Good advice. And if that's bad, other stuff on the bias and key logic
boards may be bad. It's worth pulling those boards out and checking all
the semiconductors with an analog VOM (like a 260).
Yes, I think the input VSWR is a possible clue. But also, those padder
caps are switched in parallel as the bandswitch goes lower in frequency.
Another possibility is a weak tube. The amp runs and tunes fine with
only one tube in place, so I suggest trying it with one tube at a time.
The first time I had to chase problems on the keying and bias boards, I
took the time to draw a nearly complete schematic (everything but the
tuning components in the RF section). It's on my website as several pdf
files. k9yc.com/publish.htm
73, Jim K9YC
73, Jim K9YC
On Wed,8/31/2016 11:56 AM, Joe Giacobello via Amps wrote:
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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|