[Johnson] Pacemaker RF Phasing Unit

[David C. Hallam]

Bob,

Thanks for your information about setting the oscillators.  Right now I 
have my Invader 2000 to fix.  After that there is a SX-100 sitting on 
the bench to finish rehabbing.  After that I will pull the Pacemaker out 
and try again.

A previously owner evidently was using this Pacemaker as a CB.  I had to 
replace replace the crystal and realign the 10M band.

73
David
KW4DH

On 1/1/2015 11:45 AM, Robert Nickels wrote:
> On 12/31/2014 5:37 AM, David C. Hallam wrote:
>> I have a Pacemaker sitting on the back shelf in my workshop with the
>> same problem.  It's been sitting there ignored for a couple of years
>> after I gave up on getting 90 degree phase shift on 20M
>
> Having found and fixed the problem, I thought I'd post a few notes
> here for David and others who might encounter this issue in the future.
>
> Once I overcame my trepidation and popped off the dial cord drive, the
> module can be easily accessed and tested without unsoldering
> anything.   As expected, both the coil and 68pf capacitor were good,
> and of the correct value to create  the required leading and lagging
> 45 degree phase shifts at the 11 Mhz mixing frequency used for 20
> meter SSB generation.  Since adjusting the slug had no effect, I did
> expect I might find a bent or broken switch contact but that proved
> not to be the case either.    The problem, as often is the case, was
> self-induced...and having to re-string the dial cord was suitable
> punishment.
>
> Johnson specifies that the crystal oscillator slugs be adjusted to
> produce a 1.5V level at the output of the phasing network.   I thought
> I did so when I went through it previously but I also remember using
> my HP 410C VTVM for some projects before realizing it had drifted
> badly out of calibration, so that might be a factor. In any case, the
> problem was caused by excessively high output from the 11 Mhz
> oscillator, and reducing it to the level specified allowed the
> unwanted sideband to be suppressed.   For some reason, the 11 Mhz
> crystal produced MUCH more output than the others to begin with, so
> that is why it may be more problematic.
>
> I did a little playing with the levels and found that when Johnson
> says 1.5 volts, they mean it!     Since most of us have decent scopes
> now, adjusting the output levels to 4.2V P-P on the scope is probably
> easier than messing with an RF Probe and VTVM.   I found that "less is
> more" when it comes to sideband suppression; increasing the output
> level to as little as 5 V P-P was enough to reduce opposite sideband
> suppression by 10dB or more. (Incidentally, if you're not using an SDR
> as a spectrum analyzer to align phasing rigs, it's the best $50
> investment you can make - drop me a private email for more information).
>
> After carefully setting the carrier oscillator levels to 1.5VRMS, I
> was able to obtain the specified 30dB sideband suppression on all
> bands.   So,  removing the phasing assembly wasn't reallly necessary
> after all, but if you do so, proactively replace the paper plate
> bypass cap on the cathode follower stage while you're at it.   And
> take good notes/pics of how the dial cord is routed as none of the
> dial cord patterns are documented in the manual.
>
> The Pacemaker is far from Johnson's best effort.   Maybe they were
> trying to avoid patents, but you don't even need a slipstick to see
> that the 7th harmonic of the 3 Mhz VFO is going to fall right at 21
> Mhz and will go right through the transmitter along with the desired
> signal.   30dB of sideband suppression into a KW amplifier isn't going
> to make you very popular on a crowded band but no one is going to run
> the Pacemaker as a contest station anyhow ;-)   The addition of a
> simple low-pass audio filter would have helped to restrict the audio
> bandwidth but Johnson left it out.    Based on a quick sweep, the
> output only drops about 10dB from 1 to 5 kHz (don't tell the hi-fi SSB
> guys!).   Using a Shure 444 or other type restricted range mic should
> help produce a clean SSB signal, along with proper loading and drive
> adjustment.
>
> The Pacemaker is kinda like owing an AMC Pacer - you don't do it
> because it's the greatest design or most popular, but because it's
> unique and still does the job despite its quirks.    A big thanks to
> Lee Craner for his diligent efforts in compiling useful documentation
> and for creating a vastly improved schematic of the audio/RF phasing
> generator.
>
> 73 and HNY,
>
> Bob W9RAN
>


-- 
There are two possible outcomes: if the result confirms the hypothesis, 
then you've made a measurement.
If the result is contrary to the hypothesis, then you've made a discovery.
Enrico Fermi