HP 3469A Multimeter: 70's vintage teardown and clean up - Page 1
Forum section: electronics / testgear

author: bitseeker
author_rank: Super Contributor
timestamp: July 09, 2017, 06:16:45 am
content: For my 3000thpost, I bring you a somewhat rare bit of test kit. The Hewlett-Packard 3469A digital multimeter. There was also a "B" series, which has a 7-segment LED display and examples show up on eBay in small quantities. The "A" series, however, has the very cool "dot" LED display and seems to be quite scarce, for both the device and online information.This one was one of several instruments being sold from someone's estate. Its functional condition was unknown, but it came with a service manual, so I decided to risk it. Fortunately, since it's a relatively unknown model, no one else wanted it.As you can see, it was pretty crusty looking and all the panels were sticky.BeforeOn the rear panel (click thumbnail to see larger version), there was some extra sticky stuff around the fuse holder. Really weird. This appears to be an early unit from the 1104 (1971) design revision -- serial number 54. I don't know how many were produced or how many revisions of the A series were created.Getting into the meter is pretty easy. The top, sides, and bottom panels come off with a few screws. The enclosure is what folks at HP apparently referred to as "System I". Many instruments of the era made use of this aluminum chassis and panel system. I have an HP 5216A electronic counter that uses it as well.I've included photos of the immaculate interior, but didn't remove the boards as there are looms of interconnect wires between them.The first inside photo is from the top. You can see the LED display (black box) on the left. To the right is the A3, DC circuits, board. All gold-plated traces, of course. There's another board below it, which can be seen a little better from the right side (next photo).From the right of the front panel we can see the A2, AC circuits, board. Again, the display module is the black box on the left. The large aluminum shield below the A2 board contains the mode/range switch. We'll get there shortly.In the next photo, we're looking in from the left side of the enclosure. This subsection of the A2 board is shielded. I'm not sure yet what it does, but apparently it's noise sensitive. Perhaps an input amplifier.Next, is the bottom of the meter and a shot of the power supply (left) and the mode/range switch (right). Oh, how nice and tidy it is.Finally, a few photos after cleaning it up. Of course, everyone loves to see 10V on a multimeter, right? And here it is on a 31/2-digit dot LED display. The HP E3630A it's connected to is calibrated, so the multimeter is a little off, but not bad for being over 40 years old. It's amazing it's still working.AfterGetting the sticky stuff off the panels was quite a chore. It cleaned up pretty well, overall, though. The one casualty is on the rear panel. In the process of getting most of the gunk off from around the fuse holder, the "m" in "mAT" got hit once. D'oh! So, no more cleaning the rear panel.I hope you enjoyed this look at the HP 3469A. If you have any more information about it, do post it here.

author: slurry
author_rank: Regular Contributor
timestamp: July 09, 2017, 06:40:05 am
content: Neat and tidy instrument in the old HP small formfactor like the power meters and sinewave generators, i like'emActually i think i have one of those in my attic, but i suppose it's the B-model.

author: Vgkid
author_rank: Super Contributor
timestamp: July 09, 2017, 06:40:35 am
content: That is pretty cute. Looks like they used their ubiquitous half rack chassis for that.

author: bitseeker
author_rank: Super Contributor
timestamp: July 09, 2017, 07:36:02 am
content: It's interesting how little contrast there appears to be on the display. In reality, the background isn't so bright and blends in well with the black area to the right of the digits.

author: Cubdriver
author_rank: Unknown
timestamp: July 09, 2017, 08:56:49 am
content: Quote from: bitseekerLOL - had I known about it I might have taken a crack - I have a soft spot for those dot displays, too. (I like oddball stuff, in case you all hadn't yet picked up on that.)Quote from: bitseekerNice little meter. I'm curious - are you photographing it by available light, or using on camera flash? On camera flash will often wash things out in that manner. Here's my 3490A in ambient light:-Pat

author: Cubdriver
author_rank: Unknown
timestamp: July 09, 2017, 09:02:02 am
content: And BTW, congrats on hitting 3k!-Pat

author: bitseeker
author_rank: Super Contributor
timestamp: July 09, 2017, 09:30:59 am
content: Quote from: Cubdriver on July 09, 2017, 08:56:49 amNo flash. I was using a desk lamp for additional illumination since it was already night. It probably was the angle of that light source since the E3630A's display looks better. I'll have to give it another go in the daytime.

author: xrunner
author_rank: Super Contributor
timestamp: July 09, 2017, 02:26:34 pm
content: Good catch on that one. Good example of the chassis style they used back then. I have a couple of HP gear that use the same build - the 400E AC Voltmeter and a 209A Oscillator. Built like tanks. You're lucky that the range selector knob isn't broken - that tab that sticks out cracks off a lot. Love the LED dots too. Congrats on that one.

author: TiN
author_rank: Super Contributor
timestamp: July 09, 2017, 02:53:45 pm
content: Someone was busyRevamp it with LM399

author: alm
author_rank: Super Contributor
timestamp: July 09, 2017, 03:40:52 pm
content: Quote from: bitseeker on July 09, 2017, 06:16:45 amThe 3469B (likely very similar to the 3469A) tolerance on the 10 VDC range for measuring 10 V is 0.02%. So based on the DMM reading, the true value might be between 9.998 V and 10.002 V if the 3469A is in calibration. The E3630A meter accuracy for a 9.96 V reading is 0.5 % + 2 counts, so the true value might be between 9.89 V and 10.03 V if the power supply is in calibration. Based on this, I would say that the reading is consistent with the assumption that both instruments are in calibration.

author: Cubdriver
author_rank: Unknown
timestamp: July 09, 2017, 06:07:53 pm
content: Quote from: bitseeker on July 09, 2017, 09:30:59 amThat's probably it, then. Enough light hitting almost straight on got through the filter and washed things out. The pics of the innards look great, it was just too much for the display.-Pat

author: bitseeker
author_rank: Super Contributor
timestamp: July 10, 2017, 03:41:25 am
content: Quote from: xrunner on July 09, 2017, 02:26:34 pmIt is surprising. I've seen similar breakage on old signal generators. It really looks like this meter didn't get a lot of use. However, some of the knobs do have tiny fractures developing. To be used gently.

author: bitseeker
author_rank: Super Contributor
timestamp: July 10, 2017, 03:44:01 am
content: Quote from: Cubdriver on July 09, 2017, 06:07:53 pmYeah, seems to be the case. I don't often photograph equipment; it's quite different from landscapes.I'd like to take more photos of the innards, either when I'm feeling more adventurous or something stops working. Those capacitors won't last forever. We'll see.

author: bitseeker
author_rank: Super Contributor
timestamp: July 10, 2017, 03:46:09 am
content: Quote from: alm on July 09, 2017, 03:40:52 pmThat is quite a range on the tolerances. Everything may very well be in calibration. That's amazing. Well, when the US calibration club's 10-volt standard comes around to my stop, I'll have a check on it, too.

author: alm
author_rank: Super Contributor
timestamp: July 10, 2017, 03:51:46 am
content: The E3630A was not exactly designed as a precision voltage source. You could likely to better if you use the E3630A just to provide a stable voltage, and then measure it with the 3469A and a known-good multimeter.

author: bitseeker
author_rank: Super Contributor
timestamp: July 10, 2017, 03:57:33 am
content: Good point. I'll do that when the standard arrives. I have a couple handhelds that are still within their one-year calibration. That'll be fun.