O.K., I purchased a Heathkit VTVM (IM-11) from Ebay w/ no probe for 10 bucks.
I found a ‘make your own’ Heathkit VTVM probe here:
http://www.theheathkitshop.com/vtvmprobefix.html
Great site - shows you how to make the probe and modify the AC/DC switching that has a 1 Meg resistor built into the original probe.
AND, I downloaded the IM-11 VTVM Heathkit manual from the BAMA site here:
http://bama.sbc.edu/heath.htm
AND, Apex Jr. has an inexpensive test lead set that can easily be modified for the probe test leads here, scroll down:
http://www.apexjr.com/new.htm
Here is my question, in the manual it states the hot probe lead is shielded, anyone know where good and CHEAP shielding is? Could just ordinary aluminum tape be used here since it is only providing a shield? DIY suggestions??
They hinted in the diy probe fix site that after the replacing the 1 Meg resistor from the probe and relocating it into the housing, that any straight through leads could be used, would non shielded leads be possible now with this removed resistor from the lead?
Stan
I found a ‘make your own’ Heathkit VTVM probe here:
http://www.theheathkitshop.com/vtvmprobefix.html
Great site - shows you how to make the probe and modify the AC/DC switching that has a 1 Meg resistor built into the original probe.
AND, I downloaded the IM-11 VTVM Heathkit manual from the BAMA site here:
http://bama.sbc.edu/heath.htm
AND, Apex Jr. has an inexpensive test lead set that can easily be modified for the probe test leads here, scroll down:
http://www.apexjr.com/new.htm
Here is my question, in the manual it states the hot probe lead is shielded, anyone know where good and CHEAP shielding is? Could just ordinary aluminum tape be used here since it is only providing a shield? DIY suggestions??
They hinted in the diy probe fix site that after the replacing the 1 Meg resistor from the probe and relocating it into the housing, that any straight through leads could be used, would non shielded leads be possible now with this removed resistor from the lead?
Stan
Hi Stan,
That may possibly work, the tip of my probe broke off. I am worried about the voltage rating on "phone" plugs. Mine is very different with a wider gap showing.
I'd recommend installing banana plug jacks spaced 3/4" apart. At least the capacitance is low and you can now use the BNC adapters and any other standard banana plug set. I think you can get the safety versions that allow the safety lead sets (as in Fluke or HP) to be used.
Anyone remember the solid state tube replacements Heathkit had? I'd love to build some up for my meters.
-Chris
That may possibly work, the tip of my probe broke off. I am worried about the voltage rating on "phone" plugs. Mine is very different with a wider gap showing.
I'd recommend installing banana plug jacks spaced 3/4" apart. At least the capacitance is low and you can now use the BNC adapters and any other standard banana plug set. I think you can get the safety versions that allow the safety lead sets (as in Fluke or HP) to be used.
Anyone remember the solid state tube replacements Heathkit had? I'd love to build some up for my meters.
-Chris
Hi sklimek,
That is exactly what I am suggesting. You can still use a shielded BNC type probe. They make adapters from dual banana (3/4" spacing) to BNC connectors and maybe others. At least you would be on a standard as opposed to the 1/4" phone plug. I have real concerns about the voltage ratings on those.
It would not be as pretty as the original Heathkit probe, I do hate to mess up original panels.
-Chris
The solid state tubes, that's what I want a schematic for!
That is exactly what I am suggesting. You can still use a shielded BNC type probe. They make adapters from dual banana (3/4" spacing) to BNC connectors and maybe others. At least you would be on a standard as opposed to the 1/4" phone plug. I have real concerns about the voltage ratings on those.
It would not be as pretty as the original Heathkit probe, I do hate to mess up original panels.
-Chris
The solid state tubes, that's what I want a schematic for!
anatech said:
How about this, it is shielded coax and I would just have to take out the existing jack and install only one BNC connector in the same hole saving the look of the front panel, should fly??
Good luck on finding - "the solid state tube replacements for you Heathkit...
Stan
Attachments
Hi Stan,
I remember installing them for a few friends. I'd love to know how they replaced the 12AU7. Did they just use diodes for the 6AL5 (?), and what type? This would be in the mid Seventies.
The worry about using a BNC connector is the resistance of the connection in case you use the resistance scale. It might be fine. I use leads like the one you posted for my audio generator some times.
-Chris
I remember installing them for a few friends. I'd love to know how they replaced the 12AU7. Did they just use diodes for the 6AL5 (?), and what type? This would be in the mid Seventies.
The worry about using a BNC connector is the resistance of the connection in case you use the resistance scale. It might be fine. I use leads like the one you posted for my audio generator some times.
-Chris
testlab said:
Hi testlab, which Heath VTVM will it fit, I am currently watching a IM-11, IM 13 and a IM 28. These are the ones I can modify based on the above link for the diy probe and downloadable manuals. These VTVM's are quite affordable on Ebay without the probe. If your probe can fit a regular occuring Heathkit VTVM on Ebay I would definitely be interested.
Stan
Although this is an old thread, the following information may be helpful.
Isolating Probes
(Extract from the book The V.T.V.M. How it works How to use it. By Rhys Samuels. Gernsback Library Described as a Working Man’s Book & considered to be THE book about using a VTVM. While giving detailed information about VTVM circuits & their repair, much of the book is comprehensive practical use instructions with numerous examples.)
Isolating probes are used for the measurement of d.c. voltages. For example, d.c. voltage measurements from 0 to 1,000 are made with a shielded cable terminated with an isolating resistor, as shown in Fig. 402-a (Image of direct probe with 1 megohm resistance at tip). The shielded cable eliminates the pickup of any stray magnetic fields about the test bench, which might otherwise result in an inaccurate indication by the vacuum tube voltmeter. The isolating probe resistance prevents the shielded cable from acting as a capacitive shunt across tuned & high impedance circuits, such as the local oscillator in a receiver. If it were not for the isolating effect of the resistance, connection of the v,t,v,m, test leads might detune the test circuit so greatly that circuit operation might cease altogether.
The equivalent circuit in Fig. 402-b shows the cable capacitance C is isolated by the 1 megohm resistor which cuts down the effective value of C to approximately 1 or 2 uuf. This is a large reduction, as compared with the approximately 75uuf of cable capacitance. Hence, the d.c. probe can be applied across the local oscillator grid without disturbing circuit operation.
The equivalent circuit is also a low pass filter. This is an essential feature, since d.c. voltages at the grids and plates of tubes often must be measures in the presence of large a.c. voltages. If the a.c. signal is passed along to the bridge circuit, it might be rectified and cause an erroneous d.c. reading. Most v.t.v.m.’s include another low pass filter at the grid of the v.t.v.m. to make certain that no a.c. voltage reaches the grid.
These probes consist of plastic housings containing the isolating resistor in series with the probe tip and the output end of the probe. While the value of the resistor used depends upon the d.c. input resistance of the v.t.v.m., a 1 megohm resistor having 1/2 or 1 watt rating is most generally employed.
The d.c. isolating probe should always be used for measurements of d.c. voltages because d.c. voltage scales are calibrated on the basis of the d.c. input resistance. The isolating resistor in the probe is in series with the voltage divider networks in the v.t.v.m. and is part of the d.c. input resistance of the instrument. If the isolating probe is not used, a considerable degree of error might result.
The switch type probe, which combines the functions of direct & isolating probes is finding increased popularity among v.t.v.m. manufacturers. … When set to the forward or DIRECT position, the sliding switch shorts the 1 megohm resistor so the probe can be used for ohms an a.c. measurements. When set to the rear or D.C. position, the short is removed and the resistor is placed in series with the test lead.
Isolating Probes
(Extract from the book The V.T.V.M. How it works How to use it. By Rhys Samuels. Gernsback Library Described as a Working Man’s Book & considered to be THE book about using a VTVM. While giving detailed information about VTVM circuits & their repair, much of the book is comprehensive practical use instructions with numerous examples.)
Isolating probes are used for the measurement of d.c. voltages. For example, d.c. voltage measurements from 0 to 1,000 are made with a shielded cable terminated with an isolating resistor, as shown in Fig. 402-a (Image of direct probe with 1 megohm resistance at tip). The shielded cable eliminates the pickup of any stray magnetic fields about the test bench, which might otherwise result in an inaccurate indication by the vacuum tube voltmeter. The isolating probe resistance prevents the shielded cable from acting as a capacitive shunt across tuned & high impedance circuits, such as the local oscillator in a receiver. If it were not for the isolating effect of the resistance, connection of the v,t,v,m, test leads might detune the test circuit so greatly that circuit operation might cease altogether.
The equivalent circuit in Fig. 402-b shows the cable capacitance C is isolated by the 1 megohm resistor which cuts down the effective value of C to approximately 1 or 2 uuf. This is a large reduction, as compared with the approximately 75uuf of cable capacitance. Hence, the d.c. probe can be applied across the local oscillator grid without disturbing circuit operation.
The equivalent circuit is also a low pass filter. This is an essential feature, since d.c. voltages at the grids and plates of tubes often must be measures in the presence of large a.c. voltages. If the a.c. signal is passed along to the bridge circuit, it might be rectified and cause an erroneous d.c. reading. Most v.t.v.m.’s include another low pass filter at the grid of the v.t.v.m. to make certain that no a.c. voltage reaches the grid.
These probes consist of plastic housings containing the isolating resistor in series with the probe tip and the output end of the probe. While the value of the resistor used depends upon the d.c. input resistance of the v.t.v.m., a 1 megohm resistor having 1/2 or 1 watt rating is most generally employed.
The d.c. isolating probe should always be used for measurements of d.c. voltages because d.c. voltage scales are calibrated on the basis of the d.c. input resistance. The isolating resistor in the probe is in series with the voltage divider networks in the v.t.v.m. and is part of the d.c. input resistance of the instrument. If the isolating probe is not used, a considerable degree of error might result.
The switch type probe, which combines the functions of direct & isolating probes is finding increased popularity among v.t.v.m. manufacturers. … When set to the forward or DIRECT position, the sliding switch shorts the 1 megohm resistor so the probe can be used for ohms an a.c. measurements. When set to the rear or D.C. position, the short is removed and the resistor is placed in series with the test lead.
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