That still doesn't explain the original error.
I see two possibilities:
1. Alpha designed it wrong in the first place.
or
2/ Some component changed value.
So, which was it? Or was it something else?
73, Bill W6WRT
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Thu, 19 Jan 2017 20:55:32 -0500, W9AC wrote:
>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
>>
>>
>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|