Comments mixed into the text below...
Mike WA6ILQ
At 07:58 PM 7/5/05, you wrote:
>Mathew,
>
>The Maggiore EV-1 is a very basic, crystal-controlled exciter that has none
>of the filters, power control, or SWR protection features of commercial
>units. I have one that I took off the air because of its tendency to
>produce spurs. It is perhaps not a good idea to use a relatively unstable
>exciter to feed a 200 watt (!) power amplifier, which itself may not have
>all of the controls and protections of a commercial amplifier. I've never
>heard of Vocom, so I don't know where it falls in the quality lineup.
>
>I find it interesting that the fundamental frequency of your exciter,
>12.1175 MHz, when mixed with your output frequency of 145.410 MHz, produces
>a product at 133.2925 MHz. This product is in the aviation band and could
>be slope-detected with ease by an AM aircraft radio on a nearby frequency.
>I'm not suggesting that this is the cause, but it's a coincidence.
Having a look at 157.5275, which is 12.1175 above his output would be
interesting.
If there is a spur there, it would be indicative of the 133mhz spur being
produced by the same mechanism.
Matt, I'd grab a scanner that had a AM aircraft mode and program up
the 133mhz channel and then rock each exciter adjustment and see if
anything affects it. I've seen spurs come and go with a 1/32 turn tweak
of a slug that was a 1/4 turn wide peak on a meter-type test set.
A bit of FM history.... (and yes, bear with me, it is relevant)
The local 146.22 / 82 repeater here in Los Angeles is W6FNO, and
is a very wide area system... it started out about 40 years ago as a single
system at a 3,800' northeast of downtown LA and later added an 8,000'
site with the original site used as a fill-in on the simulcast TX and voting
RX system.
Over 30 years ago, when Standard introduced the first japanese mobile
radios to the USA, the model 806 and 826, some of the users of the
146.82 system were being heard by the Torrance (one of the 86
communities that make up Los Angeles county) police department
dispatcher.
Fortunately, the Torrance radio tech was a user of the 82 system... it
seemed that almost every 82 user that were being heard by the Torrance
dispatcher had a brand-new Standard...
A bit of math showed where the problem was... the Standard
design used an 8mhz crystal times 18, and the first stage was
an oscillator-tripler (base circuit at 8mhz, collector at 24mhz)....
146.220 / 18 = 8.123333 crystal fundamental
8.123333 times 19 = 154.343333, and guess where Torrance PD was...
The simple cure was to take the 8mhz rocks and trash them
and insert a 12mhz rocks. The oscillator-tripler automagically
became an oscillator-doubler when a 12mhz rock was
selected.
The deviation in the Standard was created by a phase modulator
and while the dev was slightly reduced when running 12mhz
vs 8mhz it was not noticeable.
An agreement was worked out between Torrance and Standard,
and Standard provided the Torrance dispatcher with a Callbook
and a stack of prepaid postcards.
When the dispatcher heard a ham, they'd look up the call,
and send a postcard, which read (something like)
"Your radio was heard on Torrance PD dispatch frequencies.
Standard is aware of the problem and has arranged for a free fix.
Please contact (name of local popular 2-way shop #1) at (phone
number) or (name of local 2-way shop #2) at (phone number).
Present this card to either shop and they will fix the problem
at no charge to you, and tune your radio as well".
The shop basically swapped the 146.22 rock and one other
local channel that was a potential problem, adjusted TX and
RX frequency, and set deviation. Standard provided the crystals
and paid for an hour of bench time per radio.... the shop
stapled the postcards to an invoice to Standard and they paid
the bill, no questions asked.
Now Matts radio already is using 12mhz rocks so this "fix"
technique is not applicable UNLESS the first stage is
an oscillator-multiplier.
>You might check to see that the box containing the exciter
>is tightly shielded and its power and audio leads are properly
>bypassed.
If the exciter is in a box or shield of any kind... if not, maybe
it's time to box it up with a set of good quality feed through caps.
The Standard fix mentioned above took advantage of the lucky
design quirk that allowed a simple plug-in crystal swap to
eliminate the spur.
The real fix would have been adding more shielding and
a bypass cap or three.
This typical cheap design, plus the name of the current-
production Motorola radio of the time, the Motrac, is were
the (no longer politically correct) derogatory term "japtrac"
came from.
>Check to see that all connectors are clean and tight. Use
>a spectrum analyzer to see if your PA is generating spurs.
>Try running without the PA, or use a different and lower-
>powered PA for comparison.
Make sure all the shields are in place. If your radio came
to you as a used radio, you might want to make sure that
there isn't a missing shield that you are not aware is missing...
see if Paul Maggorie can email you a digital photo of what
your TX should look like. Or maybe Paul has seen this
problem before and can save you from reinventing the cure.
>Another brute-force measure which might be helpful in
>troubleshooting this problem, is to put a bandpass cavity
>between the exciter and the PA, and a second bandpass
>cavity at the output of the PA.
Or maybe a coax tee fitting and a shorted or open stub
to notch out the frequency.
The in-line cavity technique is what was done on one local
system that used a Moto MSY retrofitted with a TPL amplifier.
The UHF MSY was famous for having a catastrophic meltdown
in the tube PA... and so many failed that TPL began to produce
a MSY retrofit kit to convert it into a solid state PA. One local
system had a mix problem that was only solved by placing
a pass cavity between the MSY chassis and the TPL amplifier.
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