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  #1  
Old 09-19-2017, 12:58 AM
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Zenith 852-120 HV probe repair

Forget it. I no longer have the probe anyway, and I'm not going to fight with anyone, especially with hotheads who have offended me repeatedly and without cause.

There are only two people who help me regularly anyway, and I don't need the board to contact them.

Last edited by Jon A.; 09-22-2017 at 04:44 PM.
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Old 09-19-2017, 01:35 AM
old_coot88 old_coot88 is offline
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Shouldn't be any problem using 1W metal oxides. Couple of questions - Did you suspect the probe of reading high, or low?

In your schematic, you show a shunt across the paralleled 30K and 33K resistors. Is that shunt supposed to be there?
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Old 09-19-2017, 01:53 AM
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Quote:
Originally Posted by old_coot88 View Post
In your schematic, you show a shunt across the paralleled 30K and 33K resistors. Is that shunt supposed to be there?
I was just about to say... that shunt would negate the 2 resistors on the far right. In fact... why are they in parallel? Wouldn't that be like... (33+30)/2 = 31.5kohm? Oddly specific value. You sure your schematic is right?
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Old 09-19-2017, 06:38 AM
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Last edited by Jon A.; 09-20-2017 at 03:56 AM.
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Old 09-19-2017, 09:49 AM
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Quote:
Originally Posted by MadMan View Post
I was just about to say... that shunt would negate the 2 resistors on the far right. In fact... why are they in parallel? Wouldn't that be like... (33+30)/2 = 31.5kohm? Oddly specific value. You sure your schematic is right?
Your math is wrong (should be ~15.7K). Two resistors in parallel are Req=(R1*R2)/(R1+R2) Three or more are Req=1/( (1/R1) + (1/R2) + ... + (1/Rn) )
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Old 09-19-2017, 04:31 PM
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Last edited by Jon A.; 09-20-2017 at 03:57 AM.
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Old 09-19-2017, 05:32 PM
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Only if they are both rhe same value.

jr
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Old 09-19-2017, 05:50 PM
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Last edited by Jon A.; 09-20-2017 at 03:56 AM.
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Old 09-19-2017, 06:11 PM
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The equation simply says that 1/R = 1/R1 + 1/R2 and to continue adding them up for as many resistors (n means number of resistors) as you have in parallel, that's all that ....+ stuff in the mathematical series means. Do you have a calculator that inverts?

My guess is that the shunt and the two resistors across it are to be used in calibration and can be clipped out if necessary by the calibrator if the calibration pot does not have adequate range.

Why do you think the meter reads low, have you measured a known voltage standard?

jr
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Old 09-19-2017, 06:21 PM
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The only reason I can think of for the shunt is that maybe this assembly was also used in another probe model where the shunt was replaced with a switch to change the meter full scale reading.

BTW, your schematic does not show the huge resistor that is in the probe body.
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Old 09-19-2017, 06:26 PM
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Another thing, the 2.2M is apparently only a current-limiting safety provision and does not affect calibration.
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Old 09-19-2017, 11:28 PM
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Quote:
Originally Posted by Electronic M View Post
Your math is wrong (should be ~15.7K).
It was 1 am dude, cut me some slack.
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Old 09-19-2017, 11:42 PM
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Last edited by Jon A.; 09-20-2017 at 03:57 AM.
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Old 09-20-2017, 12:45 AM
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Quote:
Originally Posted by Jon A. View Post
I've had this meter for about 3 years and can recall three voltage standards I've tried to measure: I got about 25K from a 1988 Zenith with a 9-516 that zeno said seemed low, then I got a reading of about 22K from a Zenith 25DC56 before and after changing the safety caps and finally about 28K from my daily-use set which has a 31K HV supply.
Then you are assuming that the meter is wrong because it indicated that the HV on a few old TV sets was low? Vintage TVs are hardly what I would call "voltage standards" .

jr
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Old 09-20-2017, 03:13 AM
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I've seen videos made by Doug in which his meter measured accurately. Mine wouldn't even register on a tripler focus tap. Only saying this because the mods ignored my delete request which doesn't surprise me or help my opinion of them. It's not what you know but who you know.

Nothing like having a know-it-all bully around to pick apart my efforts. Now, can the patronizing attitude and go back to tube town.

Last edited by Jon A.; 09-20-2017 at 04:58 PM.
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