I must be the only person who didn’t know the Fluke DMM can source exactly 1
mA. If I read the manual in the past, it probably didn’t register. Not sure
if the internal source is regulated as the battery ages. Time to review the
manual. Anyway, that method works great.
My favorite "grab 'n go" DMM is a Fluke 8060A from the mid-1980s. I have three
of them; two are still new on the shelf in the original boxes. It's not
auto-ranging, so it doesn’t slow me down when taking multiple measurements. I
prefer to range it myself. The 8060A's lowest DC resistance range is 200 ohms.
In that position, the Modutec meter reads 34 ohms versus my manual method that
shows 32 ohms. Because of various mechanical limitations, the Fluke is
probably more believable. In the 200-ohm range and with the leads applied to
the meter terminals, it reads...almost full scale as Jim pointed out! BTW,
it's only the in the lowest resistance range that the 8060A sources 1 mA.
To Bill's question, the 1% metal film resistors all measure on the high end of
tolerance. 1% of 1-meg is 10K ea. for a total of 50K, but that still doesn’t
come close to the 500K required change. With the Fluke DMM applied, it does
not reach full scale and is off by about half the discrepancy amount. If the
Fluke is sourcing very close to 1 mA, then the meter may be contributing to the
error. I would like to believe it except that an identical Modutec meter reads
the same error. But I have to believe it because the math doesn’t make sense
otherwise. The meter's internal resistance of 32 ohms in parallel with the 15K
pull-down resistor is a miniscule resistance in the string.
Next, I need to source exactly 1 mA and observe the result. I may have two
identical meters that read low. Since the meter is used for HV and Ig, then
grid current is probably reading a bit low. Recall that my Alpha 70V uses the
same components as the 70A, yet metering in the 70V's HV position agrees with
the Fluke's HV probe.
By the way, with the meter at 4/5 scale for 4KV of HV, that results in 0.64
watt of 1-meg resistor heat dissipation. It looks like Alpha used 1-watt/1%
metal film resistors. My change brought the cold-end resistor in the string
down to 500K from 1-meg. The computed dissipation from that resistor is 0.32
watt. I used a 1/2 watt/1% metal film resistor, slightly elevated from the
PCB.
Paul, W9AC
-----Original Message-----
From: MU 4CX250B [mailto:4cx250b@miamioh.edu]
Sent: Thursday, January 19, 2017 4:46 PM
To: Paul Christensen <w9ac@arrl.net>
Cc: amps@contesting.com
Subject: Re: [Amps] Alpha Seventy HV Meter Readings
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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