[R-390] R390A- Intermittent sensitivity variation

[rbethman]

Eric,

Don't celebrate too soon.

You may have simply fixed things inadvertently by the de-soldering and 
re-soldering of the two transformers.

You won't know for sure.  It may be working without an issue of the 
silver mica capacitors.

Time will tell.

Bob - N0DGN

On 5/22/2012 9:30 AM, Eric Gauja wrote:
> Dear Group,
>                          An update:   Thanks to all those who made helpful
> suggestions -  it seems that my problem was in fact caused by aged silver
> mica capacitors.  After checking the bandswitch operation and cleaning of
> contacts, the intermittent behaviour was still present.  After reading David
> Wise's comments, I looked more closely at the circuitry  around the 16-32MHz
> RF transformer stages, and noticed that of the transformers T206,  Z206-1
> and Z206-2, only  Z206-1 has mica capacitors connected to the B+ line.  My
> hunch was that it was more likely that the Z206-1 caps had problems with
> value change or breakdown with B+ voltage stress..  Since Z206-1 and Z206-2
> have identical capacitors and coils , I decided to swap over these
> transformers.  After realignment at 17.6 and 30.4 MHz,   sensitivity was
> restored , and no more intermittent behaviour!   I'll see what happens over
> time - for a proper fix, I will replace the mica capacitors in these
> transformers with newly manufactured silver micas or ceramic capacitors at a
> future date.   I'm also slowly working my way through past posts , and
> problems with the old silver mica capacitors are well documented.
> Heartfelt thanks to all involved in archiving and maintaining this
> information!
>
> Best Wishes,
>                         Eric.
>