Vintage equipment voltage measurement

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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



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Re: Vintage equipment voltage measurement
Put the appropriate resistance across the test leads.  

Re: Vintage equipment voltage measurement
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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?



Re: Vintage equipment voltage measurement
On Sun, 19 Aug 2018 16:13:00 +0000, 698839253X6D445TD wrote:

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



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Re: Vintage equipment voltage measurement
On Sunday, 19 August 2018 16:17:51 UTC+1, Cursitor Doom  wrote:
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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

Re: Vintage equipment voltage measurement
On Sun, 19 Aug 2018 15:17:49 -0000 (UTC), Cursitor Doom

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Why not ignore the voltage notes and just fix it?


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Vintage equipment voltage measurement
On Sun, 19 Aug 2018 09:24:57 -0700, John Larkin wrote:

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



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Re: Vintage equipment voltage measurement
On Sun, 19 Aug 2018 16:41:26 -0000 (UTC), Cursitor Doom

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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  


Re: Vintage equipment voltage measurement
On 19/08/18 17:56, John Larkin wrote:
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Spot on!

Re: Vintage equipment voltage measurement
On Sun, 19 Aug 2018 09:56:08 -0700, John Larkin wrote:

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



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Re: Vintage equipment voltage measurement
Cursitor Doom wrote:
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Your nym isn't that old.  What was it before?




Re: Vintage equipment voltage measurement
John Larkin wrote:

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This thread is cross-posted to both ...


Re: Vintage equipment voltage measurement
On 08/19/2018 12:56 PM, John Larkin wrote:
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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.

Re: Vintage equipment voltage measurement
On Sun, 19 Aug 2018 09:56:08 -0700, John Larkin

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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     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: Vintage equipment voltage measurement
On 20/08/18 00:44, Jeff Liebermann wrote:
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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.

Re: Vintage equipment voltage measurement
On 8/19/2018 8:17 AM, Cursitor Doom wrote:
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What's so hard about putting a resistor in parallel with your meter?

Re: Vintage equipment voltage measurement
On Sun, 19 Aug 2018 11:28:10 -0700, mike wrote:

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Never even occurred to me.



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Re: Vintage equipment voltage measurement
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That's what you get when people lack basic understanding of the matter.

Re: Vintage equipment voltage measurement
On Sun, 19 Aug 2018 19:09:10 +0000, Rob wrote:

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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!



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Re: Vintage equipment voltage measurement
On 19/08/18 20:41, Cursitor Doom wrote:
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And that is clearly true for more than just electronics!

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