[R-390] R-390A restoration Questions

[John Kolb]

While I would be happy to sell you a replacement filter  :)
<http://jlkolb.cts.com/site/fs_mf.htm>
there are other possibilities - there could be problems with
the bandwidth switch or  the shunt  capacitors, either fixed or
variable.

Measure across the terminals at each end of the filter with an
ohmmeter. The two coils should measure between 40 and 60 ohms.
None of the 4 terminals should read zero ohms to gnd.
If the terminals read open circuit at either end, the filter
is defective and needs to be replaced. A short circuit to gnd
from any terminal might be inside the filter, in which case,
replace, but might be external also. Disconnect the wiring
to the filter and measure again.

There have been reports of foam inside the filter degrading
to cause a high insersion loss. If that's the case, I would
expect the 4 kHz position to have a much weaker signal
than the other filter positions, 30 or 40 db weaker, but
not completely dead. In this case, a few hardy souls
have managed to open the filters and clean them, restoring
them to proper operation. No reason not to try if the
filter's bad anyway.

John
http://jlkolb.cts.com

Ian Gallimore wrote:

> I am working on an R-390A which I purchased from Fair, and I wonder if I could trouble you with three questions which would help me greatly. 
>1) When I select the 4 kc bandwidth, there is no signal. There is signal at all other settings, with the results that one might expect as the bandwidth is changed. Is it correct to assume that the 4 kc filter is faulty and should be replaced?
>2) I have noticed that the RF Gain seems to function only in the last 10 degrees or so of clockwise rotation - that is, there is no signal below about 8 on the dial, and then the signal rapidly peaks as the pot is turned fully clockwise. Is this normal? If not, what should I be checking?
>3) The carrier meter seems to be at Maximum regardless of the signal. Any suggestions most welcome.
>
>Thanks for you help!
>
>Ian Gallimore
>VA3ODA
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