9
1-5-1-1, CARRIER OSCILLATOR FAULT ANALYSIS
If the equipment failed the tests in 1-5-1 the following measurements should aid in fault analysis.
Setup in “safe mode”.
You will be making measurements at the junction of R1 and R2, the bottom of R2, and V1 pin1, pin2 and pin3.
To get accurate measurements you must insure that your meter does not load the oscillator. Connect the scope
to the junction of R5 and R6 and observe the signal. Move the FUNCTION switch to CW. Now touch pins 1, 2,
and 3 with the meter probe while observing the signal on the scope. If the probe causes a significant change in
the amplitude of the signal you need a better meter.
MODE / VOLTAGE CHART
OPERATION
FUNCTION
R1
–
R2
R2 bottom
V1pin1
V1 pin2
V1 pin3
MOX
AM
-138vdc
-140
265vdc
-130vdc
0.0vdc
MOX
CW
-1.4vdc
0.0vdc
199vdc
0.0
3.0vdc
NOTEs: Some schematics have R2 listed as C2. Voltages will differ from rig to rig. In the rig used for these readings the
250vdc measured 313vdc, the bias measured -140vdc.
1-5-2 HETRODYNE OSCILLATOR.
The Het Osc is comprised of V15 and its associated circuitry. V15A is the oscillator section and V15B is the cathode
follower/buffer. This oscillator is the most troublesome of the three. There are no adjustments to pull the frequency of
each xtal. So, if you do not have a box of spare xtals you are rather limited in what you can do to put it precisely on
frequency. First thing, check the oscillator output. Connect the scope to the tie point of C121 and L31 which are located
in the cathode ckt of V15B. The minimum peak to peak voltages for each band should be: 80 meters 3.75 Vpp, 40 meters
2.5 Vpp, 20 meters 2.5 Vpp, 15 meters 1.5 Vpp, 10 meters (all 4 segments) 1.5 Vpp.
If all the bands are off in one direction you can offset some of the error with the carrier oscillator. The
frequency synthesis is: TX frequency = (Het osc) – (carrier osc + VFO). If all the bands are high by say 500Hz
then adjust the carrier osc to 1650.5KHz and you are good to go.
HET OSC VOLTAGE CHART
FUNCTION
OPERATION
V15
PIN 1
PIN 2
PIN 3
PIN 6
PIN 7
PIN 8
MOX
AM
144
0
0
302
57
53
1-5-3 VFO
The VFO is comprised of V10 A & B and associated circuitry. If it has stability problems most likely the fault is
C27, 15pf N220 and C28, 51pf N80 the temperature compensating capacitors. V10A is the oscillator section
and V10B is the buffer, cathode follower. This is an extremely stable oscillator design, if it drifts it has a fault.
Find the failed component and replace it. Do not try to “modify stability into it”.