EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: dan3460 on March 15, 2016, 07:11:05 pm
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Does anyone know how to DIY the above probes? I was looking on ebay and at $150 a pop there are a no.
I think I can play with some components but does anyone know what the connector (for the temperature probe at the DM501) is call?
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I, too, would love to find some alternate source for a compatible probe.
Anyone have details?
Thanks.
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The DM501, DM502, 7D13, DM43, and DM44 use delta Vbe temperature measurement so the sensor in the P6058 and P6430 is just a small signal transistor like a 2N3904 and no calibration is required.
The DM501A and DM502A are different and the P6601 probe contains a 100 ohm platinum RTD.
In both cases you can build the probe but the expensive part is the LEMO connector and despite looking, I have no idea what the current LEMO part number is.
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Thanks for the info, David.
I looked around for the LEMO connectors, and they are indeed around $40 if you can even find the right one.
I wonder if replacing the connector with a BNC or some other standard plug that fits in the hole (BNC might be too big) would be a good idea. I think the connector in the DM501A has only one pin plus ground.
I ordered some cheap pt100 RTDs off eBay to see how they work. I would mainly be interested in temperature changes rather than exact temps.
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Hmm, I see that the forum cookie bug has not been fixed. It only took me all day to get this reply posted.
The DM501A schematic shows that once side of the Pt100 RTD returns to chassis ground while the temperature circuits use the single point ground but I think that is wrong. I suspect the probe actually uses the single point ground as well through one of the two shown LEMO connector pins and not the body. Ah ha, I have a DM501A in my repair pile ... what does it show ...
Ok, the two LEMO pins do go to the Pt100 RTD. The body of the connector may be used as the shield but I do not have a probe to verify that. Internally, one of the pins does go to the chassis ground however that point is also connected to the LO input of the voltmeter only when temperature is being measured which itself goes to the single point ground. So what happens is that the chassis ground is joined to the single point ground *only when* in temperature measurement mode; otherwise it is allowed to float up to 1 kilovolt.
You might get away with using a BNC or other coaxial connector since the two chassis ground connections are not that far apart. The temperature probe design in this case does not take full advantage of the potential Pt100 RTD accuracy so any added error may be insignificant. What you could do I guess is cut the trace to the internal chassis ground connection and replace it with a lead doing to the BNC or coaxial connector ground.
I have a delta Vbe temperature probe for my other meters but even having the actual LEMO connector in my possession has not been enough to figure out the part number for the modern LEMO connector. Nobody else on the Yahoo Tektronix email list knows it either.
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Thanks for the detailed reply. Sorry you had trouble getting it to post.
I don't have the unit in hand, but the schematic seems to show pin 2 from the temp jack going to ground. It looks like case ground (forked ground symbol).
So if I used coax the outer ground of the coax would be tied to case ground as well. Hopefully putting the meter into temp mode disconnects the input of the meters's input LO from the internal ground?
I bet this will become more clear to me when I open it up. But I might as well wait until the RTDs arrive.
As far as the part number goes. I did see a bunch of LEMO connectors on Mouser, but I did not take the time to try to find the correct one... So it is possible they don't even have it.
Thanks again.
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Thanks for the detailed reply. Sorry you had trouble getting it to post.
I don't have the unit in hand, but the schematic seems to show pin 2 from the temp jack going to ground. It looks like case ground (forked ground symbol).
That is correct however schematics often leave out the details of the actual ground layout so it is best to check.
So if I used coax the outer ground of the coax would be tied to case ground as well. Hopefully putting the meter into temp mode disconnects the input of the meters's input LO from the internal ground?
The meter connects the signal and chassis ground only when in temperature measurement mode. One side of the probe itself is always connected to chassis ground so replacing the LEMO connector with a coaxial connector will work fine. The very small ground loop should be insignificant.
I bet this will become more clear to me when I open it up. But I might as well wait until the RTDs arrive.
As far as the part number goes. I did see a bunch of LEMO connectors on Mouser, but I did not take the time to try to find the correct one... So it is possible they don't even have it.
I believe they have it however even going through the LEMO catalog with calipers and the connector in hand did not allow me to figure out exactly what the LEMO part number is.
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Thanks again for the detailed info.
I got the meter (which works nicely, but I want to calibrate a tiny bit). I have not taken it all apart yet except to clean switch contacts (will wait for RTDs on the slow boat from China).
It looks like the Lemo connector hole is likely smaller than a BNC. So perhaps 1/8" headphone jack would be the way to go in terms of a common connector. I need to see how much space is behind it as well.
If I used a stereo (three conductor) jack I could have a separate chassis ground for an outer shield. However, I think the RTDs come with a long unshielded cable anyway. While I could shorten that and still make most of it shielded with a 3 conductor shielded cable, perhaps the shielding would just be overkill.
Since I already have a couple thermometers this is kind of a not-so-important-but-fun project to be implemented when I am avoiding doing something else :-DD
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I was recapping one of my DM501A meters, and thought I would give this a shot.
I found that a standard BNC connector would fit directly in the hole of the stupid nearly-un-obtanium LEMO connector on the DM501A. The hole actually has two flat sides just like my BNC connectors. So this was perfect. Make sure you get the polarity right, as one wire is connected to chassis ground. From the schematic it looks like on the DM501A it is always connected to chassis ground, regardless of the setting of the mode switch.
I bought a PT100 probe from eBay like this:
http://www.ebay.com/itm/2pcs-PT100-Temperature-Sensor-Waterproof-Platinum-Resister-/191674287533?hash=item2ca0ad85ad:g:ppgAAOSwLVZVjSrP (http://www.ebay.com/itm/2pcs-PT100-Temperature-Sensor-Waterproof-Platinum-Resister-/191674287533?hash=item2ca0ad85ad:g:ppgAAOSwLVZVjSrP)
Though I see some alternative like this:
http://www.ebay.com/itm/RTD-Pt100-Thermocouple-Temperature-Sensor-2m-Cable-100mm-Probe-3-Wires-50-500-/401172761628?hash=item5d67c28c1c:g:dawAAOSw6n5XtRbi (http://www.ebay.com/itm/RTD-Pt100-Thermocouple-Temperature-Sensor-2m-Cable-100mm-Probe-3-Wires-50-500-/401172761628?hash=item5d67c28c1c:g:dawAAOSw6n5XtRbi)
or this:
http://www.ebay.com/itm/1pcs-PT100-waterproof-Thermal-Probe-Temperature-Sensor-Thermometer-controller-/321045049046?hash=item4abfc6aed6:g:4AgAAOSwQJhUkud0 (http://www.ebay.com/itm/1pcs-PT100-waterproof-Thermal-Probe-Temperature-Sensor-Thermometer-controller-/321045049046?hash=item4abfc6aed6:g:4AgAAOSwQJhUkud0)
Those alternatives might save you a step, and possibly be nicer. Mine works great, though, and I have a 2 meter cable which is quite helfpull when doing temp measurements, as you can't always get them near the TM mainframe.
I also got a standard twist on test probe, Velleman style like this:
http://www.eio.com/p-23830-velleman-tlm6-1-red-1-black-solder-test-probes.aspx (http://www.eio.com/p-23830-velleman-tlm6-1-red-1-black-solder-test-probes.aspx)
Got that form my local store for $1
I then used a 75 Ohm pre-made BNC cable, and chopped off one end (I had extra 75 Ohms cables I did not need). With this method I have a good solid molded connector on the other end vs. having to crimp one on myself.
I soldered them together, and taped and heat-shrinked it. I then calibrated the meter with this probe using an ice bath and boil water per the manual.
It works great!