[Johnson] Pacemaker Problems

[David C. Hallam]

Rodger,

I do not have adequate drive to the mixer on 500 KHz and that's the problem.
At 4 MHz I have about 700 mV peak to peak at the mixer grid.  With that I
have about 0.2 mA of grid drive on the 6146 which the manual says is normal.
At 500 KHz I see only 2 or 3 mV PtoP.

I have checked alignment of switch contacts and cleaned them with DeoxIT.

When I initially aligned the RF section on 80M, I thought everything was OK
as I got 0.2mA of grid drive on the 6146.  But after closer inspection, I
found it was the 3 MHz VFO signal that was giving me the grid drive not the
mixer sum of 3 + 0.5 MHz.  The VFO and the mixer are so close that you can
align on either one.

Davie
KC2JD/4

> -----Original Message-----
> From: Rodger [mailto:wq9e at dtnspeed.net]
> Sent: Monday, September 15, 2008 8:39 AM
> To: dhallam at rapidsys.com
> Cc: 'Johnson List'
> Subject: Re: [Johnson] Pacemaker Problems
>
>
> David,
>
> Are you sure you have adequate drive to the mixers at 500 KHz?  How does
> the level compare  to the drive at 4 Mhz.?
>
> Any possibility of a bad switch contact selecting one of the capacitors?
>
> Rodger WQ9E
>
> David C. Hallam wrote:
> > 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
> >
> >
> >> -
>
>
>
>
>