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 Post subject: Sencore MU-140 / MU-150 Calibration Woes - Thermal Drift Fix
PostPosted: Mar Wed 30, 2011 5:56 pm 
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Joined: Feb Thu 24, 2011 1:29 am
Posts: 3021
Location: Dallas, TX - in the city but with bobcats and coyotes
My MU-140 was giving me fits this weekend. I would calibrate it and watch it as it heated up. The gm calibration would consistently drift low. Low enough to where the 5 KHz signal could not be turned high enough to read "100" with the 1600 Ohms in place. I pinned it down to CR12, one of those pretty green tablet diodes. I replaced it with a 1N4007, and went about calibrating, waiting, and re-checking to verify the fix. Well, son of a gun! the problem was lessened, but, still there! Hmmm. I put on my thinking cap, grabbed a cold 807, and took a few measurements. The voltage across the new CR12 was just over .4Vdc. It's a silicon diode, shouldn't be on yet, right? Well, the meter is 1mA FS, so, WRONG! It doesnt take much leakage current to start pulling the meter reading low.
My DVM puts 1mA through a diode in diode-check mode (I know because I helped design and test it when I worked at Tandy). So, I measured the forward voltage drop of the 1N4007 at 1mA and, lo and behold, it was ~.5V at room temperature. The new diode was effectively leaking too. To solve the problem, I put a schottky diode in series with CR12. This slightly raised the threshold voltage of the effective diode in CR12's position. Wow, what a change! The required setting of the 5 KHz adjustment (R24) is much lower and more stable - and gm readings on this old MU-140 are now rock-solid. I deliberately let it cool down and then went about calibration. Once the 12AU7 warmed up, the zero, emission, and gm calibration readings were all carefully re-adjusted. I then let it sit and warm up with a 6F6 running emission test for a half hour . Once it was fairly good and toasty, I rechecked the calibration - right on the money!

I know that there will be those who feel that it is just as good to remove CR12 and simply add two back-to-back diodes across the meter itself. For me, this solution keeps the designed-in meter protection (actual current limiting) in place and I am more than comfortable with the amount of protection it will afford. I hope that some of you find this useful. 8)

Anybody want to buy some green diodes? :lol:

Edit - typo/ d12 is actually CR12 and R34 is actually R24.

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Last edited by mescalero on Mar Wed 30, 2011 10:30 pm, edited 1 time in total.

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PostPosted: Mar Wed 30, 2011 8:02 pm 
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Joined: Feb Thu 08, 2007 12:36 am
Posts: 1371
Location: Hillsboro, Oregon
Thanks, I'll check this on my MU-150.

Dave Wise


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 Post subject: Re: Sencore MU-140 / MU-150 Calibration Woes - Thermal Drift
PostPosted: Oct Sun 06, 2013 7:22 pm 
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Joined: Oct Tue 01, 2013 5:21 pm
Posts: 37
What kind (or what specs) of Schottky diode would suffice for this?


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 Post subject: Re: Sencore MU-140 / MU-150 Calibration Woes - Thermal Drift
PostPosted: Oct Sun 06, 2013 8:56 pm 
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Joined: Feb Thu 24, 2011 1:29 am
Posts: 3021
Location: Dallas, TX - in the city but with bobcats and coyotes
Most any Schottky rectifier. It's (normally) a low voltage, low current application across the meter terminals.
The diode's job is to conduct (and save the meter) if the meter's voltage drop ever becomes higher than ~ 0.7V. The problem is that the diode starts to conduct well before 0.7V and it re-routes current away from the meter circuit. Thus, the meter reading becomes non-linear as the diode shunts current near full-scale. Adding a Schottky in series with the silicon rectifier essentially eliminates this parasitic leakage current under normal conditions and still allows for good meter protection in the event of a current spike.

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In a triode, no one can hear you screen.


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 Post subject: Re: Sencore MU-140 / MU-150 Calibration Woes - Thermal Drift
PostPosted: Oct Sun 06, 2013 11:53 pm 
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Joined: Oct Tue 01, 2013 5:21 pm
Posts: 37
Thanks! 0.45V forward drop,isn't too much? Looking a STMicro 1N5817 as an option.


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 Post subject: Re: Sencore MU-140 / MU-150 Calibration Woes - Thermal Drift
PostPosted: Oct Mon 07, 2013 1:06 am 
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Joined: Feb Thu 24, 2011 1:29 am
Posts: 3021
Location: Dallas, TX - in the city but with bobcats and coyotes
It won't even drop that much at "uber low" current. They are deliberately forward biased. The key is to have them kick in before the meter movement is damaged. Once they turn on, a healthy spike will only send X amount of current through the resistance of the meter movement. Any additional current will simply and safely go through the diode(s). It's a clever design and surely worked like a charm when new. As the Martian diodes aged, they probably changed. Thus the problem today.

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In a triode, no one can hear you screen.


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