Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old May 1st 21, 11:36 PM posted to sci.electronics.repair
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Default tests to do on used oscilloscope?

After selling off a prior oscilloscope nearly two decades ago, I decided
to pick up another used one. I have forgotten most of the functions.
This is a used two channel 30 Mhz Elenco S-1325. I can return if it
fails my testing.

All I did so far use a coax t adapter between an RF signal generator and
a 50 ohm dummy load to channel the remaining side into one of the scope
channels. I then set it for 28 Mhz and 100,000 uV output. After some
fiddling, mainly because I forgot most functions, I finally got a nice
sine wave of the approximate frequency. What other functions might I
perform to make sure it is functioning ok?

Thanks.

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Old May 2nd 21, 01:22 AM posted to sci.electronics.repair
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Default tests to do on used oscilloscope?

Jason Bowers wrote:
================
After selling off a prior oscilloscope nearly two decades ago, I decided
to pick up another used one. I have forgotten most of the functions.
This is a used two channel 30 Mhz Elenco S-1325. I can return if it
fails my testing.

All I did so far use a coax t adapter between an RF signal generator and
a 50 ohm dummy load to channel the remaining side into one of the scope
channels. I then set it for 28 Mhz and 100,000 uV output. After some
fiddling, mainly because I forgot most functions, I finally got a nice
sine wave of the approximate frequency. What other functions might I
perform to make sure it is functioning ok?



** You need to buy or borrow a test oscillator, function or sine /square.
Then you check each setting of the controls, one at a time.
Square waves should look "square".

Vertical calibration can be checked with the help of any DMM on AC volts and a 400Hz sine wave.
Horizontal calibration really needs a frequency counter, but the 50/60 Hz supply is very accurate .

Make sure the "trace rotation" controlactually works.


...... Phil







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Old May 2nd 21, 09:13 PM posted to sci.electronics.repair
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Default tests to do on used oscilloscope?

On 5/2/21 6:18 AM, Mike Coon wrote:
In article , says...

After selling off a prior oscilloscope nearly two decades ago, I decided
to pick up another used one. I have forgotten most of the functions.
This is a used two channel 30 Mhz Elenco S-1325. I can return if it
fails my testing.

All I did so far use a coax t adapter between an RF signal generator and
a 50 ohm dummy load to channel the remaining side into one of the scope
channels. I then set it for 28 Mhz and 100,000 uV output. After some
fiddling, mainly because I forgot most functions, I finally got a nice
sine wave of the approximate frequency. What other functions might I
perform to make sure it is functioning ok?

Thanks.


Does it offer combinations of two channels, e.g. A+B; A-B. Latter
differential could be checked with identical inputs and seeing how
accurately you get zero!


Thanks to you both. Well, all is not paradise and beginning to think
that I may return it. I could be overlooking something so looking for
some suggestions.

It seems that both frequency and voltage readings are off. I have an
old URM-25 RF signal generator that I have been feeding into it by
sampling off of the side of a t connector with one side having 50 ohm
dummy load and the other into ch 1 of the scope. Anything I've tried in
frequency and it seems like I'm reading at least double that frequency
on the scope if I count divisions from P-P. A 28 Mhz signal appears as
50 Mhz or plus on the scope. I just wonder if I'm overlooking something
here. Also, with the maximum output from the generator, 100,000 uV, I
am getting around 300,000 uV or more if I could divisions.

I don't yet have my 1/10x probes. They are due to arrive this week. I
will then immediately check the 2V P-P calibration signal on the scope
and see what happens.
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Old May 3rd 21, 12:19 AM posted to sci.electronics.repair
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Default tests to do on used oscilloscope?

Jason Bowers wrote:
================

It seems that both frequency and voltage readings are off. I have an
old URM-25 RF signal generator that I have been feeding into it by
sampling off of the side of a t connector with one side having 50 ohm
dummy load and the other into ch 1 of the scope. Anything I've tried in
frequency and it seems like I'm reading at least double that frequency
on the scope if I count divisions from P-P. A 28 Mhz signal appears as
50 Mhz or plus on the scope. I just wonder if I'm overlooking something
here. Also, with the maximum output from the generator, 100,000 uV, I
am getting around 300,000 uV or more if I could divisions.


** Using an old, tube RF gen is not the way to calibrate a scope.

You can started with known DC voltages, checked with your DMM.
Just switch the vertical amps to DC coupled.

A square wave test shows if there are response errors - in one go.
If it looks square, the response is as it should be.

I don't yet have my 1/10x probes. They are due to arrive this week. I
will then immediately check the 2V P-P calibration signal on the scope
and see what happens.


** Scope have internal trim controls for calibration.
You scope clearly works OK, it may need some adjustments to be spot on.

This would all be described in the owners manual.


...... Phil





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Old May 3rd 21, 07:49 AM posted to sci.electronics.repair
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Default tests to do on used oscilloscope?

Ralph Mowery wrote:
In article , says...

It seems that both frequency and voltage readings are off. I have an
old URM-25 RF signal generator that I have been feeding into it by
sampling off of the side of a t connector with one side having 50 ohm
dummy load and the other into ch 1 of the scope. Anything I've tried in
frequency and it seems like I'm reading at least double that frequency
on the scope if I count divisions from P-P. A 28 Mhz signal appears as
50 Mhz or plus on the scope. I just wonder if I'm overlooking something
here. Also, with the maximum output from the generator, 100,000 uV, I
am getting around 300,000 uV or more if I could divisions.




Your signal generator is probably set to give RMS voltage and you are
reading peak or peak to peak voltage on the scope. There is a
conversion factor of some number I can not recall but it is around 1.4
or 2.8 times. Also some generators will be open circuit voltage and
some into the 50 ohm load with is 6 db or someother number I can not
think of right off.


my signal generator outputs some wonky voltages unless you have a load
resistor across the outputs. It's 600ohms in a box with 1" spacing banana
plugs and receptacles. I could never figure out why that's not handled by
a pushbotton.

Also, make sure you don't have any knobs pulled out putting you in zoom
mode. My signal generator has one of silly scope knobs too for some
reason.
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Old May 3rd 21, 03:04 PM posted to sci.electronics.repair
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Default tests to do on used oscilloscope?

In article ,
says...

my signal generator outputs some wonky voltages unless you have a load
resistor across the outputs. It's 600ohms in a box with 1" spacing banana
plugs and receptacles. I could never figure out why that's not handled by
a pushbotton.

Also, make sure you don't have any knobs pulled out putting you in zoom
mode. My signal generator has one of silly scope knobs too for some
reason.



Signal generators are designed to put out a certain voltage into a
certain load. Think of them as having a signal generating source of
zero impedance in series with a resistor. Any load will devide between
the internal impedance and the load impedance.

For RF generators there were two standards. One was an open circuit Or
very high impedance) voltage and the other was for a 50 ohm load. My
service monitor has a software setting to do either one. It is from the
old analog cell phone days and 30 years ago the cost was about $ 50,000.
When the cell phones went digital the service monitor was put out for
surplus and are selling for under $ 1000 now on the used market.

When using a scope you have to allow for the peak or peak to peak ratio
to the RMS value of a sine wave. As mentioned before I don't recall the
ratio as I seldom need it and the scope I have now will calculate it and
put on the display. If your RMS meter shows abot 120 volts AC out of
the wall socket a scope will show around 170 volts peak.


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Old May 3rd 21, 06:48 PM posted to sci.electronics.repair
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Default tests to do on used oscilloscope?

On 5/3/21 10:04 AM, Ralph Mowery wrote:
In article ,
says...

my signal generator outputs some wonky voltages unless you have a load
resistor across the outputs. It's 600ohms in a box with 1" spacing banana
plugs and receptacles. I could never figure out why that's not handled by
a pushbotton.

Also, make sure you don't have any knobs pulled out putting you in zoom
mode. My signal generator has one of silly scope knobs too for some
reason.



Signal generators are designed to put out a certain voltage into a
certain load. Think of them as having a signal generating source of
zero impedance in series with a resistor. Any load will devide between
the internal impedance and the load impedance.

For RF generators there were two standards. One was an open circuit Or
very high impedance) voltage and the other was for a 50 ohm load. My
service monitor has a software setting to do either one. It is from the
old analog cell phone days and 30 years ago the cost was about $ 50,000.
When the cell phones went digital the service monitor was put out for
surplus and are selling for under $ 1000 now on the used market.

When using a scope you have to allow for the peak or peak to peak ratio
to the RMS value of a sine wave. As mentioned before I don't recall the
ratio as I seldom need it and the scope I have now will calculate it and
put on the display. If your RMS meter shows abot 120 volts AC out of
the wall socket a scope will show around 170 volts peak.



Well, I've been using a coax T. On one end, there's the 50 ohm dummy
load, other side is input from the URM 25 signal generator, and the
final side goes into channel 1 of the scope. I wonder if I should use
some sort of a buffer here, on the side leading to the scope, like a
voltage divider, etc, rather than direct? Or, perhaps actually check
from the "open circuit" output on the generator as the URM has that too?
Yes, I'm now compensating for PP by converting to RMS after I see the
waveform. It may be closer than it was, but will know with more
certainty after the probes arrive today.

A small 1x 10x probe arrives today at last so I can at least check the
scope's square wave output signal peg. See what happens then.

The frequency still seems to be showing higher than it actually is
though. Even with a 30 Mhz scope like this, should I be able to "zoom"
in to see just one or two waveforms? So far, I can't seem to get closer
than 4-5 waveforms at 28 Mhz even with the 10x magnifier on.
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Old May 3rd 21, 08:09 PM posted to sci.electronics.repair
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Default tests to do on used oscilloscope?

In article , says...

Well, I've been using a coax T. On one end, there's the 50 ohm dummy
load, other side is input from the URM 25 signal generator, and the
final side goes into channel 1 of the scope. I wonder if I should use
some sort of a buffer here, on the side leading to the scope, like a
voltage divider, etc, rather than direct? Or, perhaps actually check
from the "open circuit" output on the generator as the URM has that too?
Yes, I'm now compensating for PP by converting to RMS after I see the
waveform. It may be closer than it was, but will know with more
certainty after the probes arrive today.


Instead of the RF generator have you tried a transformer that converts
the line voltage to something between 6 volts and 50 volts AC ? Then
you can do the conversion from the RMS to peak voltage to see if the
calibration is correct.

One other thing if it is a dual trace scope go to the X-Y mode and feed
both chanels the same signal and see what the trace looks like.

As someone else mentioned feed the same signal to both inputs and use
the invert function and see if they cancel each other out.

I am thinking that URM 25 has several matching networks that go on the
end of the cable. Do you have the correct one for what you are trying
to do ? Sometimes those networks get lost over the years.





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