While running a atu between txcr and amp is not recommended, in your
case I would do this while testing to get Capacitive coupling DC isolation,
protecting the txcr finals against a a DC threat. An oscillation event
may still be an issue, I would reduce AL80 anode PSupp Q in case, to
Is the grid directly grounded ?
On 7/6/22 04:26, 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.
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!
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