[Central-Electronics] The Case of the 100V Audio Filter

[Robert Nickels]

I can toss out a mystery for the group to ponder:

I've been trying to find the reason why the output power of my 100V is 
low.  Sometimes it would start out OK at 100 watts but then dropped to 
60 watts or less a half hour or so later.

I've narrowed the problem to the Audio Filter module but it's puzzling 
because I can't find anything wrong with it.    If I bypass it by 
jumpering  pin 2-5 so the phasing system is driven directly from the 
output of the Audio Limiter module, I can get full 100 watt output with 
2.25 volts peak-to-peak out of the limiter.    The output of the filter 
is about 1/2 that level and the waveform becomes distorted as audio 
level is increased.   If I adjust the audio input to provide that same 
2.25 volts out of the limiter with the filter in place the transmitter 
power output is only 30 watts.   Even if I increase the audio level 
greatly the output never gets to 100 watts.

This makes it seem that the Audio Filter is defective, but I've run the 
module on the bench for hours, sweeping from 100 to 5000 Hz and it works 
perfectly.    The voltages in the transmitter are correct (6.5VAC fils 
and 315 VDC) but instead of the nice sine waves and consistent output 
within the filter bandpass I see on the bench, the output becomes 
distorted and triangle wave shaped as the level is increased, and will 
not drive the transmitter to full output even then.

The next stage in the 100V is the phase inverter which presents high 
impedances so it can't be a loading effect, besides the limiter will 
drive this stage just fine.

I'd like to have the audio filter inline but am getting nowhere in 
figuring out why it works on the bench but not in the transmitter and am 
about ready to bypass it and call it good. Wonder if anyone has run into 
this mystery before or has any suggestions?

73, Bob W9RAN