Paul, your method of measuring internal resistance of the meter is
just fine, though it's easier just to use a DMM and measure the
resistance directly. I imagine you have a fluke DMM and these normally
provide exactly 1.000mA in the resistance mode. Actually, you're
killing two birds with one stone because you can see if your panel
meter reads full scale.
73,
Jim w8zr
Sent from my iPhone
> On Jan 19, 2017, at 12:12 PM, Paul Christensen <w9ac@arrl.net> wrote:
>
> 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
>
>
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