[Amps] HF2500 - 15 meter problem

[gudguyham at aol.com]

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

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On Wednesday, August 24, 2022, 6:51 PM, Steve London <n2icarrl at 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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