What can one do with a piece of vintage tech like HP 6920B? I found a few uses for it already:
- testing “magic eye” tubes that require some 250V for anode/screen voltage
- testing capacitors for DC leakage up to 1kV
- reforming electrolytic capacitors
- testing gas-discharge voltage-regulator tubes
- testing Geiger–Müller radiation detectors
- testing electroluminescent backlight panels
- testing transformers (ratios, self-heating and winding insulation)
- and zapping stuff with 1kV AC/DC
I’m probably going to invent some other uses for it as well but originally I wanted it for some repair fun and one particular purpose – as a 1kV DC source to recalibrate my AimTTi 1908P multimeter:
First let me introduce the 1908P a bit. I got some other AimTTi gear as well so I expected their multimeter to be decent. Different people have different needs so here’s my personal pros and cons lists:
- 5 and 1/2 digit modern digital multimeter with reasonable specs
- it’s cheap, even taking into account Chinese offerings like Rigol and Siglent
- has Ethernet port for TCP/IP connectivity (as well as serial and USB, and GPIB/IEEE-488 if you need it)
- no fan, no noise, no holes in the case for the dust to get in
- 2s boot-up, of which 1s is the display test
- decent screen with good backlight
- manufactured in Europe
- built-in battery pack for off-line operation
- it’s only 120k counts
- not very fast, 4 readings per second at full resolution (less in dual-measurement modes)
- no rolling averaging to reduce input noise
- diode test goes only up to 1.2V due to non-selectable range even if the output voltage is about 3.5V
- only 10M impedance on input on DCV ranges (less in AC mode obviously)
- built-in battery pack for off-line operation
I’m really starting to dislike USB these days. I use it a lot and I’ve seen all kinds of issues with it – cable length problems, flaky hubs and connections in general, driver problems. In fact custom, broken, never updated drivers are the worst and can often interfere with other devices that worked perfectly before. So I always look for Ethernet ports on new equipment, I don’t trust hardware manufactures to come up with decent drivers or any at all if you aren’t on Windows or it’s not the version they support. Serial port is second best option but it’s slow and typically offers no HW error correction, so a single bit glitch in the data stream can easily bump a digit up or down in the reading and you’d never know it. GPIB/IEEE-488 is… old. And costly. And very proprietary. If you already use it a lot you won’t mind but newcomers are better off looking for something else.
The 1908P has Ethernet port and it’s a huge plus for me. And the price is still acceptable, unlike say Rigol offerings that give you cheaper USB-only models or the full-fat ones for 500 USD more. The 120k counts is more of “5 digits” than “5 and a half” in my book but I’m not going to argue with established shady marketing, it’s not just AimTTi doing this. I very much like the no-fan design and the LCD display is OK, not great but also not annoying me in any way. The battery pack is a bit of a double-edged sword – nice when you need it but keep in mind it will die at some point. Mine in fact did (was probably defective) and that most likely caused the brick I had when updating FW – AimTTi did good though and repaired this for me for free.
No averaging, 10 meg input and limited diode mode are not great but also not a dealbreaker. I might at some point need more than 4 reading/s at full 5-digit resolution but I don’t yet so that’s also not a problem. All in all I am happy with the price and performance – or rather, I was, until I discovered a glitch.
Long story short: When in 1000V DC range and below 100V (so only with manual ranging) there is a weird non-linear jump at exactly 99.00V that skips all the .01 to .05 values and goes to 99.06V (so from 99.00 to 99.06). I asked AimTTi about it and they suggested recalibration of the 1000V range. Turns out the calibration procedure requires a 100V input at one point, which is pretty much the only thing I’ve changed in the calibration data and it only made the “99V problem” worse. So clearly the jump/skip is some sort of FW issue that tries to improve the linearity of this range by having both 100V and 1000V references stored – but messes up the cross-over point. At least that’s how I see it. AimTTi has since ignored 3 of my emails – both about this issue and another one with their MX180TP PSU.
I’ve read many good things about AimTTi support but I have to say my personal experience with them is spotty at best. They will fix bricked equipment and issue FW updates for the serious bugs (not to brag but the Interface firmware version 1.07 for MX180TP exists because of me). The less obvious stuff though? Won’t even reply to the email.
And so, to conclude this – would I buy this multimeter again, knowing what I know now? Yes, it’s not like Rigol/Siglent or the Keysight/Keithley (which are often made by Rigol/Siglent in the lower price range anyway) have spotless record when it comes to FW bugs. And the price is still very good. But can I recommend AimTTI to others? No, I can’t. If you really need a good bench meter then you should probably look into the (considerably) more expensive K-brands. If on the other hand it’s for personal use then Chinese meters have more features, color LCD graphical screens and you probably won’t mind having just the USB available anyway. Unless you live in UK and can drive to AimTTi in person if need be.