CT212 sig gen desiccator reactivation

[ronbryan]

This Admiralty spec AM/FM signal generator is hermetically sealed and contains a desiccator cartridge that screws in from the outside. I've recently fixed a couple of faults on it and after putting it all back together I thought I would dry out the A.P.61043 air drier cartridge and put it back into service. The equipment has been indoors for 6 months, so has been stored in dry conditions.

I removed the cartridge, taped over the hole and dried it out for an hour at 150 degrees C in our gas oven. This turned the end disc blue, so I refitted it into the generator (whilst still warm) and used it. After a few days it turned pink again, so I repeated the drying process, this time refitting the cartridge whilst both it and the equipment was still warm. Again it went pink after a few days.

The hermetic seal is ok, as you can feel pressure being released if you remove the cartridge when the sig gen is hot, so I presume the desiccator is not properly dried out, rather than more damp air is getting in. I'm now trying Igranic's suggestion of leaving the cartridge on top of the c/h boiler for a few days, but wonder what forum members think about the following questions.

Is it a bad idea to use a gas oven to dry out a desiccator cartridge (maybe the heat is too damp)?

Was an hour far too short a drying period, even though the disc went blue?

Do desiccators eventually become worn out, so that they have limited capacity and need re-charging or replacement?

Does anybody know if replacement A.P. 61043 cartridges are available?

Ron

[Leon Crampin]

Ron, one problem with using a gas oven is that a considerable amount of water is produced when gas is burned. The levels of humidity inside a gas oven are high - this can have an adverse effect on pastry and other dishes which explains why electric ovens are preferred by some users.

I would guess that for your job, you would need to dry the desiccator and the whole instrument in a warm and really dry place. It might take quite a long time. Depending on domestic usage, an airing cupboard might do for the uncased instrument, but I think you might need dry heat at a higher temperature for the desiccator.

I once had a signal generator with such a unit fitted and never ever managed to get the desiccator to stay green. As it wasn't a very good generator, I didn't persevere.

Leon.

[medge799]

Hi,

Dunno if this helps much, but Farnell Ireland do desiccator cartridges which might fit. I googled a bit and found a data sheet (see here http://www.farnell.com/datasheets/302226.pdf). Interstingly this implies that reactivation IS possible : '4 hours in a ventilated oven at 120degC'. I would do this when your spouse is out, though

Ta,
Mark

[Hunts smoothing bomb]

Hi Ron,

Personally I would put the sig gen and desiccator (removerd from the instrument with the hole in the panel unblocked) in a room with a dehumidifier running and seal all door and window cracks up with tape. Position the desiccator so you can view it from an outside window so you can monitor it's status, once turned blue then leave for say another two or three days for the instrument to really dry out.

I don't know if it will work but I would certainly try it.

Cheers

[Hunts smoothing bomb]

One prob with just getting a new cartridge is that the original is already soaked up to capacity which also means the moisture is in the machine to the same level also, I don't think your desiccator is kaput, it is simply drawing the moisture out of the machine.

Cheers

[Sean Williams]

Put a couple of bags of silica gel inside the case for a couple of weeks, and leave the thing powered up with a vent open?

[Top Cap]

its usual to fill the instrument with Nitrogen (does not support moisture) before fitting the desiccator.
Les

[medge799]

Originally Posted by Top Cap

its usual to fill the instrument with Nitrogen (does not support moisture) before fitting the desiccator.
Les

Interesting; nearly 30 years ago I used to work for Racal making military radios. When the radio came off the production line a pressure line fitting would be screwed into the desiccator hole, a low-pressure airline attached and the whole thing was dunked in a tank of water to check for leaks. When done, the desiccator cartridge would be screwed in. Is the nitrogen fill a more recent technique, I wonder?

Ta,
Mark

[Stockden]

I can't say that I ever encountered nitrogen filling of equipment during my military service. Certainly none of the technical workshops in the units I served with were equipped to fill equipment (Larkspur, Clansman or Ptarmigan) with any gas. Somewhere I have a copy of the Army technical manual (EMER) for the CT-212. If I can find it I'll see if it has anything to say on the matter.

As to drying out of desiccators I seem to remember that the EMER for the Wireless Set No.88 had a diagram, which detailed local manufacture of an oven for drying both equipment and desiccator. The oven essentially consisted of a large metal box containing a number (6?) of electric light bulbs as the heating element. Sadly I no longer have a copy of that document.

Hugh

[rectifier]

We use nitrogen fill with silica bags at work. I don't think it has to be nitrogen - any bottled gas will be dry - but oxygen would be a really bad choice if you are trying to prevent rust.
I also agree about gas ovens - they have very high humidity compared to electric ovens.

Andy.

[Stockden]

Well, I've located the EMER and this is what it has to say:

1. The equipment is of the sealed type and should not be removed from its case unless Oven, Drying, Telecommunications Equipment and Apparatus, Seal Testing are available.

2. (not relevant)

3. Prior to repairs and resealing the equipment should be thoroughly dried out in the oven unit of the drying equipment. When time permits, the equipment together with the case should be left in the oven for at least two hours. On removal from the oven the equipment should be sealed without delay and tests carried out as detailed in para. 4.

4. After resealing, the equipment should be tested for leakage by raising the internal air pressure to 10lb./sq. in. using the Apparatus seal testing. Immersed in water no air bubbles should be visible. In accordance with EMER TELS M631 the time constant shall be at least 100 hours with a starting pressure of 5 lb./sq. in. Dry air drawn from the appropriate outlet of the drying apparatus should be used for these tests in all cases when the ambient humidity is high. After satisfactory completion of these tests an active desiccator should be fitted.


Hugh

[ronbryan]

Thank you everyone for your opinions, advice and EMER document research.

I'm now drying the desiccator with dry heat and in the absence of any space in the airing cupboard, have warmed up the equipment with the desiccator removed. I'll try refitting the desiccator once more before I try other ideas such as adding additional desiccator bags, because dismantling the case and reassembling is a bit of a pain.

Thanks again.

Ron

[jimmc101]

Get a large plastic container, something like a collapsible water carrier with a tap would be ideal.
Fill with as much silica gel as you can lay your hands on but leave plenty of air in it.
Stick a bung in in the desiccator hole of the sig-gen and connect to plastic container using a length of polythene tube.

Squeeze plastic container, some dry air will be forced into sig-gen; stop squeezing, some moist air will return to plastic container.
Pause to allow this air to dry then repeat until bored rigid.

Sig-gen will now contain dry air, replace desiccator and you're done.

This is a DIY version of the method we use at work, there the pressure cycling is automatic and continues until specified dew point is reached.

Jim

[amp_mangler]

Do you have a " paper cupboard" ?. Its the one used for keeping laser printer paper dry.
all it is, is a large resistor connected across the mains. ( I dont know the values unless I remove the element from my paper cupboard) Nitrogen is used quite a lot in modern ( and not so modern) equipment. I used work for CSIRO and all the LNB,s were filled with liquid nitrogen, but that was to keep the temperature stable, rather than keep them dry. Thats because radio telescopes work on colour temperature rather than actual "radio" signals

Joe

[DAVEHALL]

Originally Posted by Stockden

I can't say that I ever encountered nitrogen filling of equipment during my military service. Certainly none of the technical workshops in the units I served with were equipped to fill equipment (Larkspur, Clansman or Ptarmigan) with any gas. Somewhere I have a copy of the Army technical manual (EMER) for the CT-212. If I can find it I'll see if it has anything to say on the matter.
Hugh

Similarly on the 353( Clansman) production - pressure test ( from memory) was to pressurise set ,leave hose connected and dunk in a water tank .No bubbles -remove and seal .And seem to remember it was only just dry air.