[Johnson] Pacemaker

[Robert Nickels]

I thought I'd post a follow-up since there's not much info about 
troubleshooting the Pacemaker available for others who may come across 
this thread.

Unlike many other SSB exciters, the Pacemaker is a single conversion 
design where the SSB generator operates on a different frequency for 
each band, 3 Mhz below the lower band edge.  A single mixing operation 
then takes the signal to the desired frequency, e.g. for 80 meters, the 
SSB signal is generated at 500KHz, which is mixed with the 3.0-3.5 Mhz 
VFO.     This makes alignment a bit more tedious than an HT-37 or 20A 
since there are five RF phasing adjustments to make instead of just one.

The problem I had was the none of them was working right, and since I 
knew the audio phasing was OK, that meant the problem had to be 
something common in the RF phasing unit.   This circuitry is on an 
L-shaped sub-chassis which is totally inaccessible, so it must be 
removed from the transmitter.    This is does not represent EF Johnson's 
finest packaging effort, because to get access requires that the front 
panel be removed (including all knobs, control hardware, and the VFO 
bezel).  Removing the RF phasing unit requires disconnecting four wires, 
removing 3 nuts and 2 screws, and disconnecting the dial cord.   If you 
masking-tape the dial cord temporarily in position on the bandswitch 
drum you can disconnect both ends at the RF phasing switch drum and 
reconnect them afterward with minimum frustration.  Once removed from 
the transmitter, the RF phasing unit can be powered-up on the bench by 
applying filament and 300VDC.    I confirmed that coil adjust ment could 
not produce the required + and - 45 degree lead/lag phasing at the outputs.

The Pacemaker uses an R-L-C RF phasing network with values appropriate 
to each range (see page 59 of Jack Brown's SSB Techniques book for an 
example).   As I originally suspected the coils and dipped mica caps 
were fine - the problem was the 200 ohm 1% deposited carbon resistor 
feeding one side of the network was over 10 ohms high, creating a 
permanent imbalance that was beyond the range of adjustment by the 
slug-tuned inductors.    Because I was a bit afraid of self-inductance 
with modern helically-trimmed resistors, I opted to sort out a 220 ohm 
carbon comp that matched the value of the resistor in the other leg.    
I later found an article in Electric Radio #284 that described a similar 
problem with the Pacemaker where one of these original 1% resistors was 
completely open.   Like the author I'm unable to explain how this 
failure can occur with only a few volts and essentially no current 
through these components, but it seems to be a common failure point.

Reassembly and alignment using Johnson's  specified 2KHz audio tone 
produced much improved results and typically 30-35 dB of opposite 
sideband suppression.   All alignment was done using a simple RTL-SDR 
and SDR# software which provides excellent resolution and an accurate 
DBFS scale, and which makes adjusting any phasing rig vastly easier than 
any other method.

A word to the wise Pacemaker-repairman:   in addition to 110VAC for the 
Tx light, the plate supply voltage is present on the meter terminals 
which are uncomfortably close to several audio phasing adjustment 
pots.   Don't ask why I know this, (!) but use an insulated screwdriver 
to reach those pots!  Shorting HV to ground WILL get your attention (and 
hopefully the meter will survive) and if you end up needing to replace 
the plate meter shunt, it's a standard .51 ohm job that's readily 
available.    Another cautionary note - there is B+ on the exposed shaft 
of the driver neutralizing variable cap that is mounted thru the side of 
the chassis.   Some red paint or other marking will remind you to be 
extra careful around that part of the chassis until the transmitter is 
safely back in the cabinet.

I think the Pacemaker's reputation for being difficult to adjust is 
mostly undeserved, especially if you use an X-Y mode scope to set the 
audio phasing and an SDR or spectrum analyzer to adjust the RF phasing 
for maximum opposite sideband suppression for each band. Like some other 
Johnson transmitters, the RF stages are tracked with the VFO which is 
just like aligning a superhet receiver.   Both driver and PA are 
neutralized for stable operation.

I'm looking forward to having the Pacemaker on the air for vintage SSB 
and as the RF driver for my newly-acquired Desk KW.

73, Bob W9RAN