[Milsurplus] [MRCA] Working on my ART-13's

[mac]

On Feb 14, 2012, at 10:38 AM, Ray Fantini wrote:

> The tricky thing about the external power supply if you're building  
> one is to remember that the shunt for the PA current meter is  
> located in the power supply and not the radio. The plate current  
> shunt should be located on the negative side of the HV supply to  
> ground and provide a negative voltage back to the meter circuit in  
> the transmitter, don't recall off the top of my head but think you  
> use a 2.5 or 0.25 ohm resistor rated at 10 watts.

*************

There's a little more to it than that.  Metering for the PA and  
modulator plate current is done by measuring the voltage drop across a  
resistor in series with the 1150 volt supply. In the original  
dynamotor, a tapped resistor was used in the negative side of the 750  
volt winding which resulted in the PA plate  metering circuit riding  
at a +400 volt potential under normal conditions and at ground  
potential above 10K ft.

Now for the tricky part.  The meter on the front of the transmitter  
has both a 0 - 200 scale and also blocked off ranges for CW (white),  
Phone (red) and MCW (white).  As originally designed the numerical  
range does not correspond to the actual current in milliamperes.  In  
operation, the set was tuned and loaded to put the meter needle  
anywhere in the CW block.  The middle of this block is also mid-scale  
on the meter or 100 on the numerical scale but actually corresponds to  
approximately 150 ma. plate current!  In Phone and MCW both the plate  
and modulator currents are being measured together, here, and so under  
modulation, the meter will kick up into the red “Phone” area on the  
meter. Above 10K ft. altitude the PA plate voltage was reduced as  
described above and so the plate current would reduce also, and to  
keep the meter reading in the right block on the scale the value of  
the series metering resistor was increased to give a mid-scale reading  
of about 100 ma.  All this was done to make life easier for the  
operators, i.e., don’t worry about the numbers, just keep the needle  
on the right color.  To do all this, the dynamotor had a 20.1 ohm  
metering resistor tapped at 13.4 ohms.  For ham use, however, just use  
a 25 ohm pot in the negative lead of the HV supply and adjust to make  
the meter read whatever you want.

Dennis D.  W7QHO
Glendale, CA