Ken, I can't comment re L4, but high grid current is often associated
with a failure of R17 (4.7 ohm , 5% resistor) on the bias board. Why
this would only manifest on 75/80M is a mystery, but since it only
occurs after the amp warms up, it's possible that it's changing value
with temperature. The board is accessible from the removable plate
under the RF compartment. After you remove the four plugs and screws
holding it in place, it's easy to work on.
73, Joe
K2XX
On 8/31/2016 10:46 AM, Ken Claerbout wrote:
A friend asked me to look at his 425. First thing I noticed was the plate choke was
burned. I think it came from trying to operate on 12 meters. I replaced that choke
with the Ameritron model, which I understand is not resonant on 12 meters. The amp
also had a burned padder cap on the 160A position. That's replaced. The amp is
working now on every band except for 75 & 80 meters. It tunes fine to about 500
- 600 watts. Advancing the power further, the grid current goes from zero to
pegging the meter, with no in-between.
I've replaced the padder caps for those segments with no change. The band-switch and
contacts have been checked. As far as I can see they look fine. I thought I was on to
something, when I found the connection from the tap on L4 had a cold solder joint on
the band-switch. L4 is used on 160, 80 , and 75 meters. I Re-soldered that but same
thing. I'm beginning to think that the toroid L4 is wound on is fried although, I don't
see any obvious signs. I haven't taken it apart and removed the tape. After several
tests on 80 & 160, I took the cover off and felt the core. It was very warm, almost
to the point of being hot. I'm mindful that L4 also comes into play on 160. But
something appears to be heating up, or arching, once power is advanced much past 500
watts output. Also when I first turn on the amp, it's cold, I can get close to legal
limit on 80 & 75. But after several attempts the grid current problem pops up.
Seems to indicate the toroid is heating up?
I should also mention the input SWR on 80 is quite high, 3.5:1 but much better,
1.6:1 on 75. I suspect that's a separate issue and will look into that once I
get the output side squared away.
So the question, especially for those who have experience with this amp, I
don't, have you seen the core in L4 go bad? Is there something I'm overlooking?
I did try a spare set of tubes, nothing changes.
73
Ken K4ZW
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