[Amps] Alpha Seventy HV Meter Readings

[Paul Christensen]

>"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 at miamioh.edu]
>Sent: Thursday, January 19, 2017 4:46 PM
>To: Paul Christensen <w9ac at arrl.net>
>Cc: amps at 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 at 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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