Avo Multimeter 9SX Pan-climatic

[Rob Lines]

I have recently been given one of these AVOs which appears to have been stored in a very damp atmosphere for about 20 years. Despite being tropicalised it has suffered, the main damage being caused by the rubber padding in the battery compartment degrading, or melting, and giving off corrosive fumes. Some of the metal in and around the compartment has suffered, and there was some leakage into the main compartment, attacking copper parts in particular. I blame the rubber as there is no sign of any corroded battery bits in there. My problem is removing the battery compartment for proper cleaning. I've removed the front panel by removing the usual screws around the edge of the metal case, and also the two screws through the metal case hidden underneath the instruction plate. I cannot now separate the inner bakelite case from the outer metal case. Perhaps someone who has dismantled a 9SX has done this. Are the two casings meant to be separated, are they a 'jam fit', are they glued together, or is it just corrosion holding them together? I would welcome any comments or suggestions before resorting to more drastic measures.


Rob Lines

[The Philpott]

Hmm. Peter Vis might know. Also my father used a broken SX as a movement donor and still has the remains, but i can't see him getting around to digging it out and having a look for at least a couple of days

[pmmunro]

Rob,

You have followed the correct procedure for separating the metal outer case from the Bakelite inner case of a Model 9SX.

i.e. (for future reference) ;

remove 7 screws holding the front panel assembly to the inner case: 2 each top and bottom, 1 on left hand side, 2 on right hand side;

remove front panel assembly from inner case;

unscrew 2 screws holding the instruction plate to the outer metal case;

remove completely the two battery compartment lid screws;

undo two nuts with shake proof and large plain washers roughly on the "horizontal" centre line of the inner case;

It should now be possible to slide the outer case off the inner but the fit may be tight.

Mine also had been exposed to some damp but not nearly as bad as you describe. My outer casing needed some easing off the inner. I was able to press on the inner surface of the battery compartment with my fingers while holding the rim of the metal case with my thumbs just enough to start some movement. I then pulled on the lower edge of the inner case and gradually alternated between the two ends until the cases were separated.

I had the impression that there was some resistance due to minor corrosion but if your steel case is badly rusted the grip on the inner case may be much firmer, the problem being that rust has a much greater volume than the steel from which it is derived. To add to the problem, the Bakelite case is quite fragile.

It might be worth trying a lubricant such as thin oil or white spirit, possibly aided by a narrow painter's scraper slid straight down between the inner and outer case. If you decide to try this, it would probably be best to remove all sharp edges from the scraper blade to try to minimise scratching of the Bakelite. This could break the bond of rust and allow the cases to be separated.

PMM

[The Philpott]

I was going to suggest exploiting differential expansion by dunking the case in very hot water but is perhaps NOT to be an option due to potential break-down of the bakelite and liberation of it's unpleasant constituent/s. More research needed.

Nothing stopping you putting it in the freezer, taking it down to -20 then swiftly back up to a warm ambient though.

Alternatively wd40 and patience.. Last resort carefully drilling through back in places normally concealed by the instruction plate and gently tap...tap..tap.

[Rob Lines]

Thanks PMM. Now I know I'm on the right track and I haven't missed any hidden screws. I've already squirted servisol down between the two pieces and pushed down a very old very thin metal rule, both to no avail so far. The corrosion holding them together may be between the two backs which I can't reach.

Mr Philpott, Similar to your suggestion, my next attempt to separate will be to heat the metal case with a hair dryer and hope that the coefficient of expansion of the metal is greater than that of the bakelite. Drilling the case is the absolute last resort.
watch this space.

Rob.

[Boater Sam]

Is there a hole through the bakelite that you could blow compressed air into between the bakelite and steel case? If you follow my drift! Or the other way, from the steel to the bakelite?
Just blowing down the gap at the edge may produce results, its amazing what a little puff will do when nothing else works.

[Rob Lines]

After approximately 7 hours of lubrication, heating, pushing and pulling, all I had managed to achieve was 2mm of movement in a top corner reducing to 0.5mm in the other top corner and no movement at all at the bottom end. I decided that some damage would have to be acceptable if these two parts were to be separated, so I cut a 1" slot in the bottom edge of the side which was stuck. Using my scraper (thin metal rule), I then tapped out the inner box from the bottom. It came out grudgingly, a couple of mm at a time, but eventually I won. There weren't any great clods of corrosion in there as I had expected but enough traces on the side of the box to hold them together. With the battery compartment now removed, I can begin the clean up, starting with the remains of the 2 of the 6BA c/s screws holding the compartment in, which had to be drilled out. Does anybody know of a supplier of 'easy outs' which go this small? I can only find a 3mm size using interweb searches

Rob

[The Philpott]

Not sure.

When i had to drill a 6BA screw out (with a 2mm bit if i remember right) i just about got away with cutting a new thread using a steel 6BA screw as a tap.

When initially drilling the only reason i managed to get it dead straight was the tendency for the bakelite to smoke whenever the angle drifted. If brass chips kept appearing i knew it was ok to proceed

[Rob Lines]

Yes, I managed to drill down one of the screws with 2mm bit, fortunately only a little bit off centre, then used 6BA taps to finish it off. it worked reasonably well, but I still think it would have been easier if I'd ad an easy out to start with.