[TenTec] OmniVI Plus Over Drive Amp

[Ken Brown]

> Still OVERDRIVE light on Hercules II still lights up 300 watts plus out.  Power does not go into Red area.  I have to back Mic gain down to 75Watt peaks so OVERDRIVE does not light.  My conclusion is to ignore this condition.  Is this OVERDRIVE light too sensitive and needs adjustment or what do I need to do?
Do not ignore the OVERDRIVE LED. There is more going on than just an LED 
coming on. The comparators that turn on the LED also actuate relays that 
switch in attenuators in the input circuit. You don't want these 
switching routinely during your transmission. They are for protection, 
not for normal operation.

There are several conditions that will make the OVERDRIVE LED come on. 
It is a bit confusing to try to figure it out by following the 
schematics, until you realize that the schematics label it as "FAULT" 
LED and not "OVERDRIVE". The manual explains that that there are 
latching fault conditions and transient fault conditions. The manual 
says that the two transient fault conditions are overdrive and excessive 
power output. If you follow the schematics you will discover that there 
is no direct sensing of input power level, and the two inputs to the 
comparators ( U3 on the CONTROL BOARD ) come from forward output power 
sensing and collector current sensing. Either of these can be caused by 
overdrive, however it is not really overdrive directly that triggers the 
LED. The levels at which collector current or forward power trip the 
comparators are adjustable with R7 and R3.

The displayed power output and collector current is also adjustable, so 
if you doubt that the fault trigger levels are correct, you should also 
be skeptical of the collector current or forward power readings. If you 
decide to make any adjustment, you will need a a method of measuring the 
collector current and the output power that is not the internal 
adjustable metering in the Hercules II. Measuring the output power is 
easy, if you have a good RF watt meter that you can trust. Measuring the 
collector current is a bit more involved, because the four separate PA 
modules are fed DC power independently all the way from the 9420 supply. 
The displayed collector current is a sum of the four currents sensed and 
added on the DC DISTRIBUTION board. You can measure the collector 
currents one PA module at a time there, and get the internal collector 
current metering calibrated based on those measurements, using a meter 
capable of reading 10 Amperes, at roughly half full current level. Once 
the internal collector metering is and forward power metering is 
calibrated, you can adjust the OVERDRIVE trigger levels.

I recommend that you do not twiddle any of the adjustments, unless you 
are prepared to perform the whole procedure properly. By prepared I mean 
understanding what you are doing, and having the right instrumentation 
to do it.

DE N6KB