EICO 221 DC Volts probe ?

10/30/2010 1:46:06 PMMitch

Hello all,
I received my 221 with the common and AC-Ohms probes.
I need some info on the DC Volts probe ? I have the manual and schematic thanks to you on the forum.
Looks like the probe has a Phone jack using a shielded cable with the shield grounded to the probe to ground out in the meter. The test end looks like it has a 15 Meg resistor connected to it. The shield does not appear to be connected at the test end.
I can make the probe, just want to verify that this is what i should be making according to what it looks like on the schematic.
Thanks
Mitch

10/30/2010 2:26:44 PMEdd

Sir Mitch. . . . .

You'se gots tooo many brandin' irons in the far'. . . . .at one time.

That 15 megga value is confirmed by both the schematic and parts listing . . . . even though I am universally used to seeing that resistor as being present on DC mode and the feeding straight through of the measured signal in the AC mode.

AND . . . . it is invariably being of a 1 meg value.

If in question measure a reference voltage previously with a Digital meter and then investigate the comparative readings, while trying either a 15 meg or a 1 meg isolation resistor, being used for " the " isolation resistor .

The shield does not appear to be connected at the test end.

Correct, grounding is being made only at the INSTRUMENT end of the unit.

The other aspect of the units probe construction is the extending of the coax shielding right up to the VERY close proximity of that resistor.

73's de Edd

:Hello all,
:I received my 221 with the common and AC-Ohms probes.
:I need some info on the DC Volts probe ? I have the manual and schematic thanks to you on the forum.
:Looks like the probe has a Phone jack using a shielded cable with the shield grounded to the probe to ground out in the meter. The test end looks like it has a 15 Meg resistor connected to it. The shield does not appear to be connected at the test end.
:I can make the probe, just want to verify that this is what i should be making according to what it looks like on the schematic.
:Thanks
:Mitch
:

11/9/2010 12:27:13 AMMitch

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:Sir Mitch. . . . .
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:You'se gots tooo many brandin' irons in the far'. . . . .at one time.
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:That 15 megga value is confirmed by both the schematic and parts listing . . . . even though I am universally used to seeing that resistor as being present on DC mode and the feeding straight through of the measured signal in the AC mode.
:
:
:AND . . . . it is invariably being of a 1 meg value.
:
:
:If in question measure a reference voltage previously with a Digital meter and then investigate the comparative readings, while trying either a 15 meg or a 1 meg isolation resistor, being used for " the " isolation resistor .
:
:
:
:
:The shield does not appear to be connected at the test end.
:
:
:
:
:
:Correct, grounding is being made only at the INSTRUMENT end of the unit.
:
:
: The other aspect of the units probe construction is the extending of the coax shielding right up to the VERY close proximity of that resistor.
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:
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:73's de Edd
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::Hello all,
::I received my 221 with the common and AC-Ohms probes.
::I need some info on the DC Volts probe ? I have the manual and schematic thanks to you on the forum.
::Looks like the probe has a Phone jack using a shielded cable with the shield grounded to the probe to ground out in the meter. The test end looks like it has a 15 Meg resistor connected to it. The shield does not appear to be connected at the test end.
::I can make the probe, just want to verify that this is what i should be making according to what it looks like on the schematic.
::Thanks
::Mitch
::
:
Edd,
Now that i have the meter working i did try a 1 meg resistor, 10 meg and 15 meg with a regulated power supply. All results were the same in voltage measurements with the same supply measured against a DVM. Probe works well.
Thanks
Mitch

10/30/2010 2:28:17 PMWarren

The shielded end from the meter is the negative end of the test probe. Most times is it an alligator clip. The meter is grounded to earth ground, so is that alligator clip. This can be a dead short for an AA5 radio chassis. Be sure to use isolation transformer with the radio in test. Also could use a three to two pin AC cord adaptor on the meter. Leave the earth ground open.

10/30/2010 6:46:24 PMMitch

:The shielded end from the meter is the negative end of the test probe. Most times is it an alligator clip. The meter is grounded to earth ground, so is that alligator clip. This can be a dead short for an AA5 radio chassis. Be sure to use isolation transformer with the radio in test. Also could use a three to two pin AC cord adaptor on the meter. Leave the earth ground open.
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I do have a DC power supply that i can use for a reference for the resistor and a DVM.
The manual does warn about improper grounding, i will and do use an isolation transformer on these radios.
Will post the resistor test results.
Mitch