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Solatron 7150 plus

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dg:
Hello everyone,

I'd really appreciate some help on a potential problem with either my new (vintage) multimeter or my paranoia!
I have recently bought  a Solatron 7150plus DMM from ebay as I need to measure some very low (<1mV) voltages which my £10 meter couldn't do.
When I power up without any leads attached the 200mV range can read O/L (overload) - is this normal?

I suspect it could have mega high input impedance and be picking up tiny electrostatic charges.
There is just a nagging doubt in my mind so I'd appreciate a second opinion!

I did power it up straight away after I got it, but can't remember if I saw this. It only lasted a few minutes before the old Schaffner PSU block released the smoke (as I half expected) so I had to switch off quickly!

Having replaced the PSU block I now see this behaviour and am slightly paranoid that I have ESD zapped something, or otherwise damaged the thing when I had the covers off.

Observations:
All on VDC mode, manual ranging.
200mV: short leads, read all zeros (good), same with 100k "short" between leads.
200mV: open leads or 10M "short" causes offset voltage to accumulate up until reads overload.

2V: short leads, read all zeros, same with 100k and 10M "shorts".
2V: open leads, offset voltage of over 1V displayed.

          - relay switches when going 2V to 20V range -

20V: no offset.
Higher ranges: no offset observed.

At this point I'm fairly certain it is just a byproduct of a very high input impedance but your opinion would be gratefully received.

Many thanks.
David

eliocor:
I warmly suggest you to READ the ***** manuals (RTFM):
http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/Solartron
You'll learn on how to use your device and you will get the answers to your questions (see 7.3)

amspire:
From memory, for the 20V DC and higher ranges, there is something like a 10M Ohm resistor across the input which does discharge any DC voltage on the input terminals.

Below 20V, the input impedance is extremely high. So high it would be very hard to measure. This is the way the top end meters are designed to work because when you are taking precision DC measurements, the 10M Ohm input resistance can easily load many circuits enough to cause an error.

With the inputs open circuit, just walking towards the meter is enough to cause the voltage to change. leakage currents down in the pA of even fA range from internal protection diodes can easily generate an offset when open circuit. It is all good.

dg:
Thank you for your help.
I have managed to have a look at the manuals now and can see exactly what's going on.
As described there is a shunt discharge when operating on the 20V and above ranges (it's part of a resistive pot down circuit) which would indeed discharge things.
Input impedance is > 10G ohm on the bottom two ranges so no wonder!

What was jarring was how the offset voltage would suddenly jump three orders of magnitude when I heard the relay "ping" so that made me paranoid something was broken.

I'm most obliged to you for helping me.
 

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