[Elecraft] go-no go?

[John Wingard]

Thanks for the explanation, Stuart. That makes sense. You asked about my
DMM. It's a Fluke 8020A. I've had it about 25 years or so and I sent it back
to Fluke several years ago for them to put in a new LCD display. It has
served me well all these years.

Looks like I'm ready to continue with the construction. I just applied power
to the RF board and the initial voltage checks are right on. However,
that'll have to wait 'till tomorrow evening after work. I've been up since 5
a.m. today and I don't need to make any stupid mistakes, hi.

Thanks again,

de John WB4GLJ

----- Original Message -----
From: "Stuart Rohre" <rohre@arlut.utexas.edu>
To: "John Wingard" <jwin2485@bellsouth.net>
Sent: Thursday, March 21, 2002 9:58 PM
Subject: Re: [Elecraft] go-no go?


> OK, glad to elaborate.  Ohm meters test by applying a small voltage to a
> circuit.  Older analog ones used 1.5 volt battery to do this, and this is
> enough voltage to forward bias a diode which needs anywhere from 0.3 to
0.6
> volts to turn on, to have a low Resistance.  Transistors are just two
diodes
> as far as base emitter and base collector are concerned, and may be in
> parallel with the resistors and high impedance of capacitors that this
test
> was trying to check for shorts or incorrect values.  If you switched on a
> paralleling transistor with the wrong polarity of the ohm meter, you got
the
> result you first got.  Reversing the leads is reverse biasing diode
> junctions, and the transistor or diode does not turn on, and stays high
> resistance, which your test now shows.  These resistance tests are only
> approximations for gross error checking, because of complex issues the
> associated semiconductors (Note that semi, (partly conducting)) bring to
the
> circuit.  Was your meter one of the lower cost imports?  It happens
> frequently with them, while higher cost DMMs like a Fluke make, use much
> lower voltages and special circuitry to avoid that happening in ohms test.
> By the way, the convention of red wire being plus voltage side and black
> negative is often not true in simple VOM ohm meter modes.  Higher priced
> meters may do either way.
>
> Hope that explanation is clear and helps.
> 73,
> Stuart K5KVH
>
>