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THE AM BULLETIN BOARD => Technical Forum => Topic started by: WBear2GCR on February 09, 2019, 11:05:53 PM

Title: Ceramic pF Trimmers & R-390
Post by: WBear2GCR on February 09, 2019, 11:05:53 PM
Ceramic Trimmers & the R-390

(- likely applies to any rigs with these ceramic trimmers.)

Three R-390s fell out of the sky. None were in working condition. After quite a bit of effort the #1 unit was demoused and made to work. Then the plan was to determine the condition of 2 & 3.

By swapping known good modules out of #1, both 2 & 3 were made to play. Since the RF deck on #3 was very clean, but the front panel looked like graham crackers in milk (severe aluminum corrosion) the plan was to swap a fresh front panel... but. The "but" was that the chassis on the 390 does NOT have removable side panels, it's all one unit. That left making #2 "the" unit. The side panel on one side had corrosion too.

Unit #2 was fired up. All good. Except. Deaf on 4mc, 5, 6 and 7. Found the burnt up 1st can, the ANT tuning can. Pix were put up earlier. That should fix it! Nope. Cans 2 and 3 peak, but the can to the rear does not peak at all. It's one of two between 1st RF amp and 2nd RF amp, actually the 2nd.

Sherlock said, swap a can from the other deck! Swapped the can...still didn't work! Statistically what's the probability of the same can from a different deck, made by a different manufacturer in a different year not working!! Just about nil.

So, obviously there's a flaw in that RF deck that had been transmitted into enough to char that RF input coil. After all the scope and DVM checks I could figure out, the plan was to swap the RF decks. A warm 50 degree winter day meant that i could degrease the gummed up #3 gear train and swap it into #2.

After swapping it in, which went rather well, plugging in the cans, turning it on... ahhh! Nah. It still didn't work, deaf on the same bands. Believe me, everything was checked.

Last resort - took ALL the cans out of that row from the #1 unit, removed all the cans in the row from #2, and tried it again. SUCESS! But... huh? Really? So that meant, bad cans from two different RF decks!!

Later, after a whole lot of swapping of cans, testing, and the like - did I mention that I replaced the two silver micas in one of the cans? Still didn't work!

What did that leave? Yep, the TRIMMER!

I was able to dissassemble the trimmer and found that the silver had spread across the two ceramic surfaces. Contact cleaner took most of it off. I've tried an cleaned two cans. One returned to proper operation. The other had been lubed with PTFE spray lube, and did not. I may clean it again and retry.

Title: Re: Ceramic pF Trimmers & R-390
Post by: Opcom on February 10, 2019, 12:11:31 AM
Looks like someone put the RX antenna close to the TX antenna. Or had an electronic TR switch that wasn't too picky..

Title: Re: Ceramic pF Trimmers & R-390
Post by: WBear2GCR on February 10, 2019, 12:46:59 AM
Think they were testing the unit as a dummy load on 40m! :D
But, THAT wasn't the difficult problem to find or fix...

Quite an adventure...


The burnt can was the ANT, before the 1st RF.
The ones with the bad trimmer are the ones in this pic.
Between 1st and 2nd RF, then two cans out to the Mixer...

Title: Re: Ceramic pF Trimmers & R-390
Post by: KK4YY on February 10, 2019, 08:13:16 PM

This looks like a case of silver migration.

From: (

"Silver has strong tendencies to migrate, since silver oxide is water soluble. The migration of silver on electronic assemblies expresses itself in the formation of conductive paths by wandering metal ions and under the influence of humidity between traces of different potentials. Preferred direction is towards the electric field. This effect is of eminent practical importance in the case of small structures. This migration can lead to malfunction, in the worst case to the total failure of the electronics through shorts. Alloying palladium to the silver can effectively reduce if not inhibit the silver migration. Other metals like copper and tin also tend to migrate, albeit at a slower rate."


Title: Re: Ceramic pF Trimmers & R-390
Post by: WBear2GCR on February 11, 2019, 09:12:46 AM
Perhaps... but to my eye, based on the pattern, it's at least some part mechanical.
I'd guess mostly mechanical.

It may not be obvious in the images, but I can see where there is thickness of the silver reduced
in a wear pattern.

Now, how or why the trimmer would get spun around enough for that, it's unclear.
How many times does it take? 10? 25? 50? 100??

You could diddle it 10x or more doing an alignment, especially if ur having troubles.

With this in mind, I shot some "PTFE" spray lube on the surface - but that one did not
come back to "life". I was hoping it would give a very very thin slippery layer. I may have
to try it on one that is known to work... or alternately I may try cleaning the lubed up one,
and IF it works, then I know that the lube has leakage. :(

Maybe I'll try for another "teflon" lube... or just leave it alone.

Title: Re: Ceramic pF Trimmers & R-390
Post by: WD8KDG on February 11, 2019, 10:11:26 AM

Noticed the same silver smear on my R390/A's. Not uncommon to find and have to wonder if in the past someone used a drill motor and spun the trimmers for grins & giggles  ;D.

Just the same a Q-Tip with a drop of DeOxit will cure the disease.


Title: Re: Ceramic pF Trimmers & R-390
Post by: WBear2GCR on February 11, 2019, 10:28:36 PM
Yeah I was concerned about the residue from DeOxit...

But it sure shut that can down... wondering if you had similar problems?

It's unknown to me if the ceramic itself has special dielectric properties or not.
Assuming not for the moment, I wonder if it were glazed that this problem
would not manifest or not as much... but perhaps the ceramic's surface is ground
to get it flat enough.

It would be interesting to know the manufacturing process!

Title: Re: Ceramic pF Trimmers & R-390
Post by: WD8KDG on February 11, 2019, 11:48:19 PM

It has been a few years since going through the R390/A's. I seem to remember not getting a good peak on the RF cans. Just one of those things and I wondered about the trimmers. After sliding the retainer off of the shaft and getting a good eyeball of the trimmer surface it was just a shot in the dark to try DeOxit on a Q-Tip. Silver came off with ease and the peaks were better defined.

If you have an A, doesn't hurt to check the trimmers on the mechanical filters.

A bad silver mica cap in the RF cans shows up when the inductor is adjusted for a peak, almost falls out of the rack. There must be a dozen items that take hours to do, but the results are worth the effort. Done correctly once and you are good for years.

Glad I spent the time years ago, 75 pounds of receiver is now hard to lift.

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