Need some help please.
I am trying to repair an AL80B amp for a friend. Initially. there was no plate
voltage on the plate HV meter. The ham that sold him the amp said it worked
but the plate HV meter did not work. He tried to use the amp and it blew out
the Icom 7300 final transistors Q131 and Q132.
He sent the exciter back to the manufacturer and asked me for help. We tracked
the no plate voltage issue down to a missing pin #9 on J101A ribbon connector
plug and an open R110. Repaired these issues. Meters were working properly.
After the repair, he was low power tuning the amp on 40 meters using 43 watts
from exciter. He was in the process of tuning the amp for higher power when
the amps meters dimmed and the Icom transistors blew (again!). The HV plate
voltage meter is still working. I have not tried to operate the amp with
another radio. He was running a plate current of 375 mA and a 70 mA grid
current when the issue occurred.
I am at a loss to understand what is going on here.
I had removed the front panel of the amp while tracing the no HV plate meter
reading issue a couple of weeks ago as discussed above. Today I wondered if I
may have gotten the band switch incorrectly set when I reinstalled the front
panel. I put the output of a MFJ 269 SWR analyzer into the amps input. Held
the T/R relay closed (With the amp un-plugged and caps discharged, of course!)
and verified that the signals for the various ham bands were going to the
correct tuned inputs and to the 3-500Z (Eimac) tube correctly using a scope.
We did not remove the input board but I inspected what I could see and nothing
appeared to be smoked! The green 10m wire was clipped but it had taped ends.
We removed as much of this wire as we could. (I don't like taped wires hanging
around.)
He has sent the 7300 out for repair again.
Other details:
He is running an 8 band off center fed dipole. The swr is low.
I looked at the coax he was using to feed the amp. He soldered the center pins
of the coax but not the shield. The coax braid/shield was just mechanically
connected by twisting the connectors onto the braid which was placed over the
cable jacket. I always solder the coax braid/shield to the PL259 body. Today,
we re-terminated the exciter/amp coax with soldered shields and center pins
using new PL259s. No shorts were found before or after the soldering process.
He is in the process of changing all his PL 259s to soldered shield and pin
connections.
If the coax braid was intermittent, could a sudden loss of shield continuity
cause an momentary high SWR that blows out the 7300's final transistors?
I am looking for explanations and other things to check.
Of course, we cannot continue blowing out 7300 finals while we continue to work
on the amp.. I do not know if there is any type of protection circuit that can
be installed between the exciter and the amp. I do not know if a tuner between
the two would help. Perhaps a 1:1 UnUn?
I suspect that we might need to find a HW 100 or 101 (tube type finals) to use
as an exciter until we get this issue tracked down. What do professionals do
about exciters for problem amps?
How do you test a 3-500Z to see if it is a tube issue?
Are Icom 7300's compatible with AL80Bs?
Does anyone have any thoughts?
Thanks and 73!
Alton, NU8L
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