Here's a quick update. The math worked and the replacement resistor value
of 550K results in a HV reading of 4KV. This matches a Fluke DMM with HV
probe. The new HV meter divider string is now 4.55 Meg, slightly reduced
from the 5 meg design.
As previously indicated, the multimeter is a Modutec 1.0 mA DC movement. I
decided to make the effort and measure its internal DC resistance. For the
measurement, I first selected a series R and applied a few DC volts from a
bench power supply. The supply voltage was increased until the meter read
full scale (i.e., 1.0 mA DC). Next, I shunted the meter terminals with a 2K
pot as a rheostat and adjusted it until the meter read exactly 1/2 scale.
At that point, current is evenly divided between the meter coil and
rheostat. Finally, I removed the rheostat from the circuit and measured its
resulting resistance. The answer is 32 ohms. That 32 ohms consists of the
meter coil and any other internal resistance inside the meter enclosure.
So, terminal-to-terminal, DC resistance is 32 ohms.
Back to the Alpha 70A: In addition to the original five, 1-meg HV metering
divider resistors, a 15K resistor shunts the meter terminals when the
multimeter is in the HV position. At least with this amplifier, the 15K
resistor is definitely NOT being used to sample current for the HV reading.
The resistor is well more than 10x the meter's internal resistance. I again
verified HV multimeter accuracy by comparing results with the 15K resistor
in and out of the circuit. As expected, there's little change in deflection
since the meter's internal resistance is swamping the 15K shunt resistor.
In looking at other amplifier schematics from Ameritron, Heath and others,
most use the same HV metering configuration: the multimeter coil is shunted
with a resistor when the multimeter switch is in the HV position. Now, it's
possible that in those amplifiers that the resistor may be used as a sample
which has an additional benefit of stabilizing readings -- but only IF the
meter coil has a high internal DC resistance that approaches the value of
the shunt.
In the Alpha 70 series, the 15K meter shunt in the HV position is performing
only one function: The resistor is floating-down 4KV of high voltage that
would otherwise be present at the moment the multimeter switch is engaged in
the HV position. Otherwise, with no meter current, the full 4KV supply
potential appears on the multimeter switch, which may lead to arcing in
addition to it being a safety concern. Well, it's already a concern in an
openly exposed amp. But few folks would intuitively think that the full HV
potential could appear on a small multi-meter switch. With phenolic used as
the insulating material on most multimeter switches, then all the more
reason to have the resistor in place. With the shunt resistor in place on
the HV supply side, that level is brought safely down on the cold end of the
HV divider string.
This is probably way more than anyone wants to read, but I wanted to close
the loop with my findings and report a solution that now results in accurate
HV readings.
Paul, W9AC
> So, my plan is to change the value of only the 1-meg resistor on the
> cold end of the 5 megohm string. By computation, I get 500K instead
> of 1-meg for a total string value of 4.5-meg instead of 5.0-meg. Seem
> like a reasonable approach to y'all?
Yes.
Remember the internal resistance of the meter will effect the accuracy,
not just the 1 mA value.
--
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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