Resistance checks [resolved]

scotth · 1531

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Offline scotth

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on: September 23, 2018, 04:06:24 PM
Just finished soldering and started my resistance checks and I feel like I'm crazy. Could someone please set me straight?

If I'm measuring at terminal 4, how would I get a measurement of 60k when the resistor connecting T4 to T1 (ground) is 75k? Shouldn't it be 75k? Same with T2, T2 has a 10.5k resistor going to T4, so shouldn't it be 10.5k + 75k = 85.5k? T9 has a 47k resistor going to ground, why would it read 39k?

What am I missing?
« Last Edit: September 24, 2018, 09:46:57 AM by Caucasian Blackplate »


Offline scotth

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Reply #1 on: September 23, 2018, 05:29:11 PM
I also just went on a goose chase because I was getting a steady 620ohm reading between the red STP wires coming from the PSU (A4, B4, and C4) and ground, instead of the infinite listed in the resistance checks.

The infinite made sense to me because of the filter caps on C4 and C5 but after disconnecting a few things I tracked it back to the PSU, which I noticed on the schematic has Rset1 and Rset2 in series between the + and - of the 6.3v supply, which add up to 623ohm.  So isn't my reading correct?

With the + disconnected from the 6.3vdc on the PSU PCB, I do get the reading climbing to infinite resistance.

Again I feel like I am missing something in my ignorance and am looking for the explanation. It's been a loooooong time since my intro to EE class.
« Last Edit: September 23, 2018, 05:38:26 PM by scotth »


Offline Paul Birkeland

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Reply #2 on: September 24, 2018, 04:49:52 AM
Quote from: scotth on September 23, 2018, 04:06:24 PM
If I'm measuring at terminal 4, how would I get a measurement of 60k when the resistor connecting T4 to T1 (ground) is 75k? Shouldn't it be 75k?
That is not the only part that influences the DC resistance to ground from that terminal. Different meters will provide some different readings, but they should all be in the ballpark of 60K (75K would be close enough).  When there are capacitors present where you are trying to measure DC resistance, the readings can start to get imperfect.

Paul "PB" Birkeland

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Offline scotth

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Reply #3 on: September 24, 2018, 06:18:47 AM
Quote from: Caucasian Blackplate on September 24, 2018, 04:49:52 AM
That is not the only part that influences the DC resistance to ground from that terminal. Different meters will provide some different readings, but they should all be in the ballpark of 60K (75K would be close enough).  When there are capacitors present where you are trying to measure DC resistance, the readings can start to get imperfect.
Interesting, I thought that might affect it but I was getting exactly what the resistors were valued at, so should I be concerned? Or, could it just be how my meter reacts to that situation? I have a used fluke 77iii.
« Last Edit: September 24, 2018, 06:20:59 AM by scotth »


Offline Paul Birkeland

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Reply #4 on: September 24, 2018, 06:23:25 AM
Your meter is just a little better at ignoring the other junk present.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline scotth

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Reply #5 on: September 24, 2018, 07:22:43 AM
Thanks. Just did the voltage check. Looks good!


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