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Re: [TenTec] Tentec Centurion meter adjustment document?

To: "tentec@contesting.com" <tentec@contesting.com>
Subject: Re: [TenTec] Tentec Centurion meter adjustment document?
From: A R <raf_3@msn.com>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sat, 8 Oct 2016 00:53:37 +0000
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Thanks, Loyd.
All interesting info. I checked with Jewell-Modutec about one year ago 
regarding availability of the Modutec 920051 meter, and was told that it was no 
longer available. BTW, Mouser shows the 0-1ma 2MAS-AVV-150 meter as carrying 
the Modutec 920051 p/n. Mouser shows "Not Available" status for all of the 2MAS 
meters. All six of the Centurion Modutec meters that I have carry the 920051 
number molded in the movement housings. Presumably, that number defines the 
meter package form-factor alone...and not the electrical characterisrics of the 
movement. Today, www.Galco.com<http://www.Galco.com> lists the 2MAS-DMA-001-U 
1ma fs meter (~$34), but I didn't call to verify availability/stock status.

Four of the six meters that I have (or had) were 1.26-1.31 ma full scale. Two 
were 1.06-1.08 ma, and they are now living in a Centurion. My election was to 
fit the amp with  meters with correct sensitivity, rather than modify the 
resistor values...so, at least in this area of the amp, the hardware faithfully 
corresponds to the OEM-published schematic.

If one can find available Modutec 2MAS 1ma meters, the meter scales can be 
easily (but, carefully) transplanted from the old (dysfunctional) meter to the 
new (1ma fs) meter as follows:
Carefully remove the meter lenses (both old and new). Carefully extract the 
movements from the meter housings (which are integral with the scaled dial 
face), taking care to have clean meter leads straightened to be perpendicular 
to the rear surface of the housing, so they can be withdrawn thru the slots in 
the rear of the housing. The meter movement simply "snaps" into (and out of) 
the housing movement cavity, as the meter leads are attached only to the 
movement, and not attached to the housing. All of this requires that NO stress 
or force be applied to the meter needle, armature, moving parts, etc during the 
transplanting process. These movements are VERY delicate...and easily FUBAR.

I suspect that the accounts of Centurion metering grief that you found on 
forums were authored by the ham (an accomplished design/development EE) from 
whom I got 4 "new" TT-supplied meters. He provided me with the correspondence 
documentation between him and the TT service dept that insisted (to the very 
end) that the meters were 2ma fs movements. Uh-huh. This, I think, was the nail 
in the coffin of their relationship.

Hope the info regarding transplanting meter movements/scales/faces (housings) 
is useful to Centurion owners...that is, of course, only if they have good 
Modutec 2MAS 1ma fs movements, and want to preserve the original resistor 
----- Original Message -----
From: Floyd Sense<mailto:k8ac@k8ac.net>
To: tentec@contesting.com<mailto:tentec@contesting.com>
Sent: Friday, October 07, 2016 1:29 PM
Subject: Re: [TenTec] Tentec Centurion meter adjustment document?

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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