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Author Topic: Heathkit VTVM probe ?  (Read 17594 times)
N4NYY
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« on: December 23, 2011, 06:50:05 PM »

I just finished restoring an IM-13. Came out great. The probe was not all in that great condition. I was thinking of rebuilding the probe, or making a separate DC volts plug with a 1 Mohm resistor, and separate AC volts/Ohms plug.

Either way, the main probe uses a thin coax. Is this RG-174? Or it there another type that I am not familiar with?
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WD4CHP
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« Reply #1 on: December 24, 2011, 05:47:22 AM »

I just checked mine.

It is to big for RG-174.

As flexable as it is, any good quality audio cable would work at that frequency.

The RF probe uses the same cable.
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N4NYY
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« Reply #2 on: December 24, 2011, 06:08:50 AM »

Quote
I just checked mine.

It is to big for RG-174.

As flexable as it is, any good quality audio cable would work at that frequency.

The RF probe uses the same cable.

You know what? Mine must have been rebuilt before. I have the RF probe and the cable is not the same.
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W1BR
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« Reply #3 on: December 24, 2011, 07:09:51 AM »

I used dedicated probes for my IM-13 until I located another probe from a different brand that used
the same value resistor for DC measurements.

There is a modification on the web that installs the resistor internally on the switch--but the resistor
really belongs at the tip for isolation when measuring DC bias voltages in oscillators and such.

Pete
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N4NYY
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« Reply #4 on: December 24, 2011, 08:27:22 AM »

Pete,

I read that mod to put it internal, and I did not like it. So I will not perform it. If you have a vendor for that probe, let me know.

My Heathkit RF probe has RG-58. So this probe may have been rebuilt at some point. You have to see how this came out. It looks great. I calibrated it yesterday.

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AC5UP
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« Reply #5 on: December 24, 2011, 09:41:42 AM »

Building a probe that switches out the 1 meg resistor shouldn't be that difficult, and the choice of design and materials should take two things into consideration:

1) Keep all wiring outside of the coax shield as short as possible. Even though it's inside an insulating tube, the meter is sensitive enough to pick up on body capacitance or AC hum that can distort the measured signal. It's also a good plan to hold the probe as far from the tip as possible as this gives you some margin of safety away from a hot circuit.

2) RG-174 is generally more flexible and easier to probe with than RG-58 - but - also has a lower internal breakdown voltage. If you have a local shop that does avionics / airframe work you might want to contact them about a scrap of miniature PTFE (Teflon) coax for the probe. Extremely high voltage breakdown and your soldering iron won't burn it. Silver Teflon coax tends to be nylon reinforced and very durable.
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N4NYY
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« Reply #6 on: December 24, 2011, 09:46:39 AM »

I did the cal on this last night. When you do the voltage cal, I zeroed with the probe shorted to ground (resistor switched off). Or should I have pulled the plug out of the jack? The instructions did not say. However, I assumed for zeroing, you need a dead short.
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W1BR
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« Reply #7 on: December 24, 2011, 09:51:09 AM »

Shorting the probe is being very conservative when zeroing the meter on a voltage range.
It certainly is not wrong to do so.

Pete
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N4NYY
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« Reply #8 on: December 24, 2011, 10:04:13 AM »

I could not believe how accurate I got it when I double checked with the Extech. The 1% resistors must have worked well.

One other thing on the DC volts cal. I may have zeroed with the resistor not in line, and then calibrated with the resistor in line. I may have to zero with the resistor in line.
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AC5UP
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« Reply #9 on: December 24, 2011, 11:39:22 AM »

Check the instructions to confirm, but IIRC the 1 meg resistor in the probe is used only for DC voltage. Switch it OUT for AC Volts and Ohms.

VTVM's can be extremely accurate if you re-cal every time you switch scale or function. Short the probe tips and zero the meter. The advantage of 1% resistors is in the consistency from range to range and for routine troubleshooting you may find it's more than close enough with a compromise setting.

Downside to an analog meter is in waiting for the needle to settle with every reading, parallax errors and the bane of rookies... Reading the Ohms scale for volts. Been there, done that, and at least Heathshkit chose a 5x multiplier for the InstantMigraine-13 instead of 10. The jump from 150 to 500 volts full scale is a little spooky, but you won't be there too often. For a typical plate circuit 355 vdc is just as good as 350 vdc and if the scale is a little too crowded for an accurate reading that's not an issue......

BTW: Here's a kick in the buckskins: http://www.imdb.com/name/nm0002014/bio

Whooda' Thot?
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W1BR
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« Reply #10 on: December 24, 2011, 12:22:38 PM »

Quote
Check the instructions to confirm, but IIRC the 1 meg resistor in the probe is used only for DC voltage. Switch it OUT for AC Volts and Ohms.

You are right. The resistor in the DC lead probe allows the probe to measure DC grid voltages, etc. in oscillator or
RF stages without adversely loading the RF side of the circuit.

Pete
 

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N4NYY
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« Reply #11 on: December 25, 2011, 02:32:01 PM »

Hey Pete,

I replaced the Selenium with a silicon diode. The B+ was 10V higher. So I trialed drop resistors. It took a 560 ohm to drop 10V. I thought that was high, but I double and triple checked.
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KI6ZAP
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« Reply #12 on: January 13, 2012, 11:13:34 PM »

I have the IM-11, and need to check a 2400vDC line of a power supply, the IM-11 only reads to 1500vDC. How can I use the regular probe to read the 2400vDC line. Email me at devernham @ comcast.net
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N4NYY
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« Reply #13 on: January 14, 2012, 05:13:31 AM »

You are going to need a HV probe for a VTVM or a DMM.
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KI6ZAP
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« Reply #14 on: January 22, 2012, 05:08:40 PM »

OK got the HV probe, now want to do the mod to move the 1megohm resistor from the probe to inside the chassis. What probe would I need after the mod? I have looked on the internet, but have'nt found any info on which one to get.
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