Subject: Re: [TenTec] Tentec Centurion meter adjustment document?
Date: Fri, 7 Oct 2016 16:11:36 -0400
From: Floyd Sense <floyd@k8ac.net>
To: Floyd Sense <k8ac@k8ac.net>, tentec@contesting.com
Allen - thought I'd bring you up to date on the metering issue here. I
have it all worked out and the meters are now reading as they should.
First, the 2.2 VDC was correct for the Ig metering as well as the other
functions of that meter. But, it was incorrect for the Ip meter and
it's not because the meter is kaput as you suggested. The real problem
is that Tentec shipped at least a few meters that had a basic movement
of 1.33 ma (full-scale) instead of the 1.0 ma. they should have been. I
discovered that when I measured the meter resistance and then checked
the full scale current. Some google searching revealed that at least
one other person had discovered the same problem years back. With the
1.33 ma. meter, the shunt resistance (3 ohms) is too low to provide
enough voltage to drive the Ip meter to full-scale with 1 amp through
the shunt. I replaced the existing shunt with two resistors totaling 4
ohms and that allowed me to adjust the Ip trimmer for the correct plate
current. The voltage then required to drive the meter to full scale
through the pin on connector 4 was 3.138 VDC. The only way to know if
you have one of the 1.33 ma. meters in place of the correct 1.0 ma.
meter is to measure the current it takes to get to full scale, or to
measure the meter resistance. My 1.0 ma. meter has a resistance of
2.214 K ohms and the 1.33 ma. meter resistance is 2.346 K ohms.
So, the root of the problem in my case was that the factory installed
shunt was incorrect for the meter they installed. I ran across one
forum post where the person said that Tentec twice sent him replacement
meters for this very problem. It would have made more sense to just
replace the shunt resistors.
Now for good news: the exact meters used in the Centurion (minus the
correct scales) are still available from the manufacturer! If you
search for Jewell/Modutec Meters and look at the 2MAS model meters,
you'll find that's the one used. They have a 1 ma. model, so all that's
needed is to create and print a scale for the meter and replace the one
shipped in the meter from Jewell. Not a simple task, but better than
the alternatives. I found a couple of different programs online that
can be used to design the needed meter scales. I believe the cost of
the new meters is in the area of $35-45 each.
73, Floyd - K8AC
On 9/30/2016 11:04 AM, Floyd Sense wrote:
Thanks for the info, Allen. I'll give that a try and see how close
the meters are. Since there are adjustment pots in series with the
meters for each function, the most accurate approach would probably be
to pass a current through the shunt resistors from an external supply,
adjusted to the full scale meter readings, then adjust the series pots
to read that value full scale. Haven't checked the shunt resistor
values yet, but since the amp is now 25 years old, I'd suspect they've
drifted up in value a bit. Will also check the dividers for the HV
metering.
73, Floyd - K8AC
Floyd,
The procedure for calibrating meter indicated Ip and Ig on the 422
multi-meter is:
1. Insure that the unpowered meter is mechanically zeroed.
2. Unplug cable #4 located on the HV Rectifier-filter board (81538).
3. Feed 2.2vdc + to pin3 ,- to pins 1 and 4 of cable #4.
4. Select Ig metering mode, and adjust R3 (on display board 81540) for
240 ma meter indication.
5. Select Ip metering mode, and adjust R1 (on display board 81540) for
800 ma meter indication.
This procedure only calibrates the display board Ip and Ig metering
circuit components.
The values of the resistors in the source networks (located on the
81538 HV RECTIFIER-FILTER board) should be checked to confirm that
source data for the display-board metering circuits is correct/accurate.
I calibrated the Ep meter value by measuring the plate voltage with a
HV probe, and adjusting R2 on the display board accordingly. I
adjusted R4 and R5 to achieve meter readings consistent with external
wattmeter readings.
Hope this helps. If you can't achieve the prescribed Ip and Ig meter
readings per this procedure, chances are that the meter movement is
kaput...and, that would be a very bad outcome. The meter is a custom
"vane" type (not D'arsonval), and my search for a replacement was
unsuccessful.
Good luck, and 73...
Allen--W7GIF
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