A smear of Vaseline?

The amount of moisture in the atmosphere at the time of sealing an enclosure, will contribute to the condensation on temperature cycling within the enclosure; close up when the weather is warm. Also, try using a "getter" as used commonly in instrumentation cases. Alternatively, sealing is ineffective causing "breathing".  

Jaymack

The amount of moisture in the atmosphere at the time of sealing an enclosure, will contribute to the condensation on temperature cycling within the enclosure; close up when the weather is warm. Also, try using a "getter" as used commonly in instrumentation cases. Alternatively, sealing is ineffective causing "breathing".  

Jaymack

My experience of antenna housings outside with RF electronics in is that the only things with any  control penetrations you can make truly sealed are ones with an elastic wall that moves to allow the air inside to expand and contract without actually sucking water in past the seals. But in almost all cases this is impractical, and the next best thing is to arrange things so there is an outer protection like a hat or an overhang so that rain is thrown far off, and then a box nominally sealed except where it is vented at the bottom, but with a slug of fibreglass or similar in the vent tube  to prevent ants setting up home.

Anything to be buried or where water coverage is expected, to be potted or gel packed. Desiccant will work in a well sealed box (dual O-rings sort of level of well sealed with a test port to pump it up and down and a pressure test certificate. ) but you need an program of regularly inspecting and replacing or drying it out when it gets tired.

Really expensive things that must be kept working get a humidity sensor and a temperature sensor inside as part of the design, and it calls home when it it detects the onset of problem. And all PCBs get a varnish-like  concoat.

M

Back in the 1980s I was appointed to mv City of Durban a container-ship 

https://www.shipspotting.com/photos/3004847 for a 2-year period. The plan was 12 weeks on, 12 weeks off. In total, I was on that ship for 12 years on all her various re-names: Portland Bay, City of Durban, ACT8, Pegasus Bay.

Besides the conventional radio station, MF/HF WT & RT 1.5kW a compact HF transceiver for Radiotelex was also fitted circa 1985 rather than using the main equipment which was not too user friendly. The transceiver was a Skanti TRP-750 750 watts fully solid state with an auto-tuning ATU. When it was installed I was critical of the weatherproofness of the ATU https://i.ebayimg.com/images/g/vroAAOSwCFRi1~Xk/s-l500.jpg I am not certain that there were the two centre screws top & bottom of the case. I was assured by the installing engineer that there would be no problems..

Anyway, crossing the Tasman between Botany Bay (Australia) and Auckland (NZ) we had quite a thunderstorm. The next morning no reception of Sydney Radio/VIS. The ATU was on the monkey island, open up the ATU and from the tide mark could see that it had been about 4 inches deep in water. Due wind and sea spray, not just rain water.... I removed the internals - all on one PCB rinsed it off with fresh water and found a warm place for it to dry off.

I could see that the trifilar wound toroidal receiver matching transformer - top RH by the three reed relays was charred.

https://www.qsl.net/p/pe2jeb//foto/Skanti_HF_TRP-8000/TRP-8750/640%20antenna%20tuning%20unit.pdf

(The aerial was an 8 metre whip - not the Dieckmann & Klapper MAS 140 aerial, a cage vertical with top whip, you can see in the ship photo) While the pcb was drying out, found some thin wire and rewound the toroid and was back in business.

The bos'n found me some Denso Tape which was tacky enough to stick all the way around the ATU and prevented future water ingress.

The Skanti transceiver was a useful piece of kit. It covered 1.6 to 30 MHz and besides full power at 750 watts could also be used down to around 10 watts.  Playing around, I could communicate with BT's Portishead Radio (Rx)/Rugby Radio (Tx) on 12 MHz from Cape Horn just using 10 watts.

Clive

That was a good read, and the linked schematics the AATU are interesting - monitoring antenna amps as well as forward and reverse power is a nice way to detect the condition where it finds a  match where all the power goes into setting fire to the  tuner itself (ah yes.. it really can happen) Glad you were able to jury rig a new inductor and keep it going.

Mike.