[apa]

I'm installing a new output transformer to replace a blown one in an
amp with a quad of 6L6's.
I was taking some measurements on the new one before I installed it to
have as reference down the road. When I got to the DC resistance of
the primary, I ran into something strange.

On the primary side the DC resistance from either end to the center
tap measures about 60 Ohms on a Fluke 112 with the secondary unloaded.
When I try to measure from one end of the primary to the other end the
meter shows as out of range. If I load the secondary I get the
expected reading of roughly 120 Ohms. If I disconnect the load from
the secondary, the reading holds but it returns to an out of range
reading if I disconnect the meter and connect it again.

Is the inductance of the unloaded primary somehow effecting the
reading? If so, why does the reading hold after the load is detached?
Can someone explain what's going on?

Thanks, Andy

[Eeyore]

apa wrote:

I'm installing a new output transformer to replace a blown one in an
amp with a quad of 6L6's.
I was taking some measurements on the new one before I installed it to
have as reference down the road. When I got to the DC resistance of
the primary, I ran into something strange.

On the primary side the DC resistance from either end to the center
tap measures about 60 Ohms on a Fluke 112 with the secondary unloaded.
When I try to measure from one end of the primary to the other end the
meter shows as out of range. If I load the secondary I get the
expected reading of roughly 120 Ohms. If I disconnect the load from
the secondary, the reading holds but it returns to an out of range
reading if I disconnect the meter and connect it again.

Is the inductance of the unloaded primary somehow effecting the
reading?

Yes.


If so, why does the reading hold after the load is detached?

The test current has stabilised.


Can someone explain what's going on?

The meter doesn't 'like' the inductance. By adding a load on the sec,
you're damping the circuit so it's happy. Once it's happy you can remove
the load and it'll stay happy. Not 100% sure what's going on but I'll bet
it has to do with inductive flyback. You can sometimes get my Fluke to
'cycle' the reading on some transformers as it keeps re-trying the
measurement.

Graham

[apa]

On Mar 19, 2:25 pm, Eeyore
wrote:
apa wrote:
I'm installing a new output transformer to replace a blown one in an
amp with a quad of 6L6's.
I was taking some measurements on the new one before I installed it to
have as reference down the road. When I got to the DC resistance of
the primary, I ran into something strange.

On the primary side the DC resistance from either end to the center
tap measures about 60 Ohms on a Fluke 112 with the secondary unloaded.
When I try to measure from one end of the primary to the other end the
meter shows as out of range. If I load the secondary I get the
expected reading of roughly 120 Ohms. If I disconnect the load from
the secondary, the reading holds but it returns to an out of range
reading if I disconnect the meter and connect it again.

Is the inductance of the unloaded primary somehow effecting the
reading?

Yes.

If so, why does the reading hold after the load is detached?

The test current has stabilised.

Can someone explain what's going on?

The meter doesn't 'like' the inductance. By adding a load on the sec,
you're damping the circuit so it's happy. Once it's happy you can remove
the load and it'll stay happy. Not 100% sure what's going on but I'll bet
it has to do with inductive flyback. You can sometimes get my Fluke to
'cycle' the reading on some transformers as it keeps re-trying the
measurement.

Graham

That's what it looks like mine is doing - the out of range indication
keeps flashing.
This is the first transformer that's done this to mine. Nice to know
I'm not alone!
Thanks, Andy

[Scott Dorsey]

apa wrote:
I'm installing a new output transformer to replace a blown one in an
amp with a quad of 6L6's.
I was taking some measurements on the new one before I installed it to
have as reference down the road. When I got to the DC resistance of
the primary, I ran into something strange.

What kind of meter are you using?

On the primary side the DC resistance from either end to the center
tap measures about 60 Ohms on a Fluke 112 with the secondary unloaded.
When I try to measure from one end of the primary to the other end the
meter shows as out of range. If I load the secondary I get the
expected reading of roughly 120 Ohms. If I disconnect the load from
the secondary, the reading holds but it returns to an out of range
reading if I disconnect the meter and connect it again.

Okay, first of all you know that the transformer is blown, so there could
well be shorts between windings.

Secondly, you could be using a super-high-Z-meter which is seeing small
amounts of capacitance as a short. If this is the issue, waiting a few
minutes will cause the resistance to change.

Is the inductance of the unloaded primary somehow effecting the
reading? If so, why does the reading hold after the load is detached?
Can someone explain what's going on?

Get a Simpson 260. Don't use a DVM for this kind of work.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Eeyore]

Scott Dorsey wrote:

Don't use a DVM for this kind of work.

That certainly does tend to solve the problem.

Thinking about it, I reckon it's the autoranging on DVMs that causes this
trouble. If you can select a fixed range it may be OK.

Graham

[apa]

On Mar 19, 3:23 pm, (Scott Dorsey) wrote:
apa wrote:
I'm installing a new output transformer to replace a blown one in an
amp with a quad of 6L6's.
I was taking some measurements on the new one before I installed it to
have as reference down the road. When I got to the DC resistance of
the primary, I ran into something strange.

What kind of meter are you using?


Fluke 112

On the primary side the DC resistance from either end to the center
tap measures about 60 Ohms on a Fluke 112 with the secondary unloaded.
When I try to measure from one end of the primary to the other end the
meter shows as out of range. If I load the secondary I get the
expected reading of roughly 120 Ohms. If I disconnect the load from
the secondary, the reading holds but it returns to an out of range
reading if I disconnect the meter and connect it again.

Okay, first of all you know that the transformer is blown, so there could
well be shorts between windings.

The measurements are on the new replacement transformer - not the
blown one.

Secondly, you could be using a super-high-Z-meter which is seeing small
amounts of capacitance as a short. If this is the issue, waiting a few
minutes will cause the resistance to change.

Is the inductance of the unloaded primary somehow effecting the
reading? If so, why does the reading hold after the load is detached?
Can someone explain what's going on?

Get a Simpson 260. Don't use a DVM for this kind of work.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Thanks, Andy

[Scott Dorsey]

Eeyore wrote:
Scott Dorsey wrote:

Don't use a DVM for this kind of work.

That certainly does tend to solve the problem.

Thinking about it, I reckon it's the autoranging on DVMs that causes this
trouble. If you can select a fixed range it may be OK.

I have spent too much time looking for stray voltages that didn't really
exist, because the DVM showed some static or leakage charge. The Simpson 260
is my friend.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mark]

On Mar 19, 1:56*pm, apa wrote:
I'm installing a new output transformer to replace a blown one in an
amp with a quad of 6L6's.
I was taking some measurements on the new one before I installed it to
have as reference down the road. When I got to the DC resistance of
the primary, I ran into something strange.

On the primary side the DC resistance from either end to the center
tap measures about 60 Ohms on a Fluke 112 with the secondary unloaded.
When I try to measure from one end of the primary to the other end the
meter shows as out of range. If I load the secondary I get the
expected reading of roughly 120 Ohms. If I disconnect the load from
the secondary, the reading holds but it returns to an out of range
reading if I disconnect the meter and connect it again.

Is the inductance of the unloaded primary somehow effecting the
reading? If so, why does the reading hold after the load is detached?
Can someone explain what's going on?

Thanks, Andy

Andy,

I have seen digital DMMs do strange things with inductive loads. I
have seen one go into oscillation once in a similar case. If you have
a scope, you can confirm what is going on.


That's why I like a Simpson 260, I know exactly how it works and what
it is reading.

Mark

[Mike Rivers]

On Mar 19, 4:03 pm, (Scott Dorsey) wrote:

I have spent too much time looking for stray voltages that didn't really
exist, because the DVM showed some static or leakage charge. The Simpson 260
is my friend.

Mine, too (actually, I have a Triplett 630) but this is a transformer
and DVMs, at least mine, use DC for resistance measurements. You can
store enough voltage in a large capacitor to throw off a resistance
(leakage) reading for a long time, but I've never heard of a
transformer that can store energy very long.

I'm quite puzzled by the behavior that Andy describes. If the center
tap is really a center tap, then there should be resistance from one
end to the other. I can come up with a "huh?????" for every one of the
measurements he made, unless he's using an ohm meter with an AC source
rather than DC.

Is the meter set on Capacitance rather than Resistance? That might
explain some of the readings, but not all.

[Scott Dorsey]

Mike Rivers wrote:
On Mar 19, 4:03 pm, (Scott Dorsey) wrote:

I have spent too much time looking for stray voltages that didn't really
exist, because the DVM showed some static or leakage charge. The Simpson 260
is my friend.

Mine, too (actually, I have a Triplett 630) but this is a transformer
and DVMs, at least mine, use DC for resistance measurements. You can
store enough voltage in a large capacitor to throw off a resistance
(leakage) reading for a long time, but I've never heard of a
transformer that can store energy very long.

A transformer, between windings, is a capacitor. In fact, everything is
a capacitor if you look at it the right way.

I'm quite puzzled by the behavior that Andy describes. If the center
tap is really a center tap, then there should be resistance from one
end to the other. I can come up with a "huh?????" for every one of the
measurements he made, unless he's using an ohm meter with an AC source
rather than DC.

Right, unless he's actually looking between the two windings. There is
a RETMA standard for output transformer lead color-codes and the new
transformer and the old transformer should be fine.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mike Rivers]

On Mar 19, 7:14 pm, (Scott Dorsey) wrote:

A transformer, between windings, is a capacitor. In fact, everything is
a capacitor if you look at it the right way.

Yeah, but it's gotta be at least a few microfarads in order to distort
a resistance reading like that, unless he's only looking at the meter
for a few milliseconds.

Right, unless he's actually looking between the two windings. There is
a RETMA standard for output transformer lead color-codes and the new
transformer and the old transformer should be fine.

I thought about that, too, but if that's the case and he doesn't get
it sorted out before he installes the new transformer, it could very
quickly become as useless as the one it's replacing.

[apa]

On Mar 19, 6:39 pm, Mike Rivers wrote:
On Mar 19, 4:03 pm, (Scott Dorsey) wrote:

I have spent too much time looking for stray voltages that didn't really
exist, because the DVM showed some static or leakage charge. The Simpson 260
is my friend.

Mine, too (actually, I have a Triplett 630) but this is a transformer
and DVMs, at least mine, use DC for resistance measurements. You can
store enough voltage in a large capacitor to throw off a resistance
(leakage) reading for a long time, but I've never heard of a
transformer that can store energy very long.

I'm quite puzzled by the behavior that Andy describes. If the center
tap is really a center tap, then there should be resistance from one
end to the other. I can come up with a "huh?????" for every one of the
measurements he made, unless he's using an ohm meter with an AC source
rather than DC.

Is the meter set on Capacitance rather than Resistance? That might
explain some of the readings, but not all.

It is definitely the center tap and when I load the secondary I get
exactly what I would expect to see end to end (120 ohms which is the
sum of the two 60 ohm reading from the center to each end).

[apa]

On Mar 19, 3:30 pm, Eeyore
wrote:
Scott Dorsey wrote:
Don't use a DVM for this kind of work.

That certainly does tend to solve the problem.

Thinking about it, I reckon it's the autoranging on DVMs that causes this
trouble. If you can select a fixed range it may be OK.

Graham

Bingo.
Thanks Graham.
I switched it off off autoranging and it worked fine.
Best, Andy

[Mike Rivers]

On Mar 19, 9:45 pm, apa wrote:

I switched it off off autoranging and it worked fine.

This still makes absolutely no sense to me. Is there an explanatin for
why it can't find its correct range when measuring across the whole
primary without a load on the secondary? I'd love to see this
transformer, but I suppose that's impractical.

Truth, though, I doubt that I've measured a transformer with a DMM. I
always reach for the analog meter.

[Chris Hornbeck]

On Wed, 19 Mar 2008 18:45:08 -0700 (PDT), apa
wrote:

I switched it off off autoranging and it worked fine.

How do you switch it off autoranging?

Much thanks,

Chris Hornbeck
"It's for compatibility with 8-Track."
--scott

[Paul Stamler]

"Mike Rivers" wrote in message
...
On Mar 19, 7:14 pm, (Scott Dorsey) wrote:

A transformer, between windings, is a capacitor. In fact, everything is
a capacitor if you look at it the right way.

Yeah, but it's gotta be at least a few microfarads in order to distort
a resistance reading like that, unless he's only looking at the meter
for a few milliseconds.

Right, unless he's actually looking between the two windings. There is
a RETMA standard for output transformer lead color-codes and the new
transformer and the old transformer should be fine.

I thought about that, too, but if that's the case and he doesn't get
it sorted out before he installes the new transformer, it could very
quickly become as useless as the one it's replacing.

If I read the original post correctly, he's making these measurements on the
new transformer.

I concur with the general suggestion that he should be doing his
measurements with an analog meter.

Peace,
Paul

[Eeyore]

Chris Hornbeck wrote:

apa wrote:

I switched it off off autoranging and it worked fine.

How do you switch it off autoranging?

Press the button in the centre of the rotary switch IIRC. It has a
couple of functions so may need a couple of pushes.

Graham

[Eeyore]

Mike Rivers wrote:

On Mar 19, 9:45 pm, apa wrote:

I switched it off off autoranging and it worked fine.

This still makes absolutely no sense to me. Is there an explanatin for
why it can't find its correct range when measuring across the whole
primary without a load on the secondary? I'd love to see this
transformer, but I suppose that's impractical.

AIUI, the autoranging circuitry applies a different test current according
to the kind of value it's looking for. As it switches current you get a
dI/dt response from the inductance which throws the autoramging out and it
then gets into a loop.


Graham

[Eeyore]

Mike Rivers wrote:

I'm quite puzzled by the behavior that Andy describes. If the center
tap is really a center tap, then there should be resistance from one
end to the other. I can come up with a "huh?????" for every one of the
measurements he made, unless he's using an ohm meter with an AC source
rather than DC.

Oh, I assure you I have seen this behaviour when measuring the DC resistance of
transformer windings.

Graham

[Mike Rivers]

On Mar 20, 2:59 am, "Paul Stamler" wrote:

If I read the original post correctly, he's making these measurements on the
new transformer.

And hence my comment. I think he hasn't identified all the leads
correctly.

I concur with the general suggestion that he should be doing his
measurements with an analog meter.

But why doesn't a digital meter work? What's one meter measuring (or
not) that the other isn't?

[Mike Rivers]

On Mar 20, 7:27 am, Eeyore
wrote:

Oh, I assure you I have seen this behaviour when measuring the DC resistance of
transformer windings.

But can you explain it? What's happening here?

[Don Pearce]

On Thu, 20 Mar 2008 04:34:44 -0700 (PDT), Mike Rivers
wrote:

On Mar 20, 2:59 am, "Paul Stamler" wrote:

If I read the original post correctly, he's making these measurements on the
new transformer.

And hence my comment. I think he hasn't identified all the leads
correctly.

I concur with the general suggestion that he should be doing his
measurements with an analog meter.

But why doesn't a digital meter work? What's one meter measuring (or
not) that the other isn't?

I've often found trouble with digital meters on inductive loads when
using autoranging. The problem is that the measuring current changes
with range, and that can send the thing into a feedback loop that
oscillates between ranges - you can never get a stable reading. Once
you pin them down to a single manual range, they tend to be fine.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Mike Rivers]

On Mar 20, 7:25 am, Eeyore
wrote:

AIUI, the autoranging circuitry applies a different test current according
to the kind of value it's looking for. As it switches current you get a
dI/dt response from the inductance which throws the autoramging out and it
then gets into a loop.

Strange, but not out of the question. It must be right on the hairy
edge to respond that way through the full winding and not through half
the winding. And it seems that loading the secondary would make the
problem worse rather than fix it, because that would cause current to
flow in the secondary, which would then induce current back into the
primary. Unless the secondary is acting to damp the current surge in
the primary.

You'd think that after all this time, DMM manufacturers would have
recognized this problem and used a sufficiently slow ramp time rather
than an instantaneous change in current when autoranging.

[Don Pearce]

On Thu, 20 Mar 2008 04:40:15 -0700 (PDT), Mike Rivers
wrote:

On Mar 20, 7:25 am, Eeyore
wrote:

AIUI, the autoranging circuitry applies a different test current according
to the kind of value it's looking for. As it switches current you get a
dI/dt response from the inductance which throws the autoramging out and it
then gets into a loop.

Strange, but not out of the question. It must be right on the hairy
edge to respond that way through the full winding and not through half
the winding. And it seems that loading the secondary would make the
problem worse rather than fix it, because that would cause current to
flow in the secondary, which would then induce current back into the
primary. Unless the secondary is acting to damp the current surge in
the primary.

You'd think that after all this time, DMM manufacturers would have
recognized this problem and used a sufficiently slow ramp time rather
than an instantaneous change in current when autoranging.

If you load the secondary, it stops being an inductor and becomes a
resistor (to the transient that is, not the stabilized DC) - aside
from any residual leakage inductance of course.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Mike Rivers]

On Mar 20, 7:49 am, (Don Pearce) wrote:

If you load the secondary, it stops being an inductor and becomes a
resistor (to the transient that is, not the stabilized DC) - aside
from any residual leakage inductance of course.

Only if you load it properly (and exactly). We don't know what he
loaded the secondary with - a resistor? a short circuit? a
loudspeaker? (it was an amplifier output transformer)

I guess there's a plausible explanation and this issue has been
observed by others, but it's strange that I've never heard of it until
now. Maybe it's just one of those things that everybody knows but me.

I'll have to dig a couple of transformers out of the junk box and give
it a try.

[Don Pearce]

On Thu, 20 Mar 2008 04:55:42 -0700 (PDT), Mike Rivers
wrote:

On Mar 20, 7:49 am, (Don Pearce) wrote:

If you load the secondary, it stops being an inductor and becomes a
resistor (to the transient that is, not the stabilized DC) - aside
from any residual leakage inductance of course.

Only if you load it properly (and exactly). We don't know what he
loaded the secondary with - a resistor? a short circuit? a
loudspeaker? (it was an amplifier output transformer)


Not really, over a pretty wide range of values of load resistor, the
transformer will do a good job of just - well - transforming it There
is no single "correct" load.
..
I guess there's a plausible explanation and this issue has been
observed by others, but it's strange that I've never heard of it until
now. Maybe it's just one of those things that everybody knows but me.


It only happens when the inductance is right, so it isn't something
that happens every time. But once seen, never forgotten.

I'll have to dig a couple of transformers out of the junk box and give
it a try.

Quite so.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Eeyore]

Mike Rivers wrote:

But why doesn't a digital meter work?

It WILL do as long as it's not autoranging.

See my posts on the subject.

Graham

[Eeyore]

Mike Rivers wrote:

On Mar 20, 7:27 am, Eeyore
wrote:

Oh, I assure you I have seen this behaviour when measuring the DC resistance of
transformer windings.

But can you explain it? What's happening here?

See my other posts.

Graham

[Eeyore]

Mike Rivers wrote:

On Mar 20, 7:25 am, Eeyore
wrote:

AIUI, the autoranging circuitry applies a different test current according
to the kind of value it's looking for. As it switches current you get a
dI/dt response from the inductance which throws the autoramging out and it
then gets into a loop.

Strange, but not out of the question. It must be right on the hairy
edge to respond that way through the full winding and not through half
the winding.

I'm sure it is and I'm sure that different firmware in the meter could fix it
too.


And it seems that loading the secondary would make the
problem worse rather than fix it, because that would cause current to
flow in the secondary, which would then induce current back into the
primary. Unless the secondary is acting to damp the current surge in
the primary.

The 'load' is indeed damping the circuit.


You'd think that after all this time, DMM manufacturers would have
recognized this problem and used a sufficiently slow ramp time rather
than an instantaneous change in current when autoranging.

I tend to agree.

Graham

[Eeyore]

Mike Rivers wrote:

On Mar 20, 7:49 am, (Don Pearce) wrote:

If you load the secondary, it stops being an inductor and becomes a
resistor (to the transient that is, not the stabilized DC) - aside
from any residual leakage inductance of course.

Only if you load it properly (and exactly).

No.

ANY load will reflect back to the primary quite effectively.

Graham

[apa]

On Mar 20, 7:23 am, Eeyore
wrote:
Chris Hornbeck wrote:
apa wrote:

I switched it off off autoranging and it worked fine.

How do you switch it off autoranging?

Press the button in the centre of the rotary switch IIRC. It has a
couple of functions so may need a couple of pushes.

Graham

On the Fluke 112 you push the range button to switch to manual mode.
Push it again to page through the ranges.
Push and hold to return to Auto mode.

-Andy

[apa]

On Mar 20, 7:35 am, Mike Rivers wrote:
On Mar 20, 7:27 am, Eeyore
wrote:

Oh, I assure you I have seen this behaviour when measuring the DC resistance of
transformer windings.

But can you explain it? What's happening here?

When I hook the scope up in parallel with the meter in Auto Ranging
mode I see a DC spike - every 80ms or so if I'm remembering correctly.

If I measure the DC output of the Fluke 112 set to Manual Range and
measure the DC output with another DVM, the output changes as the
Range setting is changed (.6 V to 1.2V IIRC).

So the inductor isn't really seeing DC. May be the center tap to end
inductance isn't enough to foul the autosensing circuit but double
that is?

-Andy

[apa]

On Mar 20, 7:55 am, Mike Rivers wrote:
On Mar 20, 7:49 am, (Don Pearce) wrote:

If you load the secondary, it stops being an inductor and becomes a
resistor (to the transient that is, not the stabilized DC) - aside
from any residual leakage inductance of course.

Only if you load it properly (and exactly). We don't know what he
loaded the secondary with - a resistor? a short circuit? a
loudspeaker? (it was an amplifier output transformer)

I guess there's a plausible explanation and this issue has been
observed by others, but it's strange that I've never heard of it until
now. Maybe it's just one of those things that everybody knows but me.

I'll have to dig a couple of transformers out of the junk box and give
it a try.

I've got about a dozen out of circuit OPT's around - none of them (or
any others I've measured before) have acted this way.

[apa]

On Mar 19, 3:23 pm, (Scott Dorsey) wrote:
apa wrote:
I'm installing a new output transformer to replace a blown one in an
amp with a quad of 6L6's.
I was taking some measurements on the new one before I installed it to
have as reference down the road. When I got to the DC resistance of
the primary, I ran into something strange.

What kind of meter are you using?

On the primary side the DC resistance from either end to the center
tap measures about 60 Ohms on a Fluke 112 with the secondary unloaded.
When I try to measure from one end of the primary to the other end the
meter shows as out of range. If I load the secondary I get the
expected reading of roughly 120 Ohms. If I disconnect the load from
the secondary, the reading holds but it returns to an out of range
reading if I disconnect the meter and connect it again.

Okay, first of all you know that the transformer is blown, so there could
well be shorts between windings.

Secondly, you could be using a super-high-Z-meter which is seeing small
amounts of capacitance as a short. If this is the issue, waiting a few
minutes will cause the resistance to change.

Is the inductance of the unloaded primary somehow effecting the
reading? If so, why does the reading hold after the load is detached?
Can someone explain what's going on?

Get a Simpson 260. Don't use a DVM for this kind of work.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


For future reference what else would you put on the "Don't use a DVM"
list?

Thanks, Andy

[Mike Rivers]

Don Pearce wrote:
If you load the secondary, it stops being an inductor and becomes a
resistor (to the transient that is, not the stabilized DC) - aside
from any residual leakage inductance of course.

I replied:
Only if you load it properly (and exactly).

On Mar 20, 9:46 am, Eeyore
wrote:
No.
ANY load will reflect back to the primary quite effectively.

My comment wasn't that a load on the secondary wouldn't reflect back
to the primary, it was that the secondary will only appear as a
resistor (no inductance and no capacitance) with the proper complex
load.

You're arguing trivialities here, as usual. I like Andy's observation
about repeated spikes when the meter is connected. I just think it
shouldn't be allowed to DO that (to which I see you concur).

I fished out a couple of transformers this morning and measured them
with my Fluke 77 autoranging DMM. One was about a 25 watt sized output
transformer, one was a pretty hefty 600-6000 ohm line transformer, and
one was a 400 volt power transformer. All of them measured as
expected, same as with an analog meter.

I'd like to measure someone's transformer that doesn't measure
predictably on his DMM (but I won't pay for shipping g)

[Scott Dorsey]

apa wrote:

For future reference what else would you put on the "Don't use a DVM"
list?

In general, old tube gear will come with schematics with nominal voltages
measured with an analogue meter. If you measure with a DVM, they won't
match up.

On fairly high current stuff like auto wiring or electrical circuits,
you can have an corroded open that is ALMOST open but not quite, which will
measure fine with the dvm. Use a "wiggy" or a light bulb with leads
soldered to it.

If you are looking at voltages which vary, say a musical signal or a
capacitor charging or discharging, the DVM will be all over the place.
A mechanical meter will allow you to see the rate of change and you can
count seconds as you watch the needle dropto calculate time constants.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mike Rivers]

On Mar 20, 10:22 am, apa wrote:

I've got about a dozen out of circuit OPT's around - none of them (or
any others I've measured before) have acted this way.

Aha! Same here. Well, stand back with a fire extinguisher when you
install the transformer in the amplifier. g

[Leo Marx]

Scott Dorsey wrote:

Eeyore wrote:
Scott Dorsey wrote:

Don't use a DVM for this kind of work.

That certainly does tend to solve the problem.

Thinking about it, I reckon it's the autoranging on DVMs that causes this
trouble. If you can select a fixed range it may be OK.

I have spent too much time looking for stray voltages that didn't really
exist, because the DVM showed some static or leakage charge. The Simpson 260
is my friend.
--scott



Ya, I have a Triplett 630 on my bench, I only use it to test diodes,
transistors and SCRs.

JAM

[Scott Dorsey]

Mike Rivers wrote:
expected, same as with an analog meter.

I'd like to measure someone's transformer that doesn't measure
predictably on his DMM (but I won't pay for shipping g)

Get a variac. Put the meter on it. Adjust it until the meter starts
going wonky. Voila, you have found the inductance that causes the meter
to go wonky.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[apa]

On Mar 20, 11:17 am, (Scott Dorsey) wrote:
Mike Rivers wrote:

expected, same as with an analog meter.

I'd like to measure someone's transformer that doesn't measure
predictably on his DMM (but I won't pay for shipping g)

Get a variac. Put the meter on it. Adjust it until the meter starts
going wonky. Voila, you have found the inductance that causes the meter
to go wonky.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

As long as you keep disconnecting and reconnecting the meter because
once it's settled on a range it won't go wonky until it tries to
autorange again.