919 moisture tester repair. - Page 2 - The Combine Forum
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post #11 of 19 (permalink) Old 09-18-2019, 01:06 AM Thread Starter
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That's good to know about the issues with digital... I was awfully tempted to go that route if I couldn't find a solution my self.. but this makes me want to keep it as original as possible... after all the thing is older than me and worked so good, for so long...

Besides the digital issues mentioned I really didnt want to send it away because it has the original old ound needle meter on the front, I think they just look so cool, and I hate the looks of the new plastic square ones.

I suppose checking the vacuum tubes is the next logical step, i read online they should be warm... neither one is on mine but when removed, the needle fails to move at all, so they are doing something...
They don't rattle and are not discolored.

What's the chance someone would have a wiring schematic for a 919 with vacuum tubes?

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post #12 of 19 (permalink) Old 09-18-2019, 01:40 AM
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I may have vacuum tubes if you find out that is the problem. Uncle used to repair tvs back in the day. What is the brown part on the tube side? I assumed that was a pot.


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post #13 of 19 (permalink) Old 09-18-2019, 09:30 AM Thread Starter
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The round brown part in the pic is the operation/calibration switch.
I tried searching those tubes....
Most info is now all about sound amplification....
I sent off an email to a tube supplier company giving them the numbers and with any luck they can decode or even supply me with new ones.
I suppose I'll just replace them if they're not too spendy.
But I still should try to test them if I can, at least for my own knowledge.

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post #14 of 19 (permalink) Old 09-18-2019, 10:31 AM
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I agree, clean your switches and potentiometers. Tubes degrade slowly over time and if the heater goes, its obvious. All the 919 does is compare the chamber to a known capacitor value using a frequency around 250 khz thru a wheat stone bridge. When you turn the dial, you are changing the resistance of the Resistor/Cap circuit opposite the reference R/C. When they are the same, the circuit nulls (zeros out) and that is when you read the lowest value. The value on the dial (resistance) is used on the chart to show the equivalent moisture at that capacitance.



Old electronics are problematic, caps will drift, tubes get weak, contacts get oxidized, pots rub off their graphite. Best to just replace it, and put that one on the shelf for memories.


If you would like to read more about capacitive reactance - https://www.electronics-tutorials.ws.../filter_1.html So all a moisture tester is is 2 of those circuits and the dial tries to make them the same. Yes, that 2000$ meter is an oscillator and a couple resistors and a capacitor.
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post #15 of 19 (permalink) Old 09-19-2019, 12:56 AM Thread Starter
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I haven't had the opportunity to check the vacuum tubes yet, but I reached out to an online company i came across [Vivatubes.com] that specializes in vacuum tubes.
I e mailed a description of the tubes in my tester and was told they are both the same tube.
The tube type is DF92/1L4.
I went on Ebay and found a matched set for $9.00. So I just ordered them.
Since I learned they are both the same tube, this leads me to believe I have at least one failed, because I swapped them and it made the needle move the opposite direction... if they're both the same there should be no change.
I'll keep my results posted for those who are interested.

I'm determined to fix this old gurl.
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post #16 of 19 (permalink) Old 09-19-2019, 01:57 AM
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We did the retrofit in 2007, still accurate and can be calibrated on farm.

Model 919 Digital Conversion

Finally harvest near...3 weeks late, still COLD, WET(
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post #17 of 19 (permalink) Old 10-05-2019, 07:16 PM Thread Starter
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Well the vacuum tubes i ordered off Ebay arrived! I think i ended up spending $25 to $30 In total for the "matched pair" of American Navy tubes.20191004_143152_1570316458839.jpg
Stuck them in place, and, it works!!
I did a whole bunch of side by side tests with the other meter we have and I found it to be a little inconsistent when first powered up [I think this is due to the vacuum tubes needing time to warm up??]
But after powered up for half an hour or so it was within .2% compared to my other tester.
I checked canola, wheat, oats at varying moisture levels all within .2 sometimes right on.
I also noted that even dumping from one tester to another and back again you can expect varying results.
I'll most likely try to get a few samples one quite dry with a low dial number and one tough with a high dial number and verify at the elevator to know for sure, and for peace of mind.
20191004_194239_1570317374592.jpg

So in conclusion if you have an older 919 meter that all of a sudden the dial will not go below half scale when attempting to calibrate, chances are you have a bad vacuum tube.

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post #18 of 19 (permalink) Old 10-05-2019, 10:06 PM
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Now where are you going to find a really dry sample to test it out this year????????!!!!!!!! If you want some really tough $hit to see how high your meter will go, I can help you out with that!!
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post #19 of 19 (permalink) Old 10-07-2019, 05:57 PM Thread Starter
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Definitely easy to find tough stuff around here too this year.... only dry stuff is coming out of the dryer... 😞

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