Attenuator switch repair

[tkamiya]

I'm not sure if this topic is better suited in amateur radio forum or repair forum.

Over the last few years, I've been repairing over a dozen of 1980 to early 2000 HF radios, mostly Kenwood and Yaesu.  Oldest if FR/FL101.  Newest is TS940S.  They all have a common fault.  RF attenuators switch!  As you know, typically a rotary switch is located right after the T/R relay and before front end.  It switches in and out various register networks.  Incoming power is off the antenna so it is extremely weak.

Most switches have sufficient voltage across terminal so self-cleaning action happens but not here.  Deoxit or similar contact cleaner and repeated operation often fixes it but it's only temporary.  New exact replacements are long out of production.  I thought of using mercury wetted relay or something but I'd very much like NOT to modify the circuit. 

Have anyone seen isssue?  Have anyone solved it? 

To me, the band is unusually quiet means RF ATT switch is oxidized again.

[D Straney]

If it is definitely oxidation and not dust etc. getting into the contacts...

Going for minimal modifications, I wonder if a bit of DC (a volt or less maybe) through an RF choke on the switch's common terminal would give enough of a quick current spike to be useful.  This depends on the following stages being AC-coupled up-front though (probably need a large resistor from each output terminal to gnd just to bleed off the series caps), and the front-end being able to tolerate a large-enough temporary spike to be useful.  Or as an alternative, if  the downstream stages have a DC path to ground but there's some capacitance on the common side of the switch, an RF choke/1M resistor can just charge up the common-side capacitance for a nice discharge when you rotate the switch.

Or since it sounds like the switch contacts are sitting out in the open, maybe there's a decent way to remove the switch without too much trouble, and electro-plate the contacts with gold?

[tkamiya]

Thank you for your suggestion.  Attenuators are between TX/RX relay and first input stage, so it is possible to add a capacitor if it is not already there and apply some bias current.  Gold plating is a good idea though if I can find a place that can do it.  I think gold plating uses potassium cyanid, so not something I'd want to do at home....