Guess I will have to read the manual for my Fluke 77 and see if also has the 1
ma source in the low range. I do have a meter or 2 that I want to check. I
have used the variable resistor method also.
John WD5ENU
Sent from my iPad
On Jan 20, 2017, at 07:05, Paul Christensen <w9ac@arrl.net> wrote:
>> "That still doesn't explain the original error."
>
> I'm confident in the original Alpha electrical design (but see last
> paragraph). The math works in their favor and my Alpha 70V/77Dx amps report
> accurate HV.
>
> So, I can't reach a conclusion until sourcing the meter with exactly 1 mA of
> current. Jewell/Modutec specifies the meter with 2% DC accuracy. The 500V
> error is 10% of scale. As Jim pointed out, the Fluke DMM does this, but
> unless the DMM source is regulated, current will change with battery aging.
> That could be verified by changing the +9V battery voltage with an
> adjustable supply although it's a project for another day.
>
> If both Modutec panel meters read correctly with a calibrated 1.0 mA DC
> source, then there are two remaining places to look: (1) the possibility of
> a high resistance contact on the Switchcraft crossbar multimeter switch; or
> (2) a high resistance ground path on the meter's negative terminal.
>
> I can fault Alpha with an electromechanical design error: The 70A/70V use
> only the pressure of the panel to chassis contact as the ground return;
> there's no dedicated ground lead. How do I know this? 19 years ago, my 70V
> was reading erratic grid current that varied when applying hand pressure to
> the front panel. The same multimeter is used so the ground path is common
> to both the HV and grid current readings. One nice feature of these older
> Alphas is that the front panel is easily removed and disconnected with a
> 15-pin Molex connector. It has one free pin slot. On the 70V, I ran a
> dedicated return lead through the open slot and that solved the metering
> problem. Yeah, in retrospect I should have checked this before the resistor
> change. More investigating this weekend...
>
> Paul, W9AC
>
>
>
>
>
>
>
> 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
>>>
>>>
>>
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