Vintage equipment voltage measurement

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.

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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

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

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

unreal 5uA full scale.

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     ?

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.

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.

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)

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.

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 diff erent 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-ran ging DMM, and never a VTVM or similar vintage equivalent, although I have u sed 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 consist ent. d) Meaning that I can account for, and adjust to these differences without having to introduce outboard solutions.

Peter Wieck Melrose Park, PA

nd sometimes that a Thing which seemed very Thingish inside you is quite di fferent when it gets out into the open and has other people looking at it.

er 40 years now. For over 25 years of that time, I have used a Fluke auto-r anging DMM, and never a VTVM or similar vintage equivalent, although I have used same for comparative purposes. Discovering the following:

s.

stent.

t having to introduce outboard solutions.

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

made) that many discrepancies in measurements are the result of not followi ng the service manual specifics regarding test conditions. A properly prep ared 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, s etting of customer controls and service adjustments and placement of switch es or functions selected. There are others I'm sure.

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

Peter Wieck Melrose Park, PA

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.

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'/

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 K?/Volt meter on a 1000 setting will be 50 M eg but is only 250K on a 5 Volt setting. I've been using DVMs for service f or 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 wa s way too LOW to get valid readings We had to get a high voltage probe whic h is over a giga ohm to get useful readings.

I figured I have to back my statement up:

from: PDF page #1:

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.