Do these changes only impact input protection?
Fuck me!
Looking at the changes on the Rev V.02 units, its obvious they have upped their game. So I just spent $600+ on old stock that I will pretty much bet is not going to survive to the same levels this new meter will. Look at the size of those transistors compared with the parts in mine.
I opened up the second meter just to make sure as it was a later SN. No luck, the two meters that were shipped are both the old design.
So I've burned down a fair bit of time, plus the cash. I am going to have to give it some thought on how to proceed.
...
This transistor clamping circuit on the additional PCB is obviously the circuit, which Dave had explained in his videos #1157 and 1158.
...
Another remark: the use of the 1N4007, as well as a TVS at that point both affect negatively the calibration of the 5M and 50M Ohm ranges by their leakage currents. Only without them, these ranges may work linearly.
PS Afaik Dave said "we are not going to spin a different pcb revision, we'll fix problems in software". Turned out either my memory is bad, or they changed their mind.
The latest is V.02.1910 which has the transistors integrated. Although they are not in the wild yet.
Fuck me!
Looking at the changes on the Rev V.02 units, its obvious they have upped their game. So I just spent $600+ on old stock that I will pretty much bet is not going to survive to the same levels this new meter will. Look at the size of those transistors compared with the parts in mine.
I opened up the second meter just to make sure as it was a later SN. No luck, the two meters that were shipped are both the old design.
So I've burned down a fair bit of time, plus the cash. I am going to have to give it some thought on how to proceed.
Hey Joe!
This transistor clamping circuit on the additional PCB is obviously the circuit, which Dave had explained in his videos #1157 and 1158.
I criticized though, that this does not work at all to protect the pin15 of MUX 4053, U9 for the Ohm circuit.
https://www.eevblog.com/forum/blog/eevblog-1157-transistor-zener-clamp-circuit/msg2043757/#msg2043757
The circuit is able to protect symmetrical inputs, at +/- 6V clamp voltage, or +/- 20..25V, if special npn types are used.
This use case is asymmetric, as it has to be protected at -0.5V / + VDD ~ 4...16V.
I've never got a response from Dave, but obviously this not working circuit was implemented in certain HW versions of the 121GW.
It would be interesting, if in the most recent version, the BC diode of both transistors were properly used to protect the Ohm circuit, by using them as direct replacement of the original 1N4007.
Another remark: the use of the 1N4007, as well as a TVS at that point both affect negatively the calibration of the 5M and 50M Ohm ranges by their leakage currents. Only without them, these ranges may work linearly.
Frank
I just paid for two brand new meters and received the floor sweepings that are now apparently 2 PCB revisions old. I wasn't told anything about them being old stock when I made the purchase or I wouldn't have placed the order. I got fucked once. It won't happen again.
While I'd be interested in seeing every single version tested and compared, it doesn't really matter that much and is obviously impractical for a hobbyist to do so. Who knows how many more board revisions there will be in the future. In my mind, it's still the current version if that is what is being sold.
I just paid for two brand new meters and received the floor sweepings that are now apparently 2 PCB revisions old. I wasn't told anything about them being old stock when I made the purchase or I wouldn't have placed the order. I got fucked once. It won't happen again.
I am sorry to hear you got caught on the crossfire of revisions. I was looking forward for that series but, yes, I agree with you that your regular tests will not be greatly useful at this point in time. Perhaps get ahold of a modern schematics and try to implement in the new meter, à la UT61E improved protection exercise?
If only the revisions had an externally visible marker... Instead, cue the serial number merry-go-round. I wonder if UEI was avoiding an Osborne effect? If so, the cat is out of the hat. (but I confess I wasn't following the evolution that close anyways to know if this is old news).
Although any manufacturer states that "the products are subject to changes without prior notice", this reeks of a Uni-T UT61E stunt.
Frank,
About your comment, "I criticized though, that this does not work at all to protect the pin15 of MUX 4053, U9 for the Ohm circuit." I felt you (and possibly others) may have been a little confused.
I doubt I will make any changes to the hardware this time around. With the prototype, I had added two clamps in an attempt to protect the mux. If you watched the videos, may have noticed the TVS located across the supply of the mux (located on top of D7&D8). There was a second clamp that was located upstream from the resistor R82 but after the selector switch on node R_RLD. This first clamp acts like every other higher end meter I have looked at. It can withstand enough energy to overcome the PTCs thermal time constant. The PTC (PTC3) along with the surge resistor (R16) limit the current through the first stage clamp. The small resistor (R82) further limits the current. Again, the schematics are not up to date and I really didn't dig into how these meters are designed.
None of this is anything new as I went over it back in 2017 when I made the first series of videos for the prototype.
I suspect that the first stage clamp was enough to protect the mux but I never tested it. At the time my goal wasn't to optimize a production solution. That's UEI's job and that's assuming that they even wanted to try and improve it. Anyway, my point was that the meter may very well survive (my tests) without D7 & D8 present.
Hope that helps.
Hello Joe,
Frank,
About your comment, "I criticized though, that this does not work at all to protect the pin15 of MUX 4053, U9 for the Ohm circuit." I felt you (and possibly others) may have been a little confused.
I don't think at all, that I'm confused.
This critic is related to Daves videos #1157, especially 1158, not to any of your videos.
I also do not expect that you change the actual circuit, simply check what Dave has really implemented there.. as the latest revisions have some totally different transistors, and maybe a different circuit, all that already indicates, that this version you have on your table, did not work as intended.
I expect, that the DMM will fail, when you only run your usual (destructive) tests.
Frank
As a side note, I did receive a new message from Dave offering to replace the meters with the more current design. I have declined this offer as well for the reasons I mentioned earlier. If we run into a major problem that Dave knows was addressed in newer versions of the hardware, I just suggested he chime in.
Considering the confusion surrounding the prototype
But I also agree the firmware shouldn't affect so much the functionality flaws seen on the video. The slow auto range is something really obnoxious and, if this is not hardwired on the chipset, it could potentially be improved by different firmware.
I waited two years and you sent me old stock. That's on you.
Looks like she was drifting down. Don't include the settling time. Even more funny was seeing that slow Keysight lock right in and hold steady.