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GR1862-B help, please

..not a happy camper re my GR. Those two 100Meg resistors inside were rather high; one about 9% high and the other about 4% high - a good reason why the 100Gohm 1% resistor i bought (as a reference) read around 5% high on the GR. So..i bought two SlimMox 100Meg 1% 5KV rating resistors for replacement; those seem to be roughly 0.2% high and very close to each other in value (using a Mastech MY65 and a 100Meg 0.1% resistor as a reference). Used teflon sleeving for the heck of it, even tho the leads are in air after mounting. And the values of the replacement resistors read essentially the same afterwards. *BUT* That 100Gohm now reads higher..about 15% high(!!) on the GR. Using the 500V from the GR and my new Mastech as a (10Meg input Z) current meter (200nA FS), i read 5.046nA; with 498.7V supply giving a calculated 98.8Kmegs which is within the 1% spec or OK.

So thar bee sumppin rong widde GR! Any crazy ideas?

Reply to
Robert Baer
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Well, i got nasty and used LTspice and simulated the beast; found that 100pA of grid current can give/add 2% error. So...since i have already made fixes (changed a 600V cap that leaked more as temp went up, changed the two 100Meg resistors), i am now thinking of replacing the 12AU7 with a dual JFET. Granted i will have to lower the "plate" voltage from the existing

45V to something in the region of 15V. But i do not know what JFETs would do as the worst case gate current at (say) 50C (as a wild guess of warm-up temp inside the GR) needs to be 10pA or better. Any suggestions?
Reply to
Robert Baer

Here goes another classic piece of electronics ...

If you can get your hands on one of these, they are supposed to be very low leakage:

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Plus you might want to move to a place with zero humidity and no smoking within 250ft of unit :-)

--
Regards, Joerg

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Reply to
Joerg

Thanks for the reference! I have never smoked, so that is not a problem; also there is no characteristic smoke smell in the unit. For low / zero humidity, ARIDzona would seem to be the place.

Reply to
Robert Baer

Yes, Ari-"but its a dry heat"-zona would be the place. Although, when I think about Jim's problem with gnats I am not so sure about the situation right now :-)

--
Regards, Joerg

http://www.analogconsultants.com/

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Reply to
Joerg

[snip]

Yep. Really weird, we've been getting a light rain about once a week, gnats, mosquito's, flies... allergy season par excellence :-( ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
      The only thing bipartisan in this country is hypocrisy
Reply to
Jim Thompson

Our forecast says that starting tomorrow there will be another winter episode. So we schlepped some firewood to the front porch. End of April! Global warming, yeah, right ...

Next year will be the year where we up the ante WRT firewood for the umpteenth time, to five cords. If the guy who delivers happens to have an extra cord for next season on his truck we'll instantly take it. Ten years ago two cords was plenty enough :-(

--
Regards, Joerg

http://www.analogconsultants.com/

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Reply to
Joerg

Besides anteaters, there are also various bats and birds that could help.

Reply to
Robert Baer

"Grumble..grumble....damn GW!!"

Reply to
Robert Baer

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