HP 3476A DMM Repair/Refurb - Page 1
Forum section: electronics / testgear

author: xrunner
author_rank: Super Contributor
timestamp: March 10, 2015, 01:16:38 pm
content: Just came yesterday. This is a 1970's retro design, kinda cute. I've been looking for a good price for one and finally found it. One challenge will be to restore the printing on the side panel, most of it has been worn off. I have several ideas for that. I like the notice on the bottom -No operator serviceable parts inside.Refer servicing to qualified personnelThat would be me!

author: xrunner
author_rank: Super Contributor
timestamp: March 11, 2015, 01:54:01 pm
content: Some pics of the board. In the last pic you can see the intergalactic neutron beam communicator - or heat sink?

author: Vgkid
author_rank: Super Contributor
timestamp: March 11, 2015, 02:29:17 pm
content: That is a unique heatsink.

author: dom0
author_rank: Super Contributor
timestamp: March 11, 2015, 03:37:56 pm
content: The HP version of a poor man's heatsink (bolt and nut).Gotta love how HP stuffed hybrids into ... everything in the 70s and 80s. In the 3478/3468/3457 etc. they even used (part of) a hybrid just for driving a few relays!

author: Sbampato12
author_rank: Regular Contributor
timestamp: March 11, 2015, 04:37:56 pm
content: Nice heatsink! (photo too)

author: SeanB
author_rank: Super Contributor
timestamp: March 11, 2015, 04:45:35 pm
content: Those heatsinks were originally meant to fit a TO18 transistor ( or a TO39 for some) so you could run them at a higher power than the bare case. Typically a 2N2222A or a 2N2219A running at around 2W of power, way more than the 200mW or 500mW of the case alone. They had a thread to allow it to be chassis mounted with an insulator kit, so you could even use the chassis as an extra heatsink. I even had some kits to mount a TO39 device to a heatsink, using a hole pattern similar to a TO66 transistor but with 3 holes instead of 2, so you could go up to 5W of power dissipation.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 11, 2015, 05:39:17 pm
content: Quote from: xrunner on March 11, 2015, 01:54:01 pmSeveral millennia ago, the Star League of the Intergalactic Empire spread thousands of those communicators across the galaxy, in the hopes that one day it will be found by intelligent life. Now that you've opened it up, it's phoning home, and you will be contacted to join the Star League to defend the Frontier.Remember to pack a lunch when you get your free Gunstar. It's a long way to Rylos.

author: xrunner
author_rank: Super Contributor
timestamp: March 11, 2015, 09:33:23 pm
content: Few more pics. The inside of the unit looked to me to be pristine, but on the bottom of the board I saw a scratched out trace. The trace on the left of the scratch is a ground/guard trace, and the pins on the right go to the volts switch. I checked the schematic and there is not supposed to be a direct ground connection to the V switch.I've seen another picture of the bottom of the board for this model in a video, and on that board the factory had installed a bodge wire on another part of the board and covered it with some clear substance. The guy in the video said the trace it was replacing had been etched way too far and was too thin. I think the trace on mine had been scratched out at the factory due to a manufacturing error. I think they didn't have the best quality control on this model.Here's the video -

author: xrunner
author_rank: Super Contributor
timestamp: March 12, 2015, 05:04:27 pm
content: Well I'm dome with this one. It's clean and calibrated now. I attempted to restore the small thin markings on the slide cover but they are so small it's almost impossible to draw them nicely. But, it's better than nothing. Perhaps I'll find the parts on Ebay. On to a new project.

author: Muxr
author_rank: Super Contributor
timestamp: March 12, 2015, 05:46:25 pm
content: That thing looks wild. Very cool.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 12, 2015, 05:55:49 pm
content: Putting the dotinsidethe segment seems a bit.... well, unthoughtful?

author: SeanB
author_rank: Super Contributor
timestamp: March 12, 2015, 06:12:58 pm
content: Seeing as those displays are made as a deposited grown film of Gallium arsenide on a silicon wafer, it makes sense to not use any more expensive electronics grade wafer than absolutely needed, so the decimal point in the otherwise unused inner section is sensible. That you needed a whole digit for a dp was a trade off.Later versions used a slightly larger silicon slice ( cheaper as time went on and wafers got bigger) with the decimal point fitted by sloping the display to fit it on one side or the other. Later still they put a decimal point on both sides, and a comma shaped on on top as a thousands separator for calculator displays with lots of digits. then they went to light pipes and a single die per segment like the common display we see these days.

author: Thomas
author_rank: Regular Contributor
timestamp: March 12, 2015, 06:43:05 pm
content: That's a beautiful little multimeter!Everybody have to love those bubble LED displays.I think I have seen something similar, on top of a Tektronix oscilloscope back in the eighties?

author: LaurenceW
author_rank: Frequent Contributor
timestamp: March 12, 2015, 10:04:47 pm
content: Good luck finding spare 32mA fuses? One good fart, and you'd probably blow a fuse that fine!

author: xrunner
author_rank: Super Contributor
timestamp: March 12, 2015, 11:09:46 pm
content: Quote from: LaurenceW on March 12, 2015, 10:04:47 pmLOL - I know.When I first got it I checked the fuses and it appeared that the 32 mA was blown, because I had no idea how small the wire would be. I struggled to even see the tiny thin wire even under a magnifying glass! Its one of the tiniest wire I've ever seen.

author: langlv
author_rank: Contributor
timestamp: October 28, 2021, 12:52:59 pm
content: This was a genius idea. At first, it was a DMM option for HP 17xx oscilloscope.The additional 'Time Interval' function have no complicated circuitry. It simply measures the voltage between cursors of the 'dT Start time' and the 'dT Stop' potentiometers.The same trick is used in Tektronix 2336 oscilloscope.