[R-390] R-390/URR sensitivity issue

[Larry H]

Hi Don,  Nice to hear from you.  Glad all is well in Charleston.  All is
good here, now.  We had really bad smoke from the fires up north and NE.
Those poor people.  My wife has bad asthma, so the smoke really bothered
her.  It's been clear for a few days now.

Interesting problem.  Sounds like it should work.  I assume that the
adapter you built is the da-121.  That is the right one for the bal input.
You said that all is well using the unbal input.  I assume that the
secondary on the input trans tuned correctly using the unbal.  If the
correct adapter is not used for the unbal input, the readings will be
misleading.  If the unbal is fed with a low impedance source (eg: da-121),
the same level of signal may get through on all bands, even though the
circuit is not working correctly.  For the unbal, the correct adapter is
the da-124, or the 50 ohm 'impedance adapter' and a 56 pf cap in series.
IE, the unbal needs to be fed with a high impedance input so the secondary
tuning will work.

Because the primary resistance usually reads about 1 ohm, some measurements
are hard to do.  With the can in, the input should measure infinity to
ground (with no adapter plugged in to the bal) and you should measure 1 ohm
at the ant bal input between the 2 pins.

Sometimes the tuning can contacts are ok at DC, but not at RF.  Make sure
that the pins on the bottom of the cans are good and the jacks in the RF
deck for them are also good.  I've had lots of problems with both of these.

Good luck. Have fun and let me know.

Regards, Larry

On Mon, Nov 26, 2018 at 3:09 PM Don Heywood <wc4g at knology.net> wrote:

> Hello Larry, it has been a while since we communicated. I hope all is well.
> Things are going well here in Charleston.
>
> I was running some sensitivity tests on my white face meter R-390 using the
> balanced antenna input with a UG-971 adapter. I have made a resistor
> network
> to use with my URM-25. I found this network in a publication which explains
> the wiring of various receiver inputs. Anyway, I have achieved very good
> sensitivity on all bands except two. I am aligning the RF transformers with
> the ANTENNA TRIM set at zero. Also the red dot on the shaft is up
> indicating
> 1/2 mesh.
>
> I am checking sensitivity at the midway point on each band. I get 10uv
> readings on 6 megs and 12 megs, the rest of the bands are less than 1uv.
> The
> first thing I did was to swap the first transformer in both these bands 4-8
> and 8-16. I thought the input coil had been fried by a strong transmitting
> signal. I have seen this before. But different transformers made no
> difference. One last thing, if I swap the generator to the UNBALANCED WHIP
> input I get comparable good results. I also managed to drop one of the slug
> rack springs thru the hole while swapping the transformer on one band.
> (forceps slipped)
>
> So now I have the RF DECK out and I am doing a visual inspection of the
> bandswitch in the antenna input area. I don't see any problems with the
> switch leads. I realized that I could read through the transformer windings
> pins 1-2 to the coax connectors P210 & P211. These read continuity with
> short interruptions while changing the bands to get the bandswitch to move
> to the next octave. While I have the RF deck out I used DeOxit on the
> bandswitch contacts, let it work a while and again cycled the bandswitch,
> making sure to restore it to 02 000.
>
> What I am asking is for another set of eyes to try and see what else could
> be the problem. I really hate the idea of reinstalling the RF deck and
> still
> have the problem.
>
> Here is a link to the generator accessories:
>
> <http://radionerds.com/images/b/bd/TB_SIG_319.pdf>
>
> What do you think?
>
> Regards,
> Don WC4G
>
>