[Johnson] Pacemaker Problems

[David C. Hallam]

Al,

I may be wrong and maybe some Pacemaker experts will weigh in on this, but
from my examination of the schematic that is what I see.  The balanced
modulator uses a pair of 12AT7's in push-pull.  There is a tapped coil for
40M through 10M with the full length of the coil used on 40M.  On 80M an
extra 1000 pF is switched-in in parallel to the tuning capacitor for the 40M
coil.

Johnson really didn't do a very good job with the design of the Pacemaker.
That's probably why it had a short life before they brought out the Invader
which has its own set of problems.

David
KC2JD/4

> -----Original Message-----
> From: Al Parker [mailto:anchor at ec.rr.com]
> Sent: Monday, September 15, 2008 9:04 AM
> To: dhallam at rapidsys.com; Ed Tanton; 'Johnson List'
> Subject: Re: [Johnson] Pacemaker Problems
>
>
> hi DAvid,
>     Do you really mean the same coil is used at 0.5MHz & 4MHz with only a
> 1000pf cap switched in?  Seems like an awfully wide range, 8 to
> 1, for one
> coil to do the job.
> 73,
> Al, W8UT
>
> ----- Original Message -----
> From: "David C. Hallam" <dhallam at rapidsys.com>
> To: "Ed Tanton" <n4xy at earthlink.net>; "'Johnson List'"
> <Johnson at mailman.qth.net>
> Sent: Monday, September 15, 2008 7:39 AM
> Subject: RE: [Johnson] Pacemaker Problems
>
>
> > The signal goes into the grid of the 12AT7 balanced modulators but does
> > not
> > come out at the plate.  My question is why?  The way the circuit is
> > designed, the 80M and 40M plate circuits of the balanced modulators use
> > the
> > same coil and capacitor to resonate them with the single change that a
> > 1000
> > pF mica capacitor is switched in on 80M.  The 40M circuit operates at 4
> > MHz
> > and the 80M operates at 500 KHz.  The 40M output is OK but the 80M is
> > nonexistent.
> >
> > I checked the 1000 pF capacitors with my old capacitor tester.
> This thing
> > is so old that the calibration is no good so I can't trust any
> reading for
> > capacity.  However, the capacitors show no short, open, or leakage and
> > read
> > about 800 pF.
> >
> > I am going to replace the two capacitors but was wondering if there was
> > something else I was missing.
> >
> > David
> > KC2JD/4
> >
> >> -----Original Message-----
> >> From: Ed Tanton [mailto:n4xy at earthlink.net]
> >> Sent: Sunday, September 14, 2008 6:41 PM
> >> To: dhallam at rapidsys.com; 'Johnson List'
> >> Subject: RE: [Johnson] Pacemaker Problems
> >>
> >>
> >> If you have an FET oscilloscope probe, this is the very
> application they
> >> were designed for! You could see where the signal goes IN but
> >> does not come
> >> OUT, so to speak.
> >>
> >>
> >> Ed Tanton
> >>
> >> website: http://www.n4xy.com
> >>
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> >>
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> >>
> >>
> >
> >
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