Taylor VTVM 172A - probe question - UK Vintage Radio Repair and Restoration Discussion Forum

6SN7WGTB · 30th May 2021, 3:59 pm

I am a bit puzzled, regrettably through inexperience, as to what the external probe does on the Taylor 172A as it appears to have only two terminals like a 'conventional' meter.

I'm only slightly experienced with using an external home-made RF probe with a DVM but would like to know what I would lose if I acquired a Taylor 172A without its probe?

Could someone enlighten me please?

Thank you.

G4BZI Roger · 3rd Jun 2021, 10:02 am

Hi,
There are I think 3 probes that can be used with the Taylor 172A:

DC probe (has a 1Mohm resistor inside) - this forms part of the voltage divider circuit and provides a measure of isolation between the meter leads and the circuit under test. If it is not used, then dc voltage measurements will be inaccurate and you lose the isolation.

Crystal diode probe model 372 - for measuring ac volts up to 200MHz with low input capacitance.

HV probe model 472 - for measuring eht dc voltages up to 30kV.

Hope that helps.

Roger
G4BZI

stuarth · 5th Jun 2021, 10:57 am

I think the standard Taylor probe for AC, DC, and Ohms for the 172 had a 1 Meg resistor for DC and a switch to short it out for AC and Ohms.

You could use two different probes, one with the 1Meg resistor for DC and one without for AC and Ohms.

The Heathkit V7-AU uses a similar 2-probe arrangement, but with a separate 1/4 inch jack for the DC probe.

Building a probe with a resistor in the tip is easy, you just need to start with a probe which can be unscrewed to replace the cable, not a moulded cable. Building a probe with a switch requires more effort.

Stuart

6SN7WGTB · 9th Jun 2021, 7:53 pm

Thanks both. That makes sense. The 1M DC probe seems common for VTVMs, and easily made.

I have an RF probe I made recently which would be fine for that duty, although I’m not too worried about that.

30th May 2021, 3:59 pm	#1
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 442	Taylor VTVM 172A - probe question I am a bit puzzled, regrettably through inexperience, as to what the external probe does on the Taylor 172A as it appears to have only two terminals like a 'conventional' meter. I'm only slightly experienced with using an external home-made RF probe with a DVM but would like to know what I would lose if I acquired a Taylor 172A without its probe? Could someone enlighten me please? Thank you.

3rd Jun 2021, 10:02 am	#2
G4BZI Roger Pentode Join Date: Nov 2005 Location: Crewe, Cheshire, UK. Posts: 138	Re: Taylor VTVM 172A - probe question Hi, There are I think 3 probes that can be used with the Taylor 172A: DC probe (has a 1Mohm resistor inside) - this forms part of the voltage divider circuit and provides a measure of isolation between the meter leads and the circuit under test. If it is not used, then dc voltage measurements will be inaccurate and you lose the isolation. Crystal diode probe model 372 - for measuring ac volts up to 200MHz with low input capacitance. HV probe model 472 - for measuring eht dc voltages up to 30kV. Hope that helps. Roger G4BZI

5th Jun 2021, 10:57 am	#3
stuarth Heptode Join Date: Feb 2010 Location: Heysham, Lancashire, UK. Posts: 668	Re: Taylor VTVM 172A - probe question I think the standard Taylor probe for AC, DC, and Ohms for the 172 had a 1 Meg resistor for DC and a switch to short it out for AC and Ohms. You could use two different probes, one with the 1Meg resistor for DC and one without for AC and Ohms. The Heathkit V7-AU uses a similar 2-probe arrangement, but with a separate 1/4 inch jack for the DC probe. Building a probe with a resistor in the tip is easy, you just need to start with a probe which can be unscrewed to replace the cable, not a moulded cable. Building a probe with a switch requires more effort. Stuart

9th Jun 2021, 7:53 pm	#4
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 442	Re: Taylor VTVM 172A - probe question Thanks both. That makes sense. The 1M DC probe seems common for VTVMs, and easily made. I have an RF probe I made recently which would be fine for that duty, although I’m not too worried about that.