The way I'm using the word, "linear" is not a problem here. I only meant, "Do you want a dead band in the middle?" You don't. You only need the amplification factor increased. Both polarities are relevant.

Did I get the probe correctly placed on the left side of the drawing? (I know the voltage label is wrong) or is one side of the probe grounded and J2 is a different source? edit: poking around on the internet tells me one side of the ORP probe is grounded. What connects to J2?

Do you use RP1 as the, "calibration" knob?
You can dial in an offset with it, and it seems to be quite large compared to your useful range.

(R10 is brown, red, orange gold)

Ok, trying my best here. The electrode is a combination electrode (half cell electrode and reference electrode{constant potential} in one body. It connects to J1. All the new electrodes are combination electrodes. Back in the day, when this meter was made, you had a measurement electrode which goes into J1 and a reference electrode went to J2. I have an adapter so my modern combination electrode plugs into J1 and the adapter has a shorting strap which goes into J2. So J1 and J2 are connected. I only need the amplification increased. I do use RP1 as the "calibration" knob to provide an offset, but its relative mV. So, if you're measuring 300mV, you can use the cal knob to offset to -300mV if you want, but at the endpoint the needle still moves to 30-100mV, leaving you at a max of -200. It doesn't move the needle full scale like I want.

 

The best I can come up with right now is: Short TP5 to ground and adjust RP2 for more gain.

 

And how do I do that? Baby steps please. I've never touched a circuit board.

 

Which part are you having trouble with? Finding TP5, connecting it to ground, finding RP2, or adjusting RP2?

 

Which part are you having trouble with? Finding TP5, connecting it to ground, finding RP2, or adjusting RP2?

With a novice like me, this thread is worthless without pics. Attached is a pic of TP5. To short and ground TP5, do I remove the white wire (originating from RP2) connected to TP5 and put it to ground? Or do I disconnect the wire from TP5 (and let it hang), then run a wire from TP5 to ground? Or?

If it helps, I can (and with advice from an Orion superior that's been with the company for 36 years) definitely say RP1 and RP2 were messed with at that old lab I worked at to amplify the range of the meter. According to Orion's instructions, to use the model 301 as a millivolt meter, RP1 (calibration knob) needs to set the needle at 0 mV, and RP2 (temperature knob), needs to be to set to 15°C where it says "mV." We would set RP1 at -60mV or greater (-), and set RP2 to between 30-40°C.

Also, I forgot to include the trouble shooting guide for the meter, which may reveal some insights. Thanks.

 

Do not remove any wires at all. Short TP5 to TP10.
Adjust RP2 to see if you can coax some more gain out of this.

Remember, RP1 sets the zero point. After that is zero, add some input voltage and adjust RP2 to see if it does what I expect.

 

Do not remove any wires at all. Short TP5 to TP10.

With something like this?

 

Between me trying to figure out the right current path with no hope that you will be able to measure anything for me, and you thinking like a car mechanic, I think we should just stop. These meters are so sensitive that they use things like Teflon standoffs to minimize leakage currents. You can knock them out of calibration by breathing on them, and then you have to go get a hair dryer to get the humidity out of the circuit board... and you're thinking: squeeze on connectors???

I think we should just stop before you break something.

 

Between me trying to figure out the right current path with no hope that you will be able to measure anything for me, and you thinking like a car mechanic, I think we should just stop. These meters are so sensitive that they use things like Teflon standoffs to minimize leakage currents. You can knock them out of calibration by breathing on them, and then you have to go get a hair dryer to get the humidity out of the circuit board... and you're thinking: squeeze on connectors???

I think we should just stop before you break something.

Ok. If you don't want to help me, then you don't. The meter cost me $45 shipped from eBay. If I do something to it and it doesn't work, then it goes to the garbage can (or recycling bin to be earth conscious) and I buy another one. No biggie, it's not like I have $10K invested into it and as it stands now, the meter is of no use to me.

I do find your comments a tad offensive, especially the "thinking like a car mechanic" (tell that to a car mechanic). But it is just a forum, so no big deal in the grand scheme of things.

 

The best I can come up with right now is: Short TP5 to ground and adjust RP2 for more gain.

This worked beautifully. I've been wondering for a long time how to modify this meter. When titrating a sample, I could get the needle to move from -400 to +400, when in reality the sample's potential only changed by 50 mV. I can adjust RP1 and RP2 to pretty much get any needle inflection that I want. Thank you very much.