[Johnson] Pacemaker Problem

[David C. Hallam]

Roger,

I was doing some reading in the Handbook this morning and ran across an 
illustration that shows my problem.  On the lower frequency signals I am 
seeing a mixing of the fundamental and the second harmonic.  That is 
what is causing the knee in the first 90 degrees of the signal.  What I 
do about that is another matter.

David
KW4DH

Singley, Rodger wrote:
> David,
> 
> I would lift the signal leads where the go to the balanced modulator to see whether the distortion is being generated in the crystal oscillator/cathode follower/phase shift network or if it is a result of a problem in the balanced modulator stage.
> 
> If the problem remains with the circuit isolated from the balanced modulator then I suspect that there is some distortion on the signal before it gets to the phase shift network and is being exacerbated by the phase shift network.   It is difficult to see small amounts of distortion although it generally becomes evident if run through an operational amp configured in differentiation mode.
> 
> I would recheck the resistors and capacitors associated with the carrier oscillator and cathode follower to make sure that one has not shifted excessively in value.  Make sure the DC voltages are as specified.
> 
> How does adjusting the associated oscillator coil impact the waveform?
> 
> Rodger WQ9E
> 
> 
> -----Original Message-----
> From: johnson-bounces at mailman.qth.net on behalf of David C. Hallam
> Sent: Fri 1/8/2010 9:18 AM
> To: Johnson List
> Subject: [Johnson] Pacemaker Problem
>  
> I have run into a problem in the alignment of my restoration project 
> Pacemaker.  The two carrier crystal oscillator signals for the balanced 
> modulator are taken from a 12BH7 cathode follower at a common point. 
> The two signals go a switch.  One to a front wafer and the other to a 
> rear wafer.  The front wafer has switched capacitors for each band and 
> the rear wafer has switched slug tuned coils to get the 90 degree phase 
> shift.  The Pacemaker uses a different carrier oscillator frequency for 
> each band.
> 
> Then each signal is connected to the balanced modulator tubes.  The wave 
> forms on all bands from the front wafer with the capacitors look good on 
> the scope.  The ones from the rear wafer are distorted for the 80M and 
> 40M bands.  10M and 15M are good and 20M is probably OK.  The distortion 
> shows up as a knee on the first 90 degrees of the sine wave.  So far no 
> amount of twiddling with the oscillator output voltage or the phase 
> shift adjustment has cleared this up.
> 
> Any ideas?
> 
> David
> KW4DH

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