See below ...
Mike Morris WA6ILQ wrote:
>
> At 12:07 PM 6/30/05, you wrote:
>
> >Aarggh! I've pulled out most of my hair on this one! To
> >revisit something I asked about several months ago... I'm
> >still having a problem with my UHF Micor mobile converted
> >to repeater.
> >
> >It is a T34RTA3000AA with power set at 20 watts. It is on
> >444.000 (T), 449.000 (R) and is producing a spur at
> >444.910. The spur is "clean", stable, and has good
> >modulation, sounds identical to the main carrier.
> >
> >I have retuned the exciter filter per the book several
> >times. No change.
>
> One trick that I have used... the "far end" of the preselector
> tune broader than the "near end". I've found that I get better
> performance if I do the tuning then turn it around and tune
> it again. This lets the old "far end" adjustments become
> the "near end" for the second pass through the procedure.
>
> >I swapped the KXN1024A channel element
> >for one on a different frequency (443.750T/448.750R) and
> >still had a spur 910 kHz above my carrier freq. Finally I
> >tested both channel elements in a stock (unmodified) Micor
> >and STILL have a spur 910 kHz above the carrier frequency!
> >
> >I lack equipment to accurately measure the spur power level,
> >but this spur is some 80 or so dB below the carrier level,
> >perhaps a bit more. The manual specifies spurious and
> >harmonics below 85 dB so this MAY be within spec. but why
> >would it always spur 910 kHz above carrier freq.????? This
> >is an unacceptable level as it can be heard up to 5 miles
> >from the repeater on line of sight paths (and this is
> >causing a problem for some people).
> >
> >What am I missing? Is this normal behavior for a UHF Micor?
> >I have juggled some numbers around and can't see why this
> >would always be 910 kHz above the carrier freq. Thought I
> >had a bad Micor until I found identical results in the
> >second one...
> >
> >Any ideas? Should I forget about asking why and just throw
> >cavities on the thing until I knock the spur down enough?
>
> Put a test receiver on the spur frequency, and if the test
> receiver desenses due to the true frequency then add a
> cavity in line to knock down the true frequency. Then go
> poking around in the exciter and see if you can isolate it
> to one stage. I'll bet that you find a multiplier that a slight
> touch in the adjustment kills the spur. I had a Moto 63MHT
> Motrac that had an intermittent spur on 147.775 when the
> radio was on 146.220 ... I put it on an analyzer and found
> that rocking one of the multiplier slugs just a hair cleaned
> it right up. On the test set the "clean spot" was still inside
> the top part of the peak... i.e. you'd never see a difference
> between dirty and clean on the test set.
Could be the transmitter is going into self oscillation when the
adjustment is made. Another good reason to bring your transmitters
(and receivers) into the frequency range you want to use them on.
You have a Motorola T63MHT - factory in 150.8 - 162 MHz? You want
to use it at 146.46 MHz? You really need to pad down the exciter
board to get there. Ditto the Micor exciter board in the same
frequency range.
Neil - WA6KLA
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