Vintage equipment voltage measurement

C

Cursitor Doom

Guest
Gentlemen,

One of the drawbacks of attempting to fix vintage stuff is the expected
voltage readings given in the service manuals of the day. The manuals
usually state that the readings given were measured with analogue VMs of
a certain ohms-per-volt rating - most commonly IME 20k. Consequently if
you measure with a modern DVM with stupendously high Zin you're screwed
and will get unrealistically high values. That's never worried me as I
keep a vintage AVO for just such circs. All the British service manuals
seem to reference 20k OpV AVOs. However, I'm currently TS on a mid 70s Tek
scope the manual for which states the readings given are valid for a
meter with a Zin of between 100k and 200k (specifically a Triplett 630NS
see link).

Anyone come up with a solution to the problem of making voltage readings
on high impedance parts of a circuit with a meter of a different Zin to
that used by the people who wrote the service manual?

Never heard of an analogue meter with such a high Zin, but here it is:

https://tinyurl.com/ycjz9l4o



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.
 
Cursitor Doom wrote
Anyone come up with a solution to the problem of making voltage readings
on high impedance parts of a circuit with a meter of a different Zin to
that used by the people who wrote the service manual?
Click to expand...

If it is available look a the circuit under test, and see if the high impedance
does make a real difference.

Else use the scope probe... Do not some of them modern Di Gital makes also display volts?
If meter impdance is too high and no other way add a resistor in parallel to your meter?
 
On Sunday, 19 August 2018 16:17:51 UTC+1, Cursitor Doom wrote:
Gentlemen,

One of the drawbacks of attempting to fix vintage stuff is the expected
voltage readings given in the service manuals of the day. The manuals
usually state that the readings given were measured with analogue VMs of
a certain ohms-per-volt rating - most commonly IME 20k. Consequently if
you measure with a modern DVM with stupendously high Zin you're screwed
and will get unrealistically high values. That's never worried me as I
keep a vintage AVO for just such circs. All the British service manuals
seem to reference 20k OpV AVOs. However, I'm currently TS on a mid 70s Tek
scope the manual for which states the readings given are valid for a
meter with a Zin of between 100k and 200k (specifically a Triplett 630NS
see link).

Anyone come up with a solution to the problem of making voltage readings
on high impedance parts of a circuit with a meter of a different Zin to
that used by the people who wrote the service manual?

Never heard of an analogue meter with such a high Zin, but here it is:

https://tinyurl.com/ycjz9l4o
Click to expand...

100-200k is 5-10v scale on a 20k/V meter. Or use a digital & add your R.
High R meters give a more realistic reading than old analogues on high R circuitry.


NT
 
On Sun, 19 Aug 2018 15:17:49 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

Gentlemen,

One of the drawbacks of attempting to fix vintage stuff is the expected
voltage readings given in the service manuals of the day.
Click to expand...

Why not ignore the voltage notes and just fix it?


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
 
On Sun, 19 Aug 2018 09:24:57 -0700, John Larkin wrote:

> Why not ignore the voltage notes and just fix it?

You're obviously not a service engineer. ;-)



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.
 
On Sun, 19 Aug 2018 16:13:00 +0000, 698839253X6D445TD wrote:

If meter impdance is too high and no other way add a resistor in
parallel to your meter?
Click to expand...

Oh, I see. I didn't quite understand what Clive was getting at. Would
that do the trick, d'ya rechnung?



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.
 
On Sun, 19 Aug 2018 16:41:26 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Sun, 19 Aug 2018 09:24:57 -0700, John Larkin wrote:

Why not ignore the voltage notes and just fix it?

You're obviously not a service engineer. ;-)
Click to expand...

I'm an engineer, not a service technician.

This is an electronic design group. I think there is an electronic
repair group. The engineering approach to fixing things is to probe
around, understand how it's supposed to work, and figure out why it
doesn't.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
 
On 19/08/18 17:56, John Larkin wrote:
On Sun, 19 Aug 2018 16:41:26 -0000 (UTC), Cursitor Doom
curd@notformail.com> wrote:

On Sun, 19 Aug 2018 09:24:57 -0700, John Larkin wrote:

Why not ignore the voltage notes and just fix it?

You're obviously not a service engineer. ;-)

I'm an engineer, not a service technician.
Click to expand...

Spot on!
 
On Sun, 19 Aug 2018 09:56:08 -0700, John Larkin wrote:

> I'm an engineer, not a service technician.

Good Lord!! I've only been reading your comments on this group for the
last 20+ years and never really noticed that before! ;-)



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.
 
John Larkin wrote:

This is an electronic design group. I think there is an electronic
repair group.
Click to expand...

This thread is cross-posted to both ...
 
On 8/19/2018 8:17 AM, Cursitor Doom wrote:
Gentlemen,

One of the drawbacks of attempting to fix vintage stuff is the expected
voltage readings given in the service manuals of the day. The manuals
usually state that the readings given were measured with analogue VMs of
a certain ohms-per-volt rating - most commonly IME 20k. Consequently if
you measure with a modern DVM with stupendously high Zin you're screwed
and will get unrealistically high values. That's never worried me as I
keep a vintage AVO for just such circs. All the British service manuals
seem to reference 20k OpV AVOs. However, I'm currently TS on a mid 70s Tek
scope the manual for which states the readings given are valid for a
meter with a Zin of between 100k and 200k (specifically a Triplett 630NS
see link).

Anyone come up with a solution to the problem of making voltage readings
on high impedance parts of a circuit with a meter of a different Zin to
that used by the people who wrote the service manual?

Never heard of an analogue meter with such a high Zin, but here it is:

https://tinyurl.com/ycjz9l4o



What's so hard about putting a resistor in parallel with your meter?
Click to expand...
 
On 08/19/2018 12:56 PM, John Larkin wrote:
On Sun, 19 Aug 2018 16:41:26 -0000 (UTC), Cursitor Doom
curd@notformail.com> wrote:

On Sun, 19 Aug 2018 09:24:57 -0700, John Larkin wrote:

Why not ignore the voltage notes and just fix it?

You're obviously not a service engineer. ;-)

I'm an engineer, not a service technician.

This is an electronic design group. I think there is an electronic
repair group. The engineering approach to fixing things is to probe
around, understand how it's supposed to work, and figure out why it
doesn't.
Click to expand...

The engineering approach to climbing Mt. Everest is to find out where it
is. Learn to climb stuff. Get a bunch of money. Buy the stuff and hire
the people you need to climb it. Go to where it is. And then climb it
using the stuff.
 
On Sun, 19 Aug 2018 11:28:10 -0700, mike wrote:

> What's so hard about putting a resistor in parallel with your meter?

Never even occurred to me.



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.
 
Cursitor Doom <curd@notformail.com> wrote:
On Sun, 19 Aug 2018 11:28:10 -0700, mike wrote:

What's so hard about putting a resistor in parallel with your meter?

Never even occurred to me.
Click to expand...

That's what you get when people lack basic understanding of the matter.
 
On 2018/08/19 11:53 AM, Cursitor Doom wrote:
On Sun, 19 Aug 2018 11:28:10 -0700, mike wrote:

What's so hard about putting a resistor in parallel with your meter?

Never even occurred to me.
Click to expand...

Resistance used depends on the range:

https://canadianvintageradio.com/how_to/example-how-to/

better explanation:

https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/

John
 
On Sun, 19 Aug 2018 19:09:10 +0000, Rob wrote:

Cursitor Doom <curd@notformail.com> wrote:
On Sun, 19 Aug 2018 11:28:10 -0700, mike wrote:

What's so hard about putting a resistor in parallel with your meter?

Never even occurred to me.

That's what you get when people lack basic understanding of the matter.
Click to expand...

I've only recently discovered that I invariably overlook simpler
solutions. Fortunately I'm only a hobbyist and don't do this for a living!



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.
 
On 19/08/18 20:41, Cursitor Doom wrote:
On Sun, 19 Aug 2018 19:09:10 +0000, Rob wrote:

Cursitor Doom <curd@notformail.com> wrote:
On Sun, 19 Aug 2018 11:28:10 -0700, mike wrote:

What's so hard about putting a resistor in parallel with your meter?

Never even occurred to me.

That's what you get when people lack basic understanding of the matter.

I've only recently discovered that I invariably overlook simpler
solutions. Fortunately I'm only a hobbyist and don't do this for a living!
Click to expand...

And that is clearly true for more than just electronics!
 
On 8/19/2018 12:14 PM, John Robertson wrote:
On 2018/08/19 11:53 AM, Cursitor Doom wrote:
On Sun, 19 Aug 2018 11:28:10 -0700, mike wrote:

What's so hard about putting a resistor in parallel with your meter?

Never even occurred to me.




Resistance used depends on the range:

https://canadianvintageradio.com/how_to/example-how-to/

better explanation:

https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/


John
Click to expand...

Or you could just read the question:

I'm currently TS on a mid 70s Tek
scope the manual for which states the readings given are valid for a
meter with a Zin of between 100k and 200k (specifically a Triplett 630NS
see link).
 
On Sunday, August 19, 2018 at 11:30:00 AM UTC-7, mike wrote:
What's so hard about putting a resistor in parallel with your meter?
Even better would be to understand why they specified VOMs with 20k/V sensitivity, and the implications with modern instruments.
Click to expand...

Back in those days, the commonly-available multimeters were 1k/V and 20k/V VOMs and 11 Meg VTVMs. The cheaper 1k/V would load down the circuit and give an erroneously low voltage reading. With the 20k/V VOM and the VTVM, this error was usually smaller than the inaccuracy in the analog meter movement.

Resistors had 10%-20% tolerance, and the power line voltage could easily vary by 10%, so the values in the service manuals were approximate, not a specification with a precision of 0.1V. Significant discrepancies could be a hint of the source of a problem (for example, a positive voltage on the control grid and a very low plate voltage might be due to a leaky coupling capacitor).

A modern DMM without modification is fine as it is if you understand what you are doing.
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top