[Premium-Rx] WJ URR-74 (WJ-8718A) issue

[sm0aom at telia.com]

Your two questions:
The Nicad battery is for keeping the last settings of the front panel,

Without it,the frequency will be a random setting whenever the receiver is turned on.
It may be time-consuming to enter the frequency again as there is no keypad.

The 20 kHz spurious response is a consequence of the frequency generation and mixing schemes in the receiver.

There are two synthesisers in the receiver; one that provides the LO signal for the first mixer in 10 kHz steps generated by dividing a 40 kHz step signal by 4,
and another for the second mixer that interpolates between the 10 kHz steps in 10 or 1 Hz steps.

These synthesisers have a finite reference sideband suppression, so there are responses at every 10 kHz around the center frequency. In your case,it appears that a 20 kHz separated sideband is strong enough to create a response.
The reason that the response disappears when the frequency is changed by 10 Hz is that at the limits of the second LO synthesiser tuning range, when the 10 kHz figure is changed the frequency of the first LO is changed by 10 kHz, and the conditions for receiving the spurious responses are not fulfilled any longer.

These forms of responses may point to an alignment or component ageing condition in the first LO synthesiser, when the reference sideband suppression has detoriated.

It may also be a signal leakage condition between the first and second IF parts of the receiver.

The proper way of investigating these kinds of problems is with a very clean signal generator and a spectrum analyser.

73/
Karl-Arne
SM0AOM

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Från : fintronicsusa at gmail.com
Datum : 2017-02-11 - 17:17 (UTC)
Till : premium-rx at mailman.qth.net
Ämne : [Premium-Rx] WJ URR-74 (WJ-8718A) issue

Reveived my  URR-74 last night.. what a beautiful receiver!. Just about
mint condition, incredible internal construction... While inspecting the
inside, I noticed one of those nicad battery modules soldered on to one of
the plug-in PCB's. The 4 pins were green with corrosion as were the pads on
the solder side of the board. Quite the pain in the butt to remove and I
spent quite a bit of time cleaning up the mess on both sides of the board
but now the battery module is removed and the board is cleaned up with no
open or corroded traces.

Since this receiver has no microprocessor, I'm wondering what the heck a
battery backup/module would be used for? There are no memories, etc...
Figured I'd power it up without a battery and see what happens.

So on to powering it up....It didn't come with a power cord so I powered it
up with a line cord/alligator clips to that 3 pin rear panel connector.
Headphones, outside wire antenna... fired it up... It's working! ....
Switches and pots are very dirty but I'm hearing signals, all modes, all
filter positions working... but then.....

I noticed the strangest thing while tuning across some SW and AM BCB
signals. While tuned to an AM BCB station on 840khz, I noticed what appears
to be an image of that signal exactly 20khz down from 840khz.... and here's
where it gets strange... While tuned to the image signal at exactly 820khz,
if the tuning is moved even 10hz above 820khz the image signal disappears!
completely!.... but if tuned at or below the image at 820khz, the signal is
there, as if only receiving one sideband of the AM signal... This happens
on all AM signals regardless of band or frequency....

What the heck?... could this be related to the missing battery? (doubtful),
.... I did notice when I had the receiver open that some of the SMA coax
interconnect cables were loose... ... maybe this is a problem with
frequency data to the synth getting corrupted?.... don't know...

Any ideas? where to start looking?.

Thanks...

Glenn
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