23
2-2-4 DRIVER/PA TROUBLESHOOTING
You will need a tube capacitive pickup adaptor for tests in this section. It can be
constructed from an old tube shield with the base cut off so that it does not contact
chassis ground. Pick a size that fits snuggly over the 12BY7.
2-2-4-1 STATIC DC VOLTAGES
Set up in the safe mode, allow 10 minutes warm up. Voltages are measured only in
the unkeyed condition.
PIN 2
PIN 7
PIN 8
V8
-31VDC
295VDC
142VDC
V9
-35VDC
297VDC
297VDC
2-2-4-2 SIGNAL DRIVE LEVELS
Set up in the safe mode, allow 10 minutes warm up. Band 7.0.
•
Set MIC GAIN and RF LEVEL to minimum.
•
Remove the Het osc tube V15.
•
Inject 7.23
MHz from
signal
generator into
the tie point
of C35 and
L10.
•
Place the tube
pick-up on V6
and connect
the scope to
it.
•
With mic
installed key
transmitter
and increase
drive from
generator to
produce
0.5vpp on the
scope
•
If you cannot
get 0.5vpp on
V6 then the
mixer stage
has a fault.
This fault must be cleared before you can proceed.
•
Move the tube pic-up adaptor to V8 you should see 20vpp when the mic is keyed.
•
If you did not get 20vpp on V8 then the driver stage has a fault. This fault must be cleared before you can
proceed.
•
With 20vpp on V8, if you do not have close to 100 watts output from the PA then the PA has a fault (
driver,
plate, L8 and L18 may require slight adjustment
). If this test is successful go to 1-8-2.
