DX35 Plate Current

[The Pollacks]

There was a thread a while back where someone thought that their DX35 was
drawing excessive plate current.

 

The same thing had happened to mine, and before I got a chance to answer, I
had a computer crash and some other things in my life, so I dropped the
ball.

 

The situation may have been resolved, but thought I'd relate my experience.
As was the case for the guy who wrote, the output into a wattmeter was
normal, which points to the meter shunt, nominally 12 ohms per the manual.
First, to check that everything was ok, I just put my VOM set to MA across
the key (carefully!  Hi Voltage) and read the keyed current at resonance.
It was about 155 ma, which seems roughly ok, as the final should draw about
125 MA, and by reading through the key we're also reading the oscillator and
buffer draws.  The seemed in the ballpark.


Next, I checked the meter movement, which should be a 3 Ma unit.  This was
correct, measured with a resistor and battery, in series with a VOM.  Both
read the same current.

 

This leaves the meter shunt.  The 12 ohm resistor was now at about 25 ohms!
Most old resistors, especially low values, drift upward with age.  To get
the proper value from my junk box, I put a resistance decade box, which has
1 ohm increments, in place of the meter shunt, and found the value that gave
125 MA reading at proper loading.   To be sure that the decade box was not
off calibration, I measured the resistance of the box, and then went through
my resistors until I found one that read the same, and used that for a
shunt.

 

Perhaps not the "engineering" way to do it, but at least I won't beat the
meter to death by slamming up against the stop!

 

Listen for that DX35 on the classic exchange on Jan 25.  Hope to qualify it
and some other Heath gear:  AT1, DX20, Marauder, SB401 Transmitters, and
SB303 receiver.  Maybe even AR2 and AR3 if  there's enough activity!

 

Ron K2RP


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