[R-390] Ceramic Disc Caps Never Fail...

[nryan at mchsi.com]

Ceramic Disc Caps Never Fail...

...What, never?  Well, hardly ever!

After doing a painstaking replacement of the usual BBODs, off spec carbon comp resistors, electrolytic caps and a couple of bandwidth filters, etc., on a 1957 vintage R-390A, I burned it in overnight.  The next day it was deaf on all bands.  Trouble seemed to be in the RF deck, but where to start?

The depot manual provides guidance by advising checking resistances and voltages.  I did the former and found pin 6 on V401 (the crystal-oscillator deck) to be way off spec.  Normal resistance is 55K, instead it read 1.43K.

Suspicion fell on two ceramic disc caps, C401 and C410 -- both are rated .005uF @ 1000V.  After pulling the RF deck and its crystal-oscillator subchassis, I clipped a lead on C401 and the resistance reading on pin 6 of V401 went up to spec.  

Access for replacement of C401 was difficult, but in the end successful as the radio came back to life.  The shaft controlling the two rotary switch discs had to come out to permit access inside.  In a careless lapse of judgement, I slid the C-clip ASIDE the shaft rather than ALONG its axis and, of course, it broke.  A replacement was handy but I resolved not to make that mistake again.

I think our beloved rigs are getting to the point where components besides paper caps, carbon comp resistors, electrolytic caps and bandwidth filters are beginning to fail.  Ceramic disc caps still are reliable, but perhaps the passing years do not bode well for them either.

Here's hoping the burn-in presently underway goes well.

73 de Norman, KG4SWM

Any activity becomes creative when the doer cares about doing it right, or doing it better. -- John Updike