Well, it looks like it really is the plate choke. It became obvious with
the last test.
See https://www.kkn.net/~n2ic/melted_choke.jpg
Where do I get a new plate choke, or at least a new delrin form to wind my
own ?
Previously, I looked at currently manufactured 3CX800 amps and their plate
chokes are too large to fit. I may go back to that idea and see what I can
do.
73,
Steve, N2IC
On Wed, Aug 24, 2022 at 6:24 PM gudguyham@aol.com <gudguyham@aol.com> wrote:
> Steve, most of the problems with the tank circuit is from having the
> 40-160m L2 toroid burning up. I’ve got an HF2500 in my shop right now with
> a melted down L2 toroid. There is a VERY NARROW group of possibilities
> that can be causing your problem. You’ve done some good detective work so
> far and it seems you’ve covered some good possibilities. You said you
> checked the connections to the band switch and to the coil. Obviously if
> the tap going from bandswitch to L1 is bad on either side you would be
> loosing the tank circuit. But you checked that already and I assume
> accurately. So if your troubleshooting is accurate I can only think of one
> last thing that could be wrong.
> CHECK THE PLATES ON THE LOAD CAP!! The arcing you heard may have bubbled
> a plate or plates on the load cap and when the load cap is in the resonant
> area for 15 meters the rotor plate may be shorting to the stator plate.
> Use a high power flashlight and inspect the load and tune caps for any
> interference throughout the rotation of the plates when in the 15 meter
> range. I assume you checked the plate choke already and I also assume you
> are using a dummy load to rule out an antenna problem. Lou
>
>
> Sent from the all new AOL app for iOS
> <https://apps.apple.com/us/app/aol-news-email-weather-video/id646100661>
>
> On Wednesday, August 24, 2022, 6:51 PM, Steve London <n2icarrl@gmail.com>
> wrote:
>
> I have a HF2500 with a pair of 3CPX800's. It's been a great amp.
>
> Up until now, it's had a minor issue. The tuning/loading drifts of 15
> meters as it heats up. Not a big deal - just a small tweak of the tuning
> brings it back to full power. No issue with any other band.
>
> Today, while operating, it went dead on 15. Suddenly, only a few watts
> output. No drama, bangs, etc. 30 mA grid current, 750 mA plate current.
> Tweaking the tuning made no difference, until it was way out of normal,
> when it increased to 250 watts and something arc'd. All other bands are
> fine.
>
> Here's what I did, so far:
> - Examined the band switch and contact to the 15 meter tap on the coil.
> Looked good and measured good with an ohm meter.
> - Examined the tuned input network. No obvious problem. Placed a 47 ohm
> resistor across the output of the input network, instead of the tubes. SWR
> and Z looked nominal on all bands.
> - Bypassed the input network, driving the tubes directly. Still bad on 15
> meters. All other bands are good.
>
> So, that eliminates the input network as the problem.
>
> What am I missing on the PA or Pi-L side that would only affect 15 meters ?
>
> 73,
> Steve, N2IC
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