[R-1051] FIXED: Low output from 500cps synth

[paul swed]

good read
Thanks
Paul
WB8TSL

On Mon, Aug 27, 2012 at 3:24 PM, David Wise <David_Wise at phoenix.com> wrote:

> The output of the R-1051 plain's 500cps synthesizer is a CW signal at
> approximately 7.1MHz .
> The exact frequency represents the error in the 1-and-10KHz crystals, plus
> the
> 000/500/Vernier setting.  A mixer combines the 1-and-10 module's ~7.089MHz
> error output with a locally-generated signal in the 11-12kHz range which is
> the 500cps module's VCO divided by 10.)  The result goes through a narrow
> 7.1MHz crystal filter, and is then amplified and sent on its way to the
> 100kHz
> module where it becomes part of the second RF Translator injection signal.
>
> This injection signal is weak in my radio, so the 500cps module is up for
> scrutiny.
>
> I found a "13W3" hybrid-d-sub cable that works just fine to extend the
> module
> out of the six-pack.  It's the video monitor cable that Sun Microsystems
> used with
> their popular line of workstations back in the 1980's, and used ones are
> still abundant and fairly cheap.  I got mine for $12, which is about the
> price
> of two individual coax contacts new.  (I wonder if I could harvest the
> contacts
> from several cables to make other ones.)
>
> The 2N Repair Manual is the only source I have for alignment procedures.
> Does anybody have anything else?
>
> I'm one of those techs who likes to use alignment as a diagnostic aid.
> The manual says to peak A1T1 for maximum 7.1MHz output, and then adjust a
> gain trim, A1R16, for 8mV output.  I couldn't get a peak, and the output
> was only
> 5mV even though some previous tech had already turned R16 to max.
>
> Having previously noticed in the Spectrum Generator that its input coil
> A1L2 wouldn't,
> and, as far as I could tell, couldn't peak except perhaps when used with
> the earliest
> type of Frequency Standard, I wondered if the same was true here.  The
> schematic
> clearly shows a cap across the winding, however, which I figured was there
> in order to resonate.
>
> Then I wondered if the low-impedance load at the destination was swamping
> the
> resonance, so I tried again with the module powered up alone on the bench.
> I backfed the output with my tracking generator, through a 1K resistor to
> raise
> the Q.  I watched for a peak at the output using my spectrum analyzer and
> active probe.  Nothing.
>
> Disappointed, I decided I needed a break.  I started powering things off.
>  As luck
> would have it, I powered down the module first.  And Voila!  A clean,
> sharp peak
> appeared on the spectrum analyzer screen.  I turned the supply back on.
> Flat line.  Just for fun, I slowly turned the voltage down.  The peak
> reappeared
> at about 0.3V .  The germanium diode drop.
>
> I powered back up and probed output transistor A1Q3 with a DMM.  It's wired
> as a common-emitter amplifier, with bias stabilization via 1.2K emitter
> resistor R13
> and 10K/15K base voltage divider R14 and R15.  Its base and emitter are
> supposed
> to be around 11V, about half of the 20V Vcc.  (It's a PNP - a 2N1142 -
> "hanging from
> the ceiling", so the collector is 0V.)
>
> I didn't see 11V; the emitter was 0.5V, the base only 0.1V .  A B-C short.
> I replaced it with a 2N3906, and the output sprang to life with plenty of
> amplitude
> and a tunable peak.
>
> I could have called it good at this point, but the last guy did that, and
> I wanted
> to do better.  I was still worried about the amount the signal varied as I
> turned
> the vernier dial - 8dB.  Since the 7.089 was constant, it had to be the
> 11kHz.
> It was constant coming out of the divider, at A1TP6.  From there, it is
> amplified
> (Q5), filtered (T2/C17) to removed harmonics from the divide-by-ten square
> wave,
> buffered (Q4), and applied to mixer Q1.  At the buffer, TP3 showed a clear
> variation.  I wondered if the filter Q was too high.  Sure enough, swamping
> resistor R22 was out of tolerance.
>
> But only a little.  I used it as an excuse for experimentation.  I
> wondered how
> much swamping was acceptable before the filter became ineffective.  I kept
> the spectrum analyzer on TP3, and saw third harmonic about 40dB down,
> fifth about 60, and seventh just barely measurable.  Watching the third
> harmonic, I started loading the T2 primary.  At 4.7K, the harmonic had
> only risen 1dbc, and the amplitude now varied only 4dB instead of 8.
> I decided to go with this.  I also noticed that the tuning was a bit high
> due to
> tolerances, so I strung a small cap across my new resistor to pull it down.
> This might be overkill, but I won't have to worry about my 7.1 .  If
> there's still
> trouble, it's somewhere else.
>
> Dave Wise
>
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