Right you were Colin. After pulling and swapping the result was one of
them had the symptom and the other didn't. Pretty clear cut. So I guess
I'm on the hunt for another, first price i've seen was 189, hoping can
do better than that. Thanx for the suggestion, I got focused in on
possible semiconductor failure, replaced the zener in the cathode path.
Are u and owner of one of these amps?
rich
On 10/2/2014 9:07 AM, Colin Lamb wrote:
Hi Rich:
Pull one tube and try it - then the other. I suspect one tube.
Tuning will change and the input swr will increase - but you are
looking for an obviously bad tube.
Assuming the filaments are not in series and the cooling is not solely
through the socket, it will fire up fine. Watch out for hv bite.
73, Colin K7FM
On 10/2/2014 7:59 AM, k0zx Rich wrote:
Wonder if anyone has experienced this failure mode with this or a
similar amp:
This amp is 80s vintage and since then has been a solid performer,
grounded grid with two 3-500z. But it has a developed a problem where
at power up (no rf drive, ant relay input not active for tx), the
plate current rises to about 3x normal (about 300ma, 100 is normal),
stays there for 5 to 10 seconds and then drops to zero.
If enabled to transmit with antenna relay input on, plate idle
current is normal, going from 0 to 100ma.
If apply rf in, it will output but at only about .6 or .7 its normal
output.
After inspecting the schematic am not seeing an obvious source for
the problem.
Any suggestions appreciated
Rich
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