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

[Cecil]

Others can probably address the varying sensitivity issue better than I since I am at work right now and not near any of my manuals or gear.

I will comment that recently we have seen this symptom related to silver mica capacitor failures.

As for the different position of the antenna trimmer control when switching between the calibrator and the signal generator...it is probably normal due to a change in load impedance being presented to the receiver front end by the two signal sources.  I will probably be corrected by one of the folks on the list but if my memory serves me correctly the design impedance of the balanced antenna input is on the order of 120 ohms and most signal generators are 50 ohms.  The calibrator probably is not coupled at either of those impedance as it was not designed as test equipment but merely a reference so a different peak setting is probably normal and unrelated to your sensitivity problem.

Hope that helps a small amount.

Cecil
K5DL

Sent using recycled electrons.

> On May 1, 2015, at 6:46 AM, Pierfrancesco Mengacci <pf.mengacci at outlook.it> wrote:
> 
> 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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