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

[Larry H]

Hi Franco,  good work on your rx.  It sounds like you're using the J103 (big C (whip) connector) as input.  This way you will see some difference in ant trim setting when switching from cal to agc (the whip ant is switched in).  If you are using a very high impedance and very low capacitance input, that is ok.  If you are using a low impedance input (50 - 200 ohms), then you should use the balanced twinax input as Roger referred to.
Anyhow, on your changing sensitivity on warm up, if you are not experiencing this variation in calibrate, it could be the ant relay contacts.  Try measuring their resistance.  If you are, it could be many, many things.  As Roger suggests, start with the IF deck.
Regards, Larry
      From: Pierfrancesco Mengacci <pf.mengacci at outlook.it>
 To: r-390 at mailman.qth.net 
 Sent: Friday, May 1, 2015 4:46 AM
 Subject: [R-390] R390-A/URR antenna trimmer question
   
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

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