"scope problems (bad reading- way to sensible)

[hitachi8]

hello people!
i bought a scope in a garage sale a couple of years ago, its an old analog scope: Sencore minute man SP29.
i never really realize but it is reading way to high! after some test with a DC power supply and little 12v batteries i figured out that my scope
was way way too sensible.

here's some example:
Input=5volt reading @ 5v/div 100hz : 3.3 div
@ 10v/div 100hz and below i can clearly see the static/radio waveform of my body while touching the probe with my finger.

i really wish i could take some pictures but my cameras is broken, dont know the problem it just wont start.

what do you think is the problem? i never touched any of the varipot/resitor in the scope, the only thing i did i change some of the capacitor on the main power supply board.
i tried to adjust the DC voltage setting on the front, but it doesnt seems to work.

[vk6zgo]

I don't know the Sencore,but are you sure your volts/div knob is set to "cal".
If it isn't,you could have the symptoms you describe.

Another possibility is that the knob is incorrectly positioned on the switch.

Or,another,is that someone messed with it,prior to you buying it.

VK6ZGO

[hitachi8]

they do have a calibration button on both volt/div and time/div, but they dont have a "clic" position here it is preset.
even by trying to tune the volt/div setting with the cal button you dont have enough tuning capability to make the scope read right.

i checked, and the volt/div button seems to be right, it stop when you reach the maximum and it also stop at the lowest position.
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i know this is a speciality scope and its not so good for general use, but its the only one i have.

[IanB]

What do you mean by "5 V 100 Hz" exactly? Is this a 5 V RMS AC waveform? And when you examine the trace on the scope, what are you measuring?

For instance a 5 V AC signal will have a 14 V peak-to-peak amplitude, so at 5 V/div you should see the waveform spanning about three divisions (should be just less than three, but that depends on the calibration accuracy).

[hitachi8]

i mean : the input was 5volt DC, from an ATX power supply (confirmed by measuring with a multimeter).
the Scope was set to 5volt/div, DC, 1X100Hz/div.
the reading was a line, 3division higher that it was originaly.
Note: the picture i posted before was just to show you the calibration pot.
=======================

i have another picture:
i was measuring the main AC from my wall.

as you can see, the scope is adjusted to 100V/div (AC).
the input was 123~AC, but the scope read 400VAC.

[ejeffrey]

If it does this with the cal knob all the way clockwise and you have verified that V/div knob stops at the right locations for the 500 V/div and 100 mV/div then it sounds like there is a problem with the vertical amplifier.  It may be hard to solve without a service manual and a working scope for testing.  Better yet would be a working identical scope for comparison plus a working scope to use for measurements.

[hitachi8]

yep, tried to set the calibration, not worked (dont go far enough).
all the control looks ok and in the right position.

the worst thing is that those scope are quite rare (no information on them).
i had alot of problem before with old component failing but now i can resign myself on looking for another one.
thank you people, if no one has any other good idea i will be considering this case close.

Ps: looking for a 2channel Analog scope!

[vk6zgo]

Another trick is to work out what the error is at various voltage levels & just live with it.

On DC you should be able to do this fairly easily,as you can compare it to your DMM.
Set the variable gain control("CAL" is just the position on this control where the vert display should read correctly!),to give you an even number of divisions for standard voltages-- for instance,if the scale reads around 15volts for 5volts input,adjust the variable knob to read exactly 15volts.

Mark the position with a marking pen,(you can then check if the error is in the same ratio  for all the vertical ranges,which may help you to find the original fault).

VK6ZGO

[nixxon]

What does the scope show when you measure a battery? E.g 1.5V or 9V battery

[Zero999]

What do you mean by way too sensible?

Have you taken the cover off?

There may be caliubration trim potentiometers on the PCB which are clearly marked.