Here is a theory: the 3-500z in the AL80b suffered a flashover from the
plate to the grid. I don't know the details of the AL80b circuit, but
possibly the current surge resulted in a high voltage appearing on the
input coax.
This explains the meter lights dimming. The high SWR theory doesn't. It
also explains why the event happened twice, after the amp had been
running a bit. This failure isn't rare in 3-500z's.
73,
Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
https://www.qsl.net/k2vco/
On 06/06/2022 21:26, Alton McConnell via Amps wrote:
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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