Type of wire to use for shielded test lead....

Guest
I am restoring a Eico VTVM model 221. The DC requires a test lead with
shielded wire and a 15M resistor in the probe. It did not come with the
leads. I have all the connectors and the resistor, but need shielded
cord. Can I just cut the ends off a guitar cable to make one? I dont
want to buy a whole roll of cable, which willnever get used after this
project. I also considered the cable for CB radio antennas, but thats
darn stiff. And tv ant coax has that funky aluminum foil for shielding,
and is also stiff.

What else can I use?

I will use this meter for tube radios in the RF and IF stages, so I do
need the shield.
 
tub...@myshop.com wrote:
I am restoring a Eico VTVM model 221. The DC requires a test lead with
shielded wire and a 15M resistor in the probe. It did not come with the
leads. I have all the connectors and the resistor, but need shielded
cord. Can I just cut the ends off a guitar cable to make one?
Click to expand...

** Probably be fine ,just cut one end off though - the 1/4 inch plug is needed !!!

I will use this meter for tube radios in the RF and IF stages, so I do
need the shield.
Click to expand...

** DMMs don't come with shielded leads, only scopes do.



..... Phil
 
In article <dndh0e1emhi75knhs17hlbhtt5l2ebscln@4ax.com>,
tubeguy@myshop.com says...
I am restoring a Eico VTVM model 221. The DC requires a test lead with
shielded wire and a 15M resistor in the probe. It did not come with the
leads. I have all the connectors and the resistor, but need shielded
cord. Can I just cut the ends off a guitar cable to make one? I dont
want to buy a whole roll of cable, which willnever get used after this
project. I also considered the cable for CB radio antennas, but thats
darn stiff. And tv ant coax has that funky aluminum foil for shielding,
and is also stiff.

What else can I use?

I will use this meter for tube radios in the RF and IF stages, so I do
need the shield.
Click to expand...

Any shielded wire will work. Just be sure it is rated for the voltage
you are going to measure.

Not being familiar with the Eico, but the 15 meg resistor seems very
high. Maybe it is a 1.5 meg. The VTVM I have used a 1 meg resistor in
the end of the probe.
 
In article <5ac31ee4-e414-4976-9ad7-4e548c68f139@googlegroups.com>,
pallison49@gmail.com says...
I am restoring a Eico VTVM model 221. The DC requires a test lead with
shielded wire and a 15M resistor in the probe. It did not come with the
leads. I have all the connectors and the resistor, but need shielded
cord. Can I just cut the ends off a guitar cable to make one?


** Probably be fine ,just cut one end off though - the 1/4 inch plug is needed !!!


I will use this meter for tube radios in the RF and IF stages, so I do
need the shield.


** DMMs don't come with shielded leads, only scopes do.
Click to expand...

You missed the part where it is a VTVM, not a DMM.

The old VTVMs I have seen often have a shielded probe.
 
On 12/6/18 8:58 AM, Ralph Mowery wrote:
Any shielded wire will work. Just be sure it is rated for the voltage
you are going to measure.

Not being familiar with the Eico, but the 15 meg resistor seems very
high. Maybe it is a 1.5 meg. The VTVM I have used a 1 meg resistor in
the end of the probe.
Click to expand...

It's 15 Meg.
<https://stevenjohnson.com/manuals/mfiles/eico221.pdf>


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com
 
In article <tY-dnSlwVL6moZTBnZ2dnUU7-IWdnZ2d@giganews.com>,
jdangus@att.net says...
Not being familiar with the Eico, but the 15 meg resistor seems very
high. Maybe it is a 1.5 meg. The VTVM I have used a 1 meg resistor in
the end of the probe.

It's 15 Meg.
https://stevenjohnson.com/manuals/mfiles/eico221.pdf
Click to expand...

Ok, I see that the input impedance is 25 meg ohms where most are around
10 meg. That explains the difference.
 
On 12/6/18 9:49 AM, Ralph Mowery wrote:
Ok, I see that the input impedance is 25 meg ohms where most
are around 10 meg. That explains the difference.
Click to expand...

Comparatively:
That's 25,000 ohms/volt on the 1000 range.
With 11 meg, it's only 11,000 ohms/volt.

Typical multi-meters like the Simpson are 20,000/volt, but at
the 29 volt range, that's only 400 Kohms, which will load a
high impedance source considerably more than the VTVM at either
11 or 25 Meg.

However, if you're using the 20 volt range for measuring AVC
voltages, it's 1.25 meg ohms/volt.


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com
 
Ralph Mowery wrote:
I am restoring a Eico VTVM model 221. The DC requires a test lead with
shielded wire and a 15M resistor in the probe. It did not come with the
leads. I have all the connectors and the resistor, but need shielded
cord. Can I just cut the ends off a guitar cable to make one?


** Probably be fine ,just cut one end off though -
the 1/4 inch plug is needed !!!


I will use this meter for tube radios in the RF and IF stages, so I do
need the shield.


** DMMs don't come with shielded leads, only scopes do.


You missed the part where it is a VTVM, not a DMM.
Click to expand...

** I was particularly interested in the fact the Eico is a VTVM.


> The old VTVMs I have seen often have a shielded probe.

** Mainly because they are such wide band meters, covering the range up to several MHz. Most only have one input connector, a coaxial type, switched between DC and AC use.

However, the Eico has a separate DC voltage jack and that is the one the OP asked about.

Modern DMMs have similar input impedances to VTVMs ( like 10 or 11 Mohms) and full scale DC ranges starting from 2V rms.

So very comparable.



...... Phil
 
I had an EICO FET TVM. I still have it. D Cell for the ohmeter and a 9V battery for the voltmeter.

Mine had the phone plug as yours does and also. The probe was quite large in diameter and could turn to insert/remove the resistor (whatever value it was).

I think it was AC/Ohms and DC,

I doubt the cabeling matters and RG-58 would probably suffice.

If you keep your voltages down, RG174 is about 1/2 the diameter (0.1") of RG-58

RG58/59; http://www.scp-sa.es/resources/upload/files/categorias/148/esp/RG58_RG59.pdf

RG59 is TV stuff with a solid core.

RG174 https://www.awcwire.com/productspec.aspx?id=rg174-coaxial-cable is good for 1500 VRMS and it quite flexible.

If had to actually make a probe and it made a difference as to whether the resistor is in the tip or the plug, You can get a BNC for the probe end.

But then some of the standard probes could likely easily accommodate a series resistor inside. Especially some of the adjustable probes like these: https://www.fluke.com/en-us/product/accessories/test-leads/fluke-tl40
 
On Sunday, December 9, 2018 at 12:00:36 PM UTC-5, Ron D. wrote:
I had an EICO FET TVM. I still have it. D Cell for the ohmeter and a 9V battery for the voltmeter.

Mine had the phone plug as yours does and also. The probe was quite large in diameter and could turn to insert/remove the resistor (whatever value it was).

I think it was AC/Ohms and DC,

I doubt the cabeling matters and RG-58 would probably suffice.

If you keep your voltages down, RG174 is about 1/2 the diameter (0.1") of RG-58

RG58/59; http://www.scp-sa.es/resources/upload/files/categorias/148/esp/RG58_RG59.pdf

RG59 is TV stuff with a solid core.

RG174 https://www.awcwire.com/productspec.aspx?id=rg174-coaxial-cable is good for 1500 VRMS and it quite flexible.

If had to actually make a probe and it made a difference as to whether the resistor is in the tip or the plug, You can get a BNC for the probe end.

But then some of the standard probes could likely easily accommodate a series resistor inside. Especially some of the adjustable probes like these: https://www.fluke.com/en-us/product/accessories/test-leads/fluke-tl40
Click to expand...

You can get RG174 and other cable by th foot. e.g. https://www.showmecables.com/bulk-rg174-coaxial-cable-26-awg-per-ft
 
On Sunday, December 9, 2018 at 9:02:42 AM UTC-8, Ron D. wrote:
On Sunday, December 9, 2018 at 12:00:36 PM UTC-5, Ron D. wrote:
I had an EICO FET TVM. I still have it. ...
I doubt the cabeling matters and RG-58 would probably suffice.

You can get RG174 and other cable by th foot. e.g. https://www.showmecables.com/bulk-rg174-coaxial-cable-26-awg-per-ft
Click to expand...

The center conductor on RG174 is flimsy, RG-58 or RG-59 are somewhat more
mechanically robust (but the insulation will have to be replaced, or spliced to a few inches
of more flexible wire). For any non-RF work, microphone cable (shielded pair) might
also be useful.
 
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