OK there may still be a small amount of oil left in the can, but the majority of the internal assembly will be in the air inside the can. The moisture will vent out through the small tube.
To understand the black art of electronics is to understand witchcraft. Andrew.
If there is water in the oil you have a source of oxygen that can burn.
This could well be a factor why they failed even when still in the service life of the set. Certainly it would contribute to insulation breakdown.
If the EHT cable itself formed a completed insulated loop inside the can with no exposed internal terminals, then it would be far less likely a breakdown would occur.
To understand the black art of electronics is to understand witchcraft. Andrew.
Speaking of DC leakage, I am pleased to say that the main HT capacitor can comprising C114 (200uf) and C115 (100uf) has reformed quite nicely. Ditto for C99 (32uf) in the sync seperator and line drive stage HT (left side in below pic).
The same cannot be said for C26 (16uf) in the video amp. stage HT. This started to reform but after a few minutes, the case became rather hot and leakage increased considerably. The electrolyte probably has escaped in the past with evidence around the seal (bottom in pic). It will need to be replaced.
To understand the black art of electronics is to understand witchcraft. Andrew.
It will need to be replaced.
Or to be more precise, it has been re-stuffed if you pardon the expression. I removed the innards, drilled a small hole in the bottom of the case, installed a new capacitor, filled with black wax and hey presto, a new lease of life for the old banger.
To understand the black art of electronics is to understand witchcraft. Andrew.
When I had to repair small oil filled motors, I was advised to use 3 in 1 oil to refill them. On that basis any fine clear hydrocarbon oil would seem to be OK but I would heat it to over 100 degrees to drive out any moisture first.
Boater Sam
It is essential to use the correct oil for these transformers. Anything that will attack rubber/neoprene is totally out.
To understand the black art of electronics is to understand witchcraft. Andrew.
One thing I did not do previously but am doing now, is to make sure that the can is tilted so that the internal base of the little filler hole is not sitting below the level of the oil.
Since doing this, I am pleased to discover that the DC leakage, as measured by the Megger tester, has reduced by quite a margin.
564 mega-ohms becomes 1960 mega-ohms for the EHT heater winding, 125 becomes 265 for the main winding, both with respect to ground. This was measured after the LOPT was allowed to cool down to a similar ambient temperature as when first tested.
My conclusion is that it must be moisture slowly being driven off. What else would cause the change?
To understand the black art of electronics is to understand witchcraft. Andrew.
With a regular transformer working at a few hundred volts, the above readings are much less significant. But a line output transformer with an EHT winding having exposed (in the can) terminals working at around 15KV or more, I believe it is essential to get any leakage as low as practically possible to avoid internal insulation breakdown.
This may be pretty obvious stuff, but how many transformers have been spoiled by simply just operating them in the set without attempting a period of drying first I wonder? Easily done though, especially as it might be assumed that an oil filled type would always be perfectly insulated.
To understand the black art of electronics is to understand witchcraft. Andrew.
I have decided to place the transformer complete with oil in the oven at 60 degrees. This way, I am more certain of the temperature of the thing as a whole.
You won't see this on "The Great British Bake Off" I bet.
To understand the black art of electronics is to understand witchcraft. Andrew.
