Quick question about HP 428A Milliamp meter - Page 1
Forum section: electronics / repair

author: Stray Electron
author_rank: Super Contributor
timestamp: October 01, 2020, 08:43:59 pm
content: Can the probe from one instrument be used without having to recalibrate the instrument? The back story is that that I bought one with a probe but with broken glass in the meter and then found another one with good glass but no probe. I'm wondering if I can just swap the probe or if I should swap the meter movement.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 01, 2020, 09:05:32 pm
content: I would not count on a high accuracy in the full scale range of the meter movements. The overall accuracy of the old meter is relatively low. So it would not be so hard to check the calibration yourself - it does not need a high end DMM to check the calibration.

author: bob91343
author_rank: Super Contributor
timestamp: October 01, 2020, 11:11:00 pm
content: I have a 428A that I never liked after I saw the specs on the 428B. As luck would have it, I found a 428B at a swap meet that had no probe, broken glass, no tubes. It was free so I took it.I moved the tubes and probe over from my 428A. I had a spare meter from a 412A and took the glass from that. So yes you can swap the glass.The bottom line is that I now have a working 428B and it seems accurate.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 03, 2020, 03:38:24 am
content: I just tested one of the 428s that I got and it works! Not bad for something that was built in 1969! It doesn't meet spec but it's close and I need to clean the sensitivity pot. I haven't tested the 2nd one that I got yet but I checked the serial numbers and they're only 19 units apart! 085440 vs 085459.Now that I know about the B model and I know that the probe will work on both versions, I'll be looking for one of them. The 428s are pretty common around here but the probes are rare.FYI to answer my own question, yes they require a partial recalibration if you swap the probe.Thanks for letting me know that the glass is swappable. The unit with the broken glass looks like new but the meter in the other one has some mildew spots on the meter scale so I'd rather not use that meter movement.

author: bob91343
author_rank: Super Contributor
timestamp: October 03, 2020, 03:48:41 am
content: The glass I swapped in came from a different model and wasn't exactly right. But I fussed with it and managed to get it in.Yes the probes are individually calibrated to each unit but I don't think the variation is too great. Certainly within a reasonable tolerance. If necessary you can create an error table.It really surprised me to discover that the probes were the same for the A and B models. Now I can measure 1 mA to 10 A full scale. I still have the old A unit for parts.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 06, 2020, 04:17:48 pm
content: Well good news to report. It turns out that both meters are 428Bs and both work!I connected each one to a variac and applied increasing voltage slowly over about 24 hours and all of the capacitors seem to have reformed and everything is working as it should. I was told that they were A models when I bought them but I never looked until I was cleaning them up. I opened up both of them and cleaned the Zeroing pots and now both are working fine. One looks like brand new inside and the other has light corrosion on the aluminium. Both are leaking some rosin from a transformer in a can near the top of the chassis but other than making a mess, it hasn't hurt anything. Date codes on both show that they were built in 1969. You gotta love the quality of the design, engineering and construction that goes into this old HP gear.

author: mansaxel
author_rank: Super Contributor
timestamp: October 06, 2020, 05:18:03 pm
content: The 428 (A and B) is a fantastic and fascinating instrument, a true leap in capabilities when it was new. Today, one buys a Uni-T DC clamp for next to nothing, of course, but I've been mesmerised by the 428 as long as I've known about it.I've got one, a late B with a glass fibre PCB. It most probably was a "hangar queen", because it had a manufacturing defect. One of the leads to one of the primary windings in the mains transformer was shorted to ground. It had wedged between the core and the chassis when the transformer was fitted. Apparently, the instrument survived QC (the defect was impossible to detect without disassembly) but must have failed in service.It blew my mains fuse and the RCCB, and I removed the transformer to verify what was broken and there it was. Some shrink tubing around the damaged cable, and it is good as new.Yes, the probe can be moved, but, as stated upthread, the instrument must be recalibrated to a new probe.Congratulations, fellow 428 fan!

author: bob91343
author_rank: Super Contributor
timestamp: October 06, 2020, 05:44:25 pm
content: With the B model I am happy to have the output connection that lets me probe alternating signals as well. The bandwidth is limited but it still is very useful.The A and B models have very different meters. I don't know if the A model glass is able to be swapped.Is the probe recalibration simply a sensitivity adjustment, or is it more complicated than that?

author: mansaxel
author_rank: Super Contributor
timestamp: October 06, 2020, 08:01:44 pm
content: Quote from: bob91343 on October 06, 2020, 05:44:25 pmIt is a bit more complex. Themanualdoes say this on swapping probes:QuoteI have not yet gotten through the full adjustment procedure, which, in usual fashion requires half the -hp- vintage catalog, but with a bit of sensible substitutions one could use newer devices to get as close as is practical.Do keep in mind that this is a valve instrument; the most important PSU voltage is 280 Volts DC. One hand in your pocket, one holding the probe.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 08, 2020, 12:42:45 am
content: Quote from: mansaxel on October 06, 2020, 08:01:44 pmThanks for mentioning that. I didn't see it in the manual and I wondered about that. I checked the probe body but no SN. But after you posted, I went and checked the collar and it is marked "5489" with a vibrating electro-marking pencil.