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Author Topic: Question on probe for VTVM  (Read 19649 times)
WB4SNU
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Posts: 78




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« on: February 21, 2012, 07:51:43 AM »

I am making a probe for the VTVM. I am confused about how to build it. One probe ( Uni-Probe) has a 1meg resistor in it. A few other probes have a capacitor, 4.7m resistor and a 1N34a diode in it.
I don't know enough theory to understand why the difference?
I think the Uni-Probe is made for the VTVM and the others are for the DMM.
Am I correct in my thinking?
Which way do I go??  I have both meters and I am wanting to learn more than I know.

Any replies will be appreciated on this subject.

Thanks
Richard

WB4SNU
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K8AXW
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« Reply #1 on: February 21, 2012, 08:16:25 AM »

The 1meg resistor in a probe is used as an attenuator for the meter.  If you know the input resistance of the meter you can determine the correct meter reading when using this type of attenuator probe.

The probe with the cap/resistor/diode is for rectifying low level RF to a DC value for measurement.

As far as I know, they type meter isn't a factor, although I'm more familiar with both types of probes when using a VTVM.
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WD4MTW
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« Reply #2 on: February 22, 2012, 10:44:14 PM »

The probe with 1 meg resistor is there to prevent the probe cable capacitance from detuning the circuit. It also keeps any ac component of the circuit from getting into the meter. This part of the probe is for measuring DC only and the resistor isolates the meter and the probe from the circuit. Often there was a switch in the old probes that shorted out the resistor for making AC measurements where there is no isolation resistor used in the path. The other probe you're describing is an RF probe as the poster above suggested.
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WB4SNU
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« Reply #3 on: February 23, 2012, 02:34:52 AM »

Thanks. I think that answered my question. I made one with the 1 meg resistor in it and it works great. I didn't put a switch in it to take it out. I'm going to make one for rf today.

Thanks again for the replies.

Richard
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G3RZP
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« Reply #4 on: February 23, 2012, 05:54:25 AM »

Where the VTVM has a resistance range, it is necessary to switch out the series 1 meg resistor when uising the ohms range.
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WB4SNU
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Posts: 78




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« Reply #5 on: February 23, 2012, 06:39:27 AM »


Where the VTVM has a resistance range, it is necessary to switch out the series 1 meg resistor when uising the ohms range.


Thanks Peter
I have a straight lead that I can use for the resistance and AC range. I tried the 1meg one and it showed exactly 1meg on the scale. When done, I'll have 3 test leads for the vtvm.
I've heard you can't teach an old dog new tricks but I sure am having fun learning something I should have learned years ago. I like building and tinkering with electronics and I'm in the process of setting up a good building bench. Since retirement, I have a lot of time to piddle with circuits and projects. I have a whole lot to learn but that's the fun of it anyway.
I've built a Elecraft K1, 2- qrp transmitters, restored a Hammarlund recv., and am working on a simple recv. for the transmitters.
I never owned a vtvm and it was and is still a little confusing to understand the way it works.

Thanks
Richard
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K8AXW
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« Reply #6 on: February 23, 2012, 07:03:05 AM »

Quote
It also keeps any ac component of the circuit from getting into the meter

I don't think so.  If your meter requires a probe with a 1M resistor and you use a straight probe, you will be changing the input impedance of the meter which in some circumstances detune the circuit you're measuring.  Also it will affect the readings you get. 

Basically, a VTVM is a souped up VOM.... using buffer amplifiers instead of a simple resistor network to get voltage/resistance readings.  VTVMs are ususally high impedance input, around 10M or more and presents a lower "loading" of the circuit being tested.

If you start messing with homebrew probes be sure to replicate what the original was.  VTVMs were a great instrument back in the tube days.
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WB4SNU
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« Reply #7 on: February 23, 2012, 07:55:50 AM »

Quote from:  link=topic=81143.msg577962#msg577962 date=1330009385
Quote
It also keeps any ac component of the circuit from getting into the meter



If you start messing with homebrew probes be sure to replicate what the original was.  VTVMs were a great instrument back in the tube days.

I have a DMM and now the VTVM. The VTVM is an Eico 232. I went through it and replaced the paper caps with polypropylene caps and so far it works good.
The probe I am building has the .01 cap, 4.7m resistor and the 1N34A diode in it.
Should this probe be used only on transistor circuits or can I use it also on tube circuits. I know the voltage rating of the items in the probe is a factor but should I just use it for RF circuits in transistor circuits only??
So much to learn!!!!
I know the original probe was called the Uni-Probe and it only had a 1meg resistor that was switched in and out of the circuit. I'm not sure if there was another probe besides the HV probe that was available for it.
I do intend to work on/restore tube radios and this is why I want to use the VTVM. I could just use the probe I'm building with the DMM. The schematics I've seen are being used to the DMM. Is it possible to use it on both meters if I watch the voltage being tested isn't over the rating of the cap and diode in the probe??

Thanks

Richard
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G3RZP
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« Reply #8 on: February 23, 2012, 09:53:18 AM »

With the 1N34, you are limited to RF under about 15 to 20 volts. I had a Heathkit probe once that had 3 diodes in series: this made it good for higher voltages, but not so good at low voltages. The best RF probes use a thermionic diode: there was a US type and also the European EA52. Marconi Instruments and I believe HP used them, Marconi only using the European type. This allows for voltages up to 300 volts AC, although that drops down above 100 MHz. Worth looking for the complete probe assembly - they do appear from scrapped VTVMs. That's how I got mine, which I use as an AC probe for my rebuilt Heathkit VTVM.

The tube is a specail ring seal diode with the anod out of the top to connect to the probe tip via a disc capacitor.

I have seen them at Dayton, but not too common.
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K8AXW
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« Reply #9 on: February 24, 2012, 06:55:01 AM »

My limited experience with the RF probe which consists of a capacitor, resistor and diode were used for low level RF in receivers and transmitters.  They were used mostly because the person troubleshooting a piece of gear didn't have a scope. 

I've seen them used only with high impedance meters, like 10M and up.

If the probe had a switchable 1M resistor in it, the probe was used to measure voltages above the highest scale on the meter.  Switch in the resistor and you extend the voltage measuring range.  You also decrease the loading of the circuit being tested.

Yes, there is a lot to learn .... when it comes to test gear.  This is why it's best to stay with the correct probes.  Make changes to a probe and you now have a piece of test gear that may give unreliable measurements.
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WB4SNU
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Posts: 78




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« Reply #10 on: February 24, 2012, 07:40:57 AM »

Thanks Peter and Allen for the replies.
I am putting together a reasonable test and building bench. Over the years I have built a few things. Some worked, some didn't. The vtvm and a signal generator is the latest additions to it. I don't have a scope but I intend to get one in the future. I don't think at this point I would know how to use and understand it. I think I have more fun building than I do at operating. I am totally QRP on all I do operate.
I just finished a Tuna Tin 2 and it works fine. I tested the circuits with the vtvm with the probe I made ( 1meg resistor ) and it was close to the voltages stated on the schematic. The only part I couldn't check is the rf voltages. The probe I am making, ( waiting on the 1N34A diode) is the same as some of the ways I have seen on the internet. None of the watt meters I have are able to give me the output wattage I would like to know. I am talking 5 watts and less. Mostly less. The probe will give me the rf voltage and I can use the power formula to find this out with a 50 ohm dummy load.

I appreciate all that have replied.

Thanks
Richard
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W9GB
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Posts: 2616




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« Reply #11 on: February 24, 2012, 02:30:30 PM »

Quote
The VTVM is an Eico 232. I went through it and replaced the paper caps with polypropylene caps and so far it works good.
The probe I am building has the .01 cap, 4.7m resistor and the 1N34A diode in it.
Should this probe be used only on transistor circuits or can I use it also on tube circuits.

Sounds like a good way to "build up" the test instruments for your e-Bench.

DIY Builds for replacement VTVM RF probes, leads, etc.
 
N5ESE Classic RF Probe - DIY Build
http://n5ese.com/rfprobe1.htm

Building a "Sharpie" RF Probe, KC0WOX
 http://golddredgervideo.com/kc0wox/rfprobe.htm
 
You may wish to adopt this popular military surplus test lead kit.
 http://www.apexjr.com/images/TESTLEADSET.jpg
 
You can purchase the one shown from APEX Jr.
(This surplus item has been sold at many hamfests for past 10 years)
 I picked up a couple of these surplus -- several years ago for $5 each.
 http://www.apexjr.com/
 
RF Probe that Hendricks Kits introduced a few moths ago!
 http://www.qrpkits.com/rfprobe.html
 
DIY Build of the Hendricks RF Probe Kit --- for your VTVM, EICO 232
 http://www.home.comcast.net/~gregory.beat/RF_Probe.jpg
 
73 de greg, w9gb
« Last Edit: February 24, 2012, 02:38:06 PM by W9GB » Logged
WB4SNU
Member

Posts: 78




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« Reply #12 on: February 24, 2012, 03:22:13 PM »

Thanks Greg
I have looked at all the sites before and the one that I picked out was the Sharpie probe.
I have 2 of the big ones and there is plenty of room inside to do anything I want to it.
I am having a ball setting up the bench for building and testing. Due to not having very much funds to have an up to date test pieces, I have to find older things to do what I need to use. I noticed the Sharpie has " Made in USA" on it also. Very appropriate for the build.
On my next outing to a hamfest, I am looking for " their junk, my goldmine" things.

Have fun------ life is too short not too.

Richard
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W9GB
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Posts: 2616




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« Reply #13 on: February 24, 2012, 04:28:21 PM »

Quote
Due to not having very much funds to have an up to date test pieces, I have to find older things to do what I need to use.
I noticed the Sharpie has " Made in USA" on it also. Very appropriate for the build.
Richard -

Yes, they are  :-)  
Sharpie was part of the Sanford Corporation, which was purchased by Newell in 1991 and
now the Office Products division of Newell Rubbermaid that is HQ just south of me in Oak Brook, IL.
Newell also owns the Waterman and Parker pen brand names.
The Sharpie marker plant use to be in Bellwood, IL (east of QTH), but Newell shutdown that plant down in 2004,
and production moved to Shelbyville, TN
--
IF you need anything else for your growing e-Bench, let me know -- may have a few extra items to contribute.

73, greg
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WB4SNU
Member

Posts: 78




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« Reply #14 on: February 24, 2012, 05:11:51 PM »

IF you need anything else for your growing e-Bench, let me know -- may have a few extra items to contribute.

73, greg
[/quote]

Thanks Greg
I'm not sure where Shelbyville, TN is but I'm about a good hour drive from the TN. line from NC.
Thanks for the offer.
 I think all I need is for someone to download knowledge to my brain. I have a limited knowledge on troubleshooting but I am learning.
 You can teach an old dog new tricks.
 But what the heck, I don't make a living from it so it's all fun at this stage of life.

Thanks

Richard
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