Vintage equipment voltage measurement

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:

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Cursitor Doom
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k

add a resistor to the meter and be done with it?

Reply to
Lasse Langwadt Christensen

Not that simple, though AFAIK. You can't add a fixed resistance in series with the probe because you will be measuring voltages with all sorts of different source impedances.

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Cursitor Doom

with the probe "

DOH ! ! !

No, across the leads, in parallel. A resistor that will LOAD the circuit like an old VOM would,

Or you could use knowledge to compensate, look at the print and see what the source resistance is and just figure out how much loading would take off of the voltage.

Reply to
jurb6006

20k per volt doesn't really mean exactly that. It means 20k on the 1V range, 200k on the 10V range etc. So a parallel resistor appropriate to the meter range is all you need.

Cheers

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Clive
Reply to
Clive Arthur

Cursitor Doom wrote

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?

Reply to
<698839253X6D445TD

nge, 200k on the 10V range etc. "

I thought that went without saying. Ugh, what was I thinking ? Nothing goes without saying these days. I mean we are getting to the point where Mapque st instructions start with "Wake up, get dressed, find your car keys...".

And thank you for spelling "etc." correctly. An abbreviation for "et cetera ", for some reason people think it is "ect" and they don't even bother with the period. Actually many people use it when "et alia" or "et alii" would be more appropriate, both abbreviable to "et al.". Not exactly sure right n ow but I think "alia" and "alii" mean others and more while "cetera" means and so forth or and so on.

Something like that.

(hey, BS like that is what Sundays is for)

Reply to
jurb6006

Why not ignore the voltage notes and just fix it?

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Good grief. You need a /parallel/ resistor.

Traditionally at FSD there is 50uA through the meter, so that defines the resistor value.

Reply to
Tom Gardner

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

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Cursitor Doom

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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Cursitor Doom

'Cetera' (KAY-ter-ah) means 'the rest', so 'and the rest'. ('O' level Latin circa 1970)

Cheers

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Clive
Reply to
Clive Arthur

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.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Spot on!

Reply to
Tom Gardner

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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Reply to
Andy Burns

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

Reply to
mike

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

Well I think I can pretty assuredly claim that title and I can compensate f or bad readings. In fact there are quite a few circuits that I can just gue ss what the voltage, or even waveform should look like.

Know why I don't use my transistor checker ? Because from the waveforms on the base, emitter and collector I can tell if it is bad.

A tube amp, the plates are going to be about the DC of the output from the rectifier, the G2s a bit lower with some exceptions like a Williamson circu it or being used as triodes. The G1s will be negative or the cathodes will be positive. A phase splitter you want the cathode at about 1/3 of the B+ a nd the plate at 2/3, the grid voltage will be somewhat lower. For common st ages the cathode will usually be +DC within reason, and if it is intended t o use the full output range the plate will be at about 1/2 B+. that is not always true, sometimes the engineer will put it off center into the point o f the curve where it is the most linear. Still it needs headroom both ways.

Tube tester ? If the stage has a cathode resistor and there is positive vol tage on it the tube is conducting. The plate voltage will not be affected m uch by the DC bias current and even a 100:1 scope probe will throw the tuni ng off some.

Most tube circuitry is so simple I can damn near feel it. My problem now is following point to point. I used to not like circuit boards but after the years I have found them easier to follow. I am spoiled in that regard.

Back when you had to BUY prints for this stuff I was famous for not needing them. Some really complicated issues I actually sat there and drew the sch ematic of the circuit by reverse engineering.

And I have noticed mistakes on the voltages given. One line, which is a wir e, 12 volts on one side and ground on the other. Something does not compute .

The main thing I have needed the specified waveforms for were the SMPS, if any. I know my Tek is off a bit and I wish I had a clearer picture of the w aveform on the choppers. It is too old for an SMPS but is an AC/DC model.

My point ? Figure out what those voltages should be on your own.

Reply to
jurb6006

Latin circa 1970)"

Well we pronounce it "Set-er-a" around here. "The rest" sounds right, closer than what I said. It has been a while since I have paid attention to things like that. I would just be less unhappy if they wouldn't use "ect.".

Reply to
jurb6006

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.

Reply to
bitrex

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