Air Ministry type 234A Power supply

[bill knox]

Hi all

I have managed to get hold off a type 234A power supply.

The problem that I have with it is the switch below the meter, it reads mains supply and the HT output voltage, it is at rest in the centre position and spring loaded and when pressed to the right it reads the HT output voltage, the problem is, that the switch should not be able to be moved to the left hand side, but my one does move about 50% of the way, in rather a floppy manner, I have tried to remove the round knob off the front of the switch but I cannot move the grub screw, at first I thought it was rusted on but I now think that it has some form of screw lock on the threads.
Would anyone know what I can use to soften the screw lock compound, bearing in mind that its been there approx. 60 years, I have tried WD40 and a bit of heat from a soldering iron but to no avail.
As a short term "bodge" I have Araldited a screw in the slot to prevent the lever being accidently pushed to the left.

Regards

Bill

[G3VKM_Roger]

Hi Bill,

Sometimes WW2 era gear had a shellac coating on screws etc to stop them coming loose and I have found that methylated spirits will loosen the gluey properties.

HTH,

Roger

[bill knox]

Hi Roger

Thanks for the reply.

The "gluey" mess over the screws is not gluey, it is solid as a rock, but bearing in mind that it's been on there for 60 years or so, that maybe how it sets, but I will get hold of some Meths and give that a go, I will apply a layer or two and see what happens.
I don't really mind if the screws snaps on the switch mounting but there is no way the grub screw holding the round knob on would snap and I am thinking that possibly the only way I would be able to remove the switch would be to break the knob up, file the grub screw down, then assuming that I can remove/snap the mounting screws get the switch out and attempt to repair the switch itself, but to obtain another knob is, I feel going to be downright impossible.

Regards

Bill

[G4XWDJim]

Bill,

If I remember correctly the knob on the switch is made from Bakelite and cylindrical, about 10mm diameter rather like the aural sense switch on the R1155. The screw in it will be 4 BA but hardened as all proper grub screws are so not a prospect for drilling out.
I've usually found that if I dribble oil into the grub screw hole and leave it for a good few hours, then heat the knob with a hot air gun or simply place a bench lamp with a 60 W bulb in close proximity to it so that it becomes just too hot to touch, the expansion created will allow the grub screw to be loosened. It's important that the screwdriver you use is in good condition, preferably freshly ground so that it's a good fit.

The screws holding the switch body in place will be 6BA. As a last resort with a well fitting nut driver they're usually easy to over tighten and shear off.

This gluey stuff you mention is usually shellac and large lumps of it will respond to being broken by pressure from pliers if you can gain access. If you can slip a small piece of rubber sleeving over the nut you can then fill the little tube created with meths. Although it evaporates very quickly the larger quantity in the tube works more efficiently than simply painting it on.

Good luck,

jim

[Keith]

Hi Bill,

If the shaft that the knob is on doesn't have a flat, it may be possible to "screw" the knob off by twisting against the stop and simutaneously pulling outwards. I have managed this in the past and, once off, it can be put in a vice and the grub screw accessed more easily.

[bill knox]

Hi Keith

I think the shaft is flat not round, the shaft has a hole in it, the knob is slid on and the grub screw done up.
I have trying to get time to get some meths, when I do I will be having another go at it.
As I said a flat shaft gives no chance to twist the knob off.
Many thanks for all the replies, please, keep them coming, hopefully something will work.

Regards

Bill

[bill knox]

Hi Jim

That sounds quite possible, I have a good heat gun and when I have finished this reply I am going into the garage to start the oil dribble.

I will keep all informed.

Regards

Bill

[Ed_Dinning]

Hi Bill, a soldering iron, well tinned and a good blob of solder on it applied to the offending screw will often release it. This works on a smaller area than a heat gun so less potential heat damage.

Ed

[bill knox]

Hi Jim

I can only say YIPEE, instead of just dribbling oil into the screw, I found a small container ( actually, one of the containers that come with a bottle of medicine, about an inch diameter and about an inch high, I filled it with 3 in 1 for about 36 hours and then applied the heat gun and "voila" the knob is off.
The two screws that held the switch to the front panel, I could not shift by using the heat gun as I could not quite get to it, so I carefully drilled them out, as soon as I turned the switch over I could see what was wrong, the metal circular plate on the back that has "2 fingers" on that mechanically stop the switch from moving to far, had obviously been knocked sometime in the past and it had come adrift.
I repositioned it in the correct place and soldered it back into place to hold everything in place.
I don't know if it will hold, only time will tell.

Thanks once again for yours and every bodies help, it was all gratefully appreciated.

A very merry Christmas and a prosperous New year to every one.

Regards

Bill KNOX

[G4XWDJim]

Good news Bill. A relief at last I imagine.

Jim