[R-390] R390-A/URR antenna trimmer question

[Pierfrancesco Mengacci]

Hi!

 

-I have completed the restoring of my R390-A/URR (a Stewart-Warner) and  I
succeeded in getting at least 40 db carrier meter readings on all bands,
while in calibration position.

- The fact is that I still face a certain lack of sensitivity, as I notice
when I check it through a signal injected via the antenna socket (impedance
matching, contacts, wirings, tubes, cams and coils alignments, voltages etc.
are all OK. definitely)

 

- The inconvenient (as may be observed in AGC position)  is discontinuous,
it varies slowly, while chassis temperature varies, and I can lose up to  10
db (i.e. to get a 5 volts diode load reading, in some conditions I must set
the antenna signal at 5 microvolts rms, other times only 1 microvolt is
needed for the same reading, and same frequency. I tested also the
continuity of the cable connected from the stabilized RF  generator to the
receiver; it's Ok, and no shorts nor losses)

 

- The only "a little strange" thing I notice is that the position of the
antenna trimmer for maximum reading is different when I tune it in
calibrator position (marker reading)  as compared to AGC position (signal
through the antenna).

In the 29 Mcl band I must rotate the knob of 2-3 graduations to reset the
max reading, when I change from calibrator to AGC (antenna signal).

 

I do not know whether this circumstance is relevant or not. Looking at the
diagram, it seems that no change or switching is involved in the RF and
antenna circuit, so no change of capacity or tuning should be necessary
(it's true that the antenna relay becomes disengaged in calibration
position, but its internal capacity seems irrelevant (if I disconnect  J105
from the relay, internal noise  does not varies, nor the trimmer position
for max noise). But, may be, this also  is not a relevant circumstance, if a
variation of 1st RF tube internal capacity is involved  with bias variation.


 

I'm just trying to understand where to search to remove the discontinuous
sensitivity loss, while in receiving position. After all, sensitivity is the
essential feature of the radio..

 

 

Has anyone any idea!?

 

Thanks, Franco Mengacci,

Italy