[Test-Equipment] Test-Equipment Digest, Vol 119, Issue 2

Sun Mar 9 17:04:46 EDT 2014

Ed, yes, I am 'sure' this is the problem. Unfortunately, these Chinese SM parts do not appear to have standard markings. That's OK, I will bulldog my way through it.
I understand what you stated, today, I thought that if said resistor was 'shorted', no voltage would be developed across it.
It just seems that the failure mode of a resistor would be 'open' rather than 'shorted'?.......
Before I retired, I had my Techs do all my SM repairs, etc. These 68 year old eyes ain't what they were! ha ha

ron
N4UE

-----Original Message-----
From: test-equipment-request <test-equipment-request at mailman.qth.net>
To: test-equipment <test-equipment at mailman.qth.net>
Sent: Sun, Mar 9, 2014 12:04 pm
Subject: Test-Equipment Digest, Vol 119, Issue 2

Send Test-Equipment mailing list submissions to
	test-equipment at mailman.qth.net

To subscribe or unsubscribe via the World Wide Web, visit
	http://mailman.qth.net/mailman/listinfo/test-equipment
or, via email, send a message with subject or body 'help' to
	test-equipment-request at mailman.qth.net

You can reach the person managing the list at
	test-equipment-owner at mailman.qth.net

When replying, please edit your Subject line so it is more specific
than "Re: Contents of Test-Equipment digest..."

Today's Topics:

   1. "possessed'" VOM(s) (bonddaleena at aol.com)
   2. Re: "possessed'" VOM(s) (David)
   3. Re: "possessed'" VOM(s) (k4pf at juno.com)
   4. Re: "possessed'" VOM(s) (hwhall at compuserve.com)

----------------------------------------------------------------------

Message: 1
Date: Sat, 8 Mar 2014 18:00:11 -0500 (EST)
From: bonddaleena at aol.com
To: test-equipment at mailman.qth.net
Subject: [Test-Equipment] "possessed'" VOM(s)
Message-ID: <8D109394B0FA23C-2A3C-1D5E2 at webmail-m216.sysops.aol.com>
Content-Type: text/plain; charset="us-ascii"

OK, I'm almost embarrassed to post this, but it's driving me crazy....
I have a LOT of VTVMs, FET VOMs, VOMs, etc. At one of my test benches, I use an 
el-cheapo Chinese VOM that gets a lot of use. Although for serious work, I have 
a dedicated VTVM there, the VOM gets a lot of use.
Recently, I noticed that the X1 ohms function didn't work correctly. (all other 
functions/ranges are OK). When selecting X1, no matter what resistor is being 
measured, it indicates ZERO Ohms. Of course, when not connected, the meter reads 
infinity.
ie: putting a 1K resistor across the leads, it reads 0 Ohms, but when switching 
to X10, X100, X1K, the readings are OK.
So, I pull it apart and other than some sm resistors and a couple of diodes for 
AC/Audio, there's nothing in there. I have looked at the contacts, the board, 
etc, and can see nothing wrong.

The thing that bothers me, is that this problem is exactly the opposite of what 
would seem logical.......If a resistor in the circuit were open, there wouldn't 
be ANY movement on X1. If a resistor were shorted, wouldn't there be an 
indication as soon as X1 was selected?

Tomorrow, I'll try to check the sm resistors but they have some strange 
markings.

What was REALLY crazy was this... I pulled out one of my Triplett 310 VOMs and 
it did the SAME thing. OK, the 310s are beat up and have been around the world 
with me, but at this point, it was time to quit.

Is there ANYTHING in the circuit (other than the fuse) on X1? Sure doesn't look 
like it.
All my other meters check the resistor OK.

ron
N4UE

------------------------------

Message: 2
Date: Sat, 08 Mar 2014 17:13:13 -0600
From: David <davidwhess at gmail.com>
To: Discussion of Electronic Test Equipment
	<test-equipment at mailman.qth.net>
Subject: Re: [Test-Equipment] "possessed'" VOM(s)
Message-ID: <ph8nh9tke3b1i9m0jedu2d1vc9k6rqedl3 at 4ax.com>
Content-Type: text/plain; charset=us-ascii

I have seen a similar but not the same problem when repairing a Tektronix DM502.

The DM502 uses the same terminal connection for volt/ohms and amps so it is easy
to accidentally short the output when changing ranges.  The switch contacts for
the 2A range where welded together which shunted large back to back protection
diodes across the output causing odd behavior in ohms mode.

In your case it sounds like the X1 switched output from the ohms converter
current source is almost completely open so the current is low on that range
making any resistor look like a short.

On Sat, 8 Mar 2014 18:00:11 -0500 (EST), you wrote:

>OK, I'm almost embarrassed to post this, but it's driving me crazy....
>I have a LOT of VTVMs, FET VOMs, VOMs, etc. At one of my test benches, I use an 
el-cheapo Chinese VOM that gets a lot of use. Although for serious work, I have 
a dedicated VTVM there, the VOM gets a lot of use.
>Recently, I noticed that the X1 ohms function didn't work correctly. (all other 
functions/ranges are OK). When selecting X1, no matter what resistor is being 
measured, it indicates ZERO Ohms. Of course, when not connected, the meter reads 
infinity.
>ie: putting a 1K resistor across the leads, it reads 0 Ohms, but when switching 
to X10, X100, X1K, the readings are OK.
>So, I pull it apart and other than some sm resistors and a couple of diodes for 
AC/Audio, there's nothing in there. I have looked at the contacts, the board, 
etc, and can see nothing wrong.
>
>The thing that bothers me, is that this problem is exactly the opposite of what 
would seem logical.......If a resistor in the circuit were open, there wouldn't 
be ANY movement on X1. If a resistor were shorted, wouldn't there be an 
indication as soon as X1 was selected?
>
>Tomorrow, I'll try to check the sm resistors but they have some strange 
markings.
>
>What was REALLY crazy was this... I pulled out one of my Triplett 310 VOMs and 
it did the SAME thing. OK, the 310s are beat up and have been around the world 
with me, but at this point, it was time to quit.
>
>Is there ANYTHING in the circuit (other than the fuse) on X1? Sure doesn't look 
like it.
>All my other meters check the resistor OK.
>
>ron
>N4UE

------------------------------

Message: 3
Date: Sun, 9 Mar 2014 03:35:23 GMT
From: "k4pf at juno.com" <k4pf at juno.com>
To: test-equipment at mailman.qth.net
Subject: Re: [Test-Equipment] "possessed'" VOM(s)
Message-ID: <20140308.223523.32759.0 at webmail09.vgs.untd.com>
Content-Type: text/plain; charset=windows-1252

> Ron N4UE bonddaleena at aol.com wrote
<snip>
What was REALLY crazy was this... I pulled out one of my Triplett 310 VOMs and 
it did the 
SAME thing. OK, the 310s are beat up and have been around the world with me, but 
at this 
point, it was time to quit.
Is there ANYTHING in the circuit (other than the fuse) on X1? Sure doesn't look 
like it.
All my other meters check the resistor OK.

Hi, Ron

On the RX1 scale of a VOM, there is a resistor in series
with the meter and battery; its value = half-scale of the RX1 scale.
For instance, with a Triplett 310 type 3 or type 5 VOM, 
1/2 scale on RX1 is 200 Ohms, and that's the value of R10 of the schematic. This 
resistor is used for RX1 scale only.

The RX1 and the RX10 series resistors are the most likely ones to fail,
if the VOM accidentally is put across a live circuit when set to Ohms.

The resistor used for RX10 scale should also be checked, "R9", 2.2K
for the Triplett 310 types 3 and 5

73,
Ed Knobloch 

------------------------------

Message: 4
Date: Sat, 8 Mar 2014 23:57:20 -0500 (EST)
From: hwhall at compuserve.com
To: test-equipment at mailman.qth.net
Subject: Re: [Test-Equipment] "possessed'" VOM(s)
Message-ID: <8D1096B2FFDAF30-2480-1F1EA at webmail-m289.sysops.aol.com>
Content-Type: text/plain; charset="us-ascii"

Based on the limited description of the innards, it sounded like the possessed 
meter is a digital VOM, unlike the (may I say more easily diagnosed & repaired?) 
Triplett 310.

 Wayne
WB4OGM

-----Original Message-----
From: k4pf &lt;k4pf at juno.com&gt;
To: test-equipment &lt;test-equipment at mailman.qth.net&gt;
Sent: Sat, Mar 8, 2014 7:37 pm
Subject: Re: [Test-Equipment] "possessed'" VOM(s)

&gt; Ron N4UE bonddaleena at aol.com wrote 
&lt;snip&gt; 
What was REALLY crazy was this... I pulled out one of my Triplett 310 VOMs and  
it did the  
SAME thing. OK, the 310s are beat up and have been around the world with me, but  

at this  
point, it was time to quit. 
Is there ANYTHING in the circuit (other than the fuse) on X1? Sure doesn't look  

like it. 
All my other meters check the resistor OK. 

Hi, Ron 

On the RX1 scale of a VOM, there is a resistor in series 
with the meter and battery; its value = half-scale of the RX1 scale. 
For instance, with a Triplett 310 type 3 or type 5 VOM,  
1/2 scale on RX1 is 200 Ohms, and that's the value of R10 of the schematic. This  

resistor is used for RX1 scale only. 

The RX1 and the RX10 series resistors are the most likely ones to fail, 
if the VOM accidentally is put across a live circuit when set to Ohms. 

The resistor used for RX10 scale should also be checked, "R9", 2.2K 
for the Triplett 310 types 3 and 5 

73, 
Ed Knobloch  
______________________________________________________________ 
Test-Equipment mailing list 
Home: http://mailman.qth.net/mailman/listinfo/test-equipment 
Help: http://mailman.qth.net/mmfaq.htm 
Post: mailto:Test-Equipment at mailman.qth.net 

This list hosted by: http://www.qsl.net 
Please help support this email list: http://www.qsl.net/donate.html 

------------------------------

______________________________________________________________
Test-Equipment mailing list
Home: http://mailman.qth.net/mailman/listinfo/test-equipment
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:Test-Equipment at mailman.qth.net

End of Test-Equipment Digest, Vol 119, Issue 2
**********************************************