[R-390] Low sensitivity on 7-8 Mhz band

[Tim Shoppa]

"Bill Feldmann" <n6py at qnet.com> wrote:
> However I'm now beginning to notice after the receiver has run a few hours 
> and fully warmed up, the sensitivity drops 15 to 20db on the carrier meter 
> for a 100uv signal on all or most of the bands below 8mc where it is triple 
> conversion.  So there is most likely some bad caps or other components under 
> the RF deck, most likely in the first mixer and oscillator circuit.
>
> I'm not looking forward to pulling the RF deck but after reading my 2K 
> manual copy doesn't look too bad.  I wonder if there is a list of bad caps 
> or components on the RF deck published anywhere to help me be sure the RF 
> module is in good shape after I work on it?  Don't want to have to pull it a 
> second time.

My suspicion: leaky ceramic cap shifting the DC operating point of
some tube in the first mixer/oscillator and drastically reducing your
gain. Or... carbon resistor opening up after running warm for a while
(also shifting the DC point of a mixer).

If you can measure the voltages around the tubes a few minutes after
turn-on and compare this with readings after the sensitivity has
dropped, this will really reduce your need to "shotgun" all the RF
deck caps/resistors.

Ceramic caps do not OFTEN go leaky but there's a lot of them (mostly
cathode and screen bypasses, 0.005mf) in the RF deck. Resistors
can be flaky when warm too. There are some brown beauties in the
RF deck: two for bypassing the crystal oven heater (these are entirely
non-critical unless they're arcing over or something) and one that
bypasses the RF-IF B+ line. It's unlikely that your specific problem
is one of the brown beauties.

Lots of other parts can go leaky when warm: tube sockets, standoffs,
coil forms, etc. But find the voltage that's shifting first, before
you tear off the RF deck.

It could also be that nothing is leaky but you just have a bad
trimmer or low-value ceramic tuning capacitor opening up when warm
and giving you heartache.

> I'm new to the R-390A, had more experience with my earlier Collins non-A 
> which had all good caps in it from the factory.  Surprised I found so many 
> bad ones in the later Stewart Warner and no sign of any bad ones in the 
> non-A Collins

While in the RF deck, look for carbon resistors that have drifted
up in value too.

Tim.