Vintage equipment voltage measurement

[Jeff Liebermann]

On Sun, 19 Aug 2018 19:41:48 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

I've only recently discovered that I invariably overlook simpler
solutions.

Nothing is simple or stays simple.
Everything becomes more complexicated.

>Fortunately I'm only a hobbyist and don't do this for a living!

I know the feeling. Three times in my life I've turned my hobby into
a business. Now, I'm getting ready to begin to start planning to
retire and I'm turning my business into a hobby with at least one of
my hobbies into a potential business. Perhaps it would be better for
the skools to teach hobbies instead of professions?

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Jeff Liebermann]

On Sun, 19 Aug 2018 09:56:08 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

The engineering approach to fixing things is to probe
around, understand how it's supposed to work, and figure out why it
doesn't.

Nope.

The difference between engineering and repair is that the engineer
assumes that the problem is due to a design error and fixes the
problem by redesigning the circuit. The repair tech assumes that it
was designed and built correctly, therefore something has blown.

The engineers needs to make production lots of an instrument work. The
repair technician usually needs to make one work.

The engineer tries to determine how the circuit should work. The
repair tech tries to determine what the engineer was thinking when he
designed the circuit.

The engineer understands how the circuit should work. The technician
understands what the circuit actually does.

The engineer has experience making the instrument work under
laboratory conditions. The technician has experience making it work
in the rather nasty "real world" environment.

The engineer selects components based on availability, performance,
price, and lifetime. The repair tech substitutes whatever can be
found in his junk box.

The engineer writes the documentation partly to demonstrate to the
world the cleverness and greatness of his design. The repair tech
doesn't read the documentation unless he's desperate.

The engineer makes measurements in order to find problems. The repair
tech looks for smoke, burned parts, bulging capacitors, broken
connections, manufacturing errors, and mechanical damage. Well, maybe
he does take a few measurements like the power supply voltages.

All this works very nicely as long as engineers don't try to act like
technicians and technicians don't try to act like engineers.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Cursitor Doom]

On Sun, 19 Aug 2018 16:22:39 -0700, Jeff Liebermann wrote:

I know the feeling. Three times in my life I've turned my hobby into a
business. Now, I'm getting ready to begin to start planning to retire
and I'm turning my business into a hobby with at least one of my hobbies
into a potential business. Perhaps it would be better for the skools to
teach hobbies instead of professions?

Not sure about the US, but schools in the UK are teaching kids *what* to
think rather than how to. Very little of a typical school day is now
spent learning anything genuinely useful. The kids the schools turn out
into the world of work nowadays are mostly only suited to flipping
burgers or delivering pizzas.





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[Tom Del Rosso]

Cursitor Doom wrote:

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! ;-)

Your nym isn't that old. What was it before?

 

[Tom Gardner]

On 20/08/18 01:14, Cursitor Doom wrote:

On Sun, 19 Aug 2018 16:22:39 -0700, Jeff Liebermann wrote:

I know the feeling. Three times in my life I've turned my hobby into a
business. Now, I'm getting ready to begin to start planning to retire
and I'm turning my business into a hobby with at least one of my hobbies
into a potential business. Perhaps it would be better for the skools to
teach hobbies instead of professions?

Not sure about the US, but schools in the UK are teaching kids *what* to
think rather than how to. Very little of a typical school day is now
spent learning anything genuinely useful.

And if the ex Secretary of State for Education (and poisonous
brexiteer) Michael Gove got his way, they won't be taught to
think at all, only to regurgitate "useful" facts such as when
King Henry II reigned. But then he also wanted more madrassahs,
under the guise of wanting more religion in education. He
even approved the creation of three Creationist schools.

But of course the success and failure is far more nuanced
than CD states.

The kids the schools turn out
into the world of work nowadays are mostly only suited to flipping
burgers or delivering pizzas.

Old farts have always said that, and always will.

When doing A-level pure maths, homework often consisted of
doing questions from past exams. The older ones were more
difficult than the recent ones, with those from the
early 50s being bloody hard.

 

[Jasen Betts]

On 2018-08-19, 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. 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?

if too high add parallell resistance.
if too low select a higer voltage range.

> https://tinyurl.com/ycjz9l4o

unreal 5uA full scale.



--
ŘŞ

 

[Rob]

Jasen Betts <jasen@xnet.co.nz> wrote:

On 2018-08-19, 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. 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?

if too high add parallell resistance.
if too low select a higer voltage range.

In the old analog meters, there basically was only a current meter,
often 50uA full scale, and when measuring voltage a suitable series
resistor is switched in to make it draw 50uA at the full scale reading.

So a 10V range would have a total resistance of 10V/50uA = 200k
(which would be the resistance of the meter itself plus the series R).
At 10V measured voltage there is 50uA through the 200k resistance.

When looking at this, any range will have a resistance of 1V/50uA per
volt of range, hence "20K per volt". The 100V range will be 2M.

This is no longer true for a modern DVM. They usually have a 10M
series resistor on the input with selectable resistors to ground to
make a voltage divider that outputs the desired voltage for the ADC.

So, depending on the range you select, the input resistance will be
10M plus a small value that will get smaller when you select a higher
range.

Therefore there is no fixed "K per volt" input resistance anymore, and
selecting a higher range will not result in a higher resistance.


However, as already can be seen, the "20K per volt" is not really
telling the input resistance to be used in the measurement.
It depends on the selected range, and available ranges vary between
meters. One may have ranges of 10-30-100 and another maybe 10-50-200.
When you need to measure a 24V testpoint, on one meter it may be on
the 30V range (and thus 600k resistance) and on another meter it would
be the 50V range (and thus 1M resistance).
It is expected that the person doing the measurement understands how
this could affect the result, if it does at all.
(when measuring a supply voltage, there should not be a noticable
difference. when measuring in a high-impedance signal circuit, there
could be)

 

[Tom Gardner]

On 20/08/18 00:44, Jeff Liebermann wrote:

On Sun, 19 Aug 2018 09:56:08 -0700, John Larkin
jjlarkin@highlandtechnology.com> wrote:

The engineering approach to fixing things is to probe
around, understand how it's supposed to work, and figure out why it
doesn't.

Nope.

The difference between engineering and repair is that the engineer
assumes that the problem is due to a design error and fixes the
problem by redesigning the circuit. The repair tech assumes that it
was designed and built correctly, therefore something has blown.

The engineers needs to make production lots of an instrument work. The
repair technician usually needs to make one work.

The engineer tries to determine how the circuit should work. The
repair tech tries to determine what the engineer was thinking when he
designed the circuit.

The engineer understands how the circuit should work. The technician
understands what the circuit actually does.

The engineer has experience making the instrument work under
laboratory conditions. The technician has experience making it work
in the rather nasty "real world" environment.

The engineer selects components based on availability, performance,
price, and lifetime. The repair tech substitutes whatever can be
found in his junk box.

The engineer writes the documentation partly to demonstrate to the
world the cleverness and greatness of his design. The repair tech
doesn't read the documentation unless he's desperate.

The engineer makes measurements in order to find problems. The repair
tech looks for smoke, burned parts, bulging capacitors, broken
connections, manufacturing errors, and mechanical damage. Well, maybe
he does take a few measurements like the power supply voltages.

All this works very nicely as long as engineers don't try to act like
technicians and technicians don't try to act like engineers.

There is, of course, one domain where that engineer/technician
distinction is almost completely meaningless: software,
particularly "enterprise" software.

Worse, they are proud of it, and actively seek to merge
all development phases. The end result is that many sticky
fingers poke at the various parts of the system, and eventually
nobody knows a system's specification or what it actually does.

Provided it doesn't fail the tests, it is defined as working.

 

[pfjw@aol.com]

OK - Let me start off with the statement, paraphrased from A.A. Milne:

When you are a Bear of Very Little Brain, and you Think of Things, you find sometimes that a Thing which seemed very Thingish inside you is quite different when it gets out into the open and has other people looking at it.

I have been repairing and restoring vintage equipment, as a hobby, for over 40 years now. For over 25 years of that time, I have used a Fluke auto-ranging DMM, and never a VTVM or similar vintage equivalent, although I have used same for comparative purposes. Discovering the following:

a) Yes, I have gotten different voltage readings 'from the book'.
b) Many (but not all) of which are explained by higher wall-plate voltages.
c) The differences not explained by wall-plate voltages are usually consistent.
d) Meaning that I can account for, and adjust to these differences without having to introduce outboard solutions.

Peter Wieck
Melrose Park, PA

 

[John-Del]

On Monday, August 20, 2018 at 9:14:49 AM UTC-4, pf...@aol.com wrote:

OK - Let me start off with the statement, paraphrased from A.A. Milne:

When you are a Bear of Very Little Brain, and you Think of Things, you find sometimes that a Thing which seemed very Thingish inside you is quite different when it gets out into the open and has other people looking at it.

I have been repairing and restoring vintage equipment, as a hobby, for over 40 years now. For over 25 years of that time, I have used a Fluke auto-ranging DMM, and never a VTVM or similar vintage equivalent, although I have used same for comparative purposes. Discovering the following:

a) Yes, I have gotten different voltage readings 'from the book'.
b) Many (but not all) of which are explained by higher wall-plate voltages.
c) The differences not explained by wall-plate voltages are usually consistent.
d) Meaning that I can account for, and adjust to these differences without having to introduce outboard solutions.

Peter Wieck
Melrose Park, PA

Just to add (and not referring directly to any measurements you may have made) that many discrepancies in measurements are the result of not following the service manual specifics regarding test conditions. A properly prepared SM will include the conditions of measurement - such as line voltage, warm up time, impedance of the meter, whether or not a signal is applied, setting of customer controls and service adjustments and placement of switches or functions selected. There are others I'm sure.

 

[pfjw@aol.com]

On Monday, August 20, 2018 at 10:21:20 AM UTC-4, John-Del wrote:

> Just to add (and not referring directly to any measurements you may have made) that many discrepancies in measurements are the result of not following the service manual specifics regarding test conditions. A properly prepared SM will include the conditions of measurement - such as line voltage, warm up time, impedance of the meter, whether or not a signal is applied, setting of customer controls and service adjustments and placement of switches or functions selected. There are others I'm sure.

Yep. Try aligning any of several multi-band radios without following the manual - and see how far you get.

Peter Wieck
Melrose Park, PA

 

[Ralph Mowery]

In article <073eaadd-5130-4d68-84ec-df3260d997b9@googlegroups.com>,
peterwieck33@gmail.com says...

Just to add (and not referring directly to any measurements you may have made) that many discrepancies in measurements are the result of not following the service manual specifics regarding test conditions. A properly prepared SM will include the conditions of measurement - such as line voltage, warm up time, impedance of the
meter, whether or not a signal is applied, setting of customer controls and service adjustments and placement of switches or functions selected. There are others I'm sure.

Yep. Try aligning any of several multi-band radios without following the manual - and see how far you get.

Sometimes the manual is wrong. I have an old Hammurlaund hq140 and one
alignment step says hook a meter across the speaker and apply a
unmodulated signal and tune for maximum. Only problem is it should be a
modulated signal or there will not be any speaker output.

Many times on the old equipment there is nothing more than a schematic
and one just has to make the best guess.

Reminds me of a fellow at work. He had a good memory and could go right
through the calibration of many instruments. However if something went
wrong he had no idea how to correct it and would have to call on others.

My memory is not that good,so I had several notebooks with my steps on
how to do the calibration. If something went wrong, it was usually easy
for me to find the problem. Many times I have found out there are
simple steps to take for repair that is not in manuals. Finding that,I
have often written my own 'service manual'/

 

[Ron D.]

When calibrating tube testers it really matters. Otherwise a parallel resistace should work. Many DMM's have a 10 M input Z.

A "Typical" scope might have a 1 M shunted by 22 pf or so. When you add a x10 probe the input Z should be 10 ohms resistive.

Many DVM' had say a 50 uA movement with some sort of resistance (meter movement+series), Ohms/V is the reciprocal of current. So, your actually measuring a current through a fixed resistance with a scale in volts.`

 

Most DVMs I've worked with are 11 Meg regardless of voltage settings. This is not 'stupendous'. A 50 KΊ/Volt meter on a 1000 setting will be 50 Meg but is only 250K on a 5 Volt setting. I've been using DVMs for service for nearly 40 years and have not run into any faulty readings because of an 11 meg load with one exception. In a Sony BVP30 broadcast camera they use a 200 Volt power supply for the electrostatic deflection. The 11 meg load was way too LOW to get valid readings We had to get a high voltage probe which is over a giga ohm to get useful readings.

G²

 

[Ron D.]

I figured I have to back my statement up:

from: PDF page #1: https://www.desmith.net/NMdS/Electronics/Hickok/Hickok%20539C%20Calibration%20V4.1.pdf


Use this procedure to test and calibrate the Hickok
Model 539C mutual conductance (AKA
transconductance) tube tester. Except as noted, all
of the readings are taken with a 1000 ohms per vol
t
meter. If an accurate 1000 ohms per volt meter is n
ot available a modern high impedance analog or
digital voltmeter can be used with appropriate shun
t resistors in parallel with the input to simulate
proper loading. The following resistor values shoul
d be used: 10 volt scale use 10K, 50 volt scale use
51K,
250 volt scale use 250K. All resistors are 1/2 watt
5% carbon composition. Calibration will be easier
if
you supply AC power through a constant voltage regu
lation type transformer to do the tests, but this i
s
not essential. Recalibrate the tester any time eith
er rectifier tube is replaced. The correct type #81
(#63
for 230VAC mains) fuse lamp must be installed in th
e tester or false readings can result.