Electronics > Repair

HP 3437A System Voltmeter - first half of teardown, pending repair

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c4757p:
HP 3437A System Voltmeter teardown and repair, part one

Still quite a way to go, this is a complex piece of kit and not in the best shape.

Never mind the mostly uninitialized blog, it's brand new.

The analog people here might appreciate this preview:



Edit - added a couple photos I missed.

echen1024:
Delightful. Some interesting construction quirks though. However, those white ceramics are absolutely beautiful.

c4757p:
I tracked them down in the schematic (just updated the text to reflect it) - the two in the analog section are actually just precision matched resistor networks! The one in the digital section is an unobtainium microprocessor made by a company nobody's ever heard of, which specialized in MPUs for T&M applications. (I can't remember the name anymore) made by HP themselves, as free_electron pointed out - don't recall where I read that but I don't think it's true.

There are definitely some quirks. The digital board in particular looks like it was laid out rather carelessly - all sorts of pointless kinks and twists in traces, etc. The black jumper wire you can see in the picture of the MPU installed in its socket? There's a perfectly good space on the back of the board to run a trace for it. (Possibly it's there to allow disconnecting for service, but why a long wire with crimped connectors when a two-pin header-style jumper will do?) Parts of it are clearly designed with servicing in mind, but others are totally not - for instance, most of the connectors are directly soldered/crimped. Removing the GPIB port to get the digital board out was interesting.... the ribbon cable is mounted into the IDC connector and soldered directly to the board. I had to work the connector itself out through the back panel cutout (it was that or desolder twenty-something wires).

Of course, if you don't need to fully remove the boards, both of them are mounted in with flexible plastic hinges. Just unscrew on one side and fold out. :-+

And I'm curious about that triax connector on the rear panel. It's directly paralled with a standard coax connector on the front panel, and the guard is undriven, so what's the point? :-// Unless it was a junk box replacement by a clueless person. That is a possibility - there was one screw missing from the chassis, so I don't think I am the first one inside this.

I don't think I'm going to have to dig far into troubleshooting this one. I think it's just some stuck keys. But does anyone have access to a schematic that is actually legible? Agilent's copy is ridiculously smudged, as usual. (How the hell does the company that made it not have access to decent-looking schematics??)

sync:
That turrets are teflon insulators. High impedance stuff.

c4757p:
Ah yes, I see that now! Thank you. (That's pretty clever, actually.) I never noticed the ring around the metal, I saw the traces encircling them and thought they were connected to those traces. I guess they are actually guard traces.

By the way - am I right to think that the "crinkly" look to the top of this ROM is due to heat? The underside of the PCB around them is darkened a good bit, too. I'm really hoping there's no thermal damage to this thing, once I get the panel fixed - I don't remember hearing the fan spin up when I turned it on. :-BROKE

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