Anyone have a suggestion how I should repair a problem of extremely low output
on 160m with an AL-82?
All bands produce full output except for 160m. I see only 2-4 watts of output
on 160m with 20 watts of drive. Plate tuning shows a peak at approximately the
correct setting of Plate. When Load is readjusted, the power suddenly drops
from 4W to about 2 W. Neither capacitor is at an end of its adjustment range.
- Input match looks good, with the driver reporting 1.2:1 SWR.
- Plate current increases with drive.
- No grid current is shown (but this isn’t surprising at the low drive level).
Output tank dips properly with an MFJ-269 running “backwards” from the output
connector with a 2k ohm resistor from plate lead to ground. I see a 1.0:1
indicated SWR on the ‘269. To the test equipment, 160m looks the same as all
the other bands—except it produces virtually no output.
“The rest of the story”
This amp was acquired used, having been severely beaten by its original owner,
who used it with a random wire and blew up many components. Blown components
included both bias zeners shorted, the B- diode opened, the plate choke has
several burned turns 1/3 of the way down from the plate end, one of the soft
start relay’s contacts burned closed, and a few other little problems.
Fortunately, the band switch shows no carbon or any other damage. I repaired
the amp and confirmed full output, then converted the 3-500s to a 8877, adding
grid current protection, startup delay timer, and EBS along the way.
The burned plate choke is an obvious problem, even though the MFJ-269 properly
shows a 1.0:1 match on all bands. I installed a replacement choke alongside the
burned one. The replacement choke is connected to the old one at the B+ end
(bottom). The plate end wire was removed from the burned choke and installed on
the new choke. The two chokes stand up right next to each other, but the burned
one is not connected to anything at the top. Amp tunes a bit differently on all
bands with the new choke, as expected, but still produces no usable power on
160m. The MFJ-269 shows a good match on all bands with the new plate choke
mounted this way, including 160m.
Obviously I messed up something. Any ideas how to fix this?
Thanks for any assistance.
73 de Bob, K6XX
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