[R-390] deaf below 8MHz (R-390A)

[atfu]

Dear Friends,

recently an exceptionally good looking R-390A came my way. It even
performed reasonably well on all bands but I knew, before I bought it,
that the radio would require a little overhauling and care.

So I cleaned contacts and recapped the AF and IF decks. Then, within a
few days, I had this problem:

                        nil < 8 =< fb

In words: The radio wouldn't even play the calibration signal below 8
MHz, but from 8 upwards, all was alright. Browsing the net, I found that
this is a not so uncommon problem.

I first suspected some mishap during recapping but couldn't find any
fault. At this point I needed help. Larry, well-known list contributor,
was very kind in steering me through the rest of the trouble-shooting.

Obvious suspects were the bandswitch S208, the 1st mixer, the crystal
Y201, the connection P221/J221. All turned out to be OK. Ground checks
showed no shorts in B+. But values at F102 and F103, and at E601 were so
much down that the mixer tube couldn't possibly do its job.

Checking the pwr supply quickly uncovered the fault: one of the
rectifier tubes had a bad filament (didn't heat up, no glow). A very
simple solution to the problem; perhaps never mentioned in the net
because it is so unspectacular or because there are so few rectifier
tubes left (see below).

Rest of the story: 26Z5 are virtually unobtainable in Europe, expensive
in the US and the shipping costs prohibitive. So Field Change 6 (diode
rectifiers) and a resistor (220R/11W was at hand) to drop the voltage
were in order. As I was at it, I also inserted an NTC-thermistor for
soft-starting. After a re-alignment I am the happy owner of a beautiful
radio that I now understand a little better.  I almost look fwd to the
next problem ;-) Many thanks agn, Larry!

Regards,
Andre