ScienceGeek - 29-4-2008 at 09:39
I simply have no idea where to go with this besides here.
My school is redoing the lab, so there is a lot of stuff to throw away. Among those things, a "broken" Mettler PL200.
You just don't throw away a piece of equipment like this, so if anyone of you have experience with this, I will welcome any thoughts!
Diagnosis:
It starts up, but after its initiation period, the display goes blank and there is just a + (plus) sign in the display. Obviously this would mean that
the scale is overloaded, so I "strip" it of the weighing plate and the other thing (the one that distributes weight evenly).
Nothing happens. As I pull the "pin" the plate sits on top on, it jumps right to "underload".
This is not all. Sometimes as I start it, its display weight gradually increases until it reaches overload again. As I again pull the "pin", it
gradually drops again until it reaches underload again.
To sum it up: the scale seems to have a mind of its own, being both supersensitive and completely insensitive at the same time.
Does anyone have experience with any of this, or maybe how to reset or calibrate a scale like this...
Thank you for reading all of this 
woelen - 29-4-2008 at 12:05
Most likely the capacitor, used in the integrating circuit is broken.
This type of balances uses a so-called integrating AD-converter, which integrates the measured input signal, until some threshold value is exceeded or
until some preset time has elapsed. In the latter case, it does backwards integration as well. The time, required for the total operation is a measure
of the applied input signal. Most likely, the capacitor in the integrating AD-converter is broken. In that case, it immediately switches to +inf or
-inf. Integrating AD-converters can achieve amazingly high accuracy, combined with very low noise (noise is averaged out by the integration process).
The process is slow, but for a balance this is not a problem at all.
If you can locate the electronics of this device and take it out (it might be connected through some flatcable or band-cable) then you could try to
locate the capacitor and place some large capacitor in parallel to this. If there is some readout (albeit nonsensical and possibly drifting) then it
indeed is the capacitor.
[Edited on 29-4-08 by woelen]
ScienceGeek - 29-4-2008 at 13:07
Thank for your response, woelen!
Do you know where I can get a capacitor? Did the capacitors back then (30 yars ago) look like the ones we have now?
If it indeed is the capacitor, what do you suggest I do?
Once again: thanks
chemrox - 29-4-2008 at 22:35
pulling the old one out and going to (an independant) electronics store and getting one with the same specs is one way to go about it.
MagicJigPipe - 4-5-2008 at 23:03
No offense, but if you're not very experienced at this type of thing, maybe you should ask a friend to do it for you. I speak from experience when I
say that soldering on a small, crowded circuit board that you don't want to break, without any experience is asking for trouble.
Or, you could practice on a spare board a few times before you actually do it. If you burn a piece of the board (or another component) that will just
cause more problems and more work.
Good luck! Also, if you end up trashing it, don't! I (and I'm sure others here) "collect" equipment like this with interesting components.
[Edited on 5-5-2008 by MagicJigPipe]
woelen - 4-5-2008 at 23:42
The capacitors you can get now are not really much different than the ones of 30 years ago. Nowadays they may be more compact.
A bigger problem most likely will be the fact that this capacitor is by no means a simple thing you can buy at every electronics shop. The capacitors,
used in ultra high precision integrating AD converters must be of exceptional quality. The leakage of charge must be very low, and also its linearity
must be exceptionally well, otherwise you'll introduce all kinds of systematic errors. I would try to locate it on the board and see if you can find
the capacitor online through eBay. Another option might be to replace the entire board. A lot of old surplus electronics is offered on eBay and
sometimes it is more economical to replace an old board completely.
ScienceGeek - 5-5-2008 at 01:57
MagicJigPipe:
I agree! And I have NO experience whatsoever with soldering and stuff...
Woelen:
Thank you very much for your help, but considering what needs to be done (and it's not even certain it would work) I think I might just sell it...