[HN Gopher] Repairing an HP 3478A Multimeter with a Hacksaw
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Repairing an HP 3478A Multimeter with a Hacksaw
Author : picture
Score : 24 points
Date : 2021-11-26 20:10 UTC (2 hours ago)
(HTM) web link (tomverbeure.github.io)
(TXT) w3m dump (tomverbeure.github.io)
| Maven911 wrote:
| That's impressive. I've heard and the product manuals of most
| multi meters talk about the risk of shock when opening it up.
| Does anyone know how likely or rare that is?
| Aloha wrote:
| Effectively Near Zero.
|
| Most multimeters are battery powered - this one is the
| exception, this one is a bench meter and is supplied by an IEC
| cable on the rear, but so long as you unplug in and let it sit
| for 30 seconds or so before opening it up, you're just fine.
|
| You generally only have to worry about capacitors being charged
| still on tube-type gear (anything with high voltage circuits),
| anything with solid state parts, will self discharge within
| moments.
|
| Obviously, do not have the meter hooked up to a circuit under
| test while working on it.
| schoen wrote:
| I'd imagine that risk, if it's present, is from capacitors
| inside the meter. Capacitors inside tools and appliances can
| hold a large charge for a relatively long time, even after the
| circuit is no longer externally powered.
|
| You can discharge a capacitor by shorting it with a screwdriver
| (with an insulated handle), or by clipping a resistor to the
| capacitor leads for a less abrupt discharge. I think people who
| do a lot of appliance repairs learn to look carefully for
| capacitors when opening appliances, and make sure that any
| moderately large ones (or ones that there was reason to believe
| could have been charged to a high voltage) are discharged
| before beginning work.
|
| I don't know how common large capacitors are in multimeters. I
| think there's a much bigger risk when opening a CRT monitor or
| a microwave.
| Aloha wrote:
| You generally only have to worry about capacitors being
| charged still on tube-type gear (anything with high voltage
| circuits), anything with solid state parts, will self
| discharge within moments. This doesnt have a motor starting
| cap, or high voltage, so the risk is near zero.
| tverbeure wrote:
| After unplugging the power cord, I always switch the device
| back on with the main power switch. That should be sufficient
| to discharge the caps. It's never been a problem. Other than
| that, I just avoid touching anything close to the power
| circuitry. And, finally, I've survived many 220V shocks as a
| kid when playing with retired washing machines and such (I
| don't recommend it), 110V should be less shocking. :-)
| hvgk wrote:
| Risk is zero if it's powered down and disconnected from the
| circuit under test. The main risk is from the circuit under
| test leaving dangerous voltages inside the multimeter or the
| power supply of the multimeter only if it's a mains powered
| bench meter.
| [deleted]
| Animats wrote:
| _there is a element with a 90 degree angle that consists of tiny
| individual metal wires._
|
| That looks like an early version of what is now called a "Zebra
| connector".[1][2][3] Those are a stack of alternating conductive
| and insulating layers, and look striped, hence the name. These
| are used like gaskets between two PC boards. There are several
| conductive layers per pin, so you get a connection without
| carefully lining everything up. They're vibration-resistant, so
| they tend to be used with keypads and in automotive dashboards.
|
| I had no idea those were still around. I saw them mentioned once
| in some article about early digital watches.
|
| This would be a useful technology for things that stack together
| mix-and-match like Arduino shields, but you want something less
| bulky and more reliable than 0.10 header pins.
|
| [1] https://www.zebraconnectorsmfg.com/product/elastomeric-
| conne...
|
| [2] https://www.fujipoly.com/usa/products/zebra-elastomeric-
| conn...
|
| [3] https://en.wikipedia.org/wiki/Elastomeric_connector
| duskwuff wrote:
| > I had no idea those were still around.
|
| Zebra connectors are still extremely common for interconnects
| between PCBs and passive-matrix LCD glass.
| nabilhat wrote:
| Not the best solution in this case, but coincidentally one that I
| used to revive a handheld multimeter last year: A pencil eraser
| will polish up old copper contacts very nicely. It's the gentlest
| way I've found to cut through the oxide layer and a nice hack to
| have for cleaning up delicate contacts in old electronics.
| dtgriscom wrote:
| A classic trick. Works, too.
| colinng wrote:
| Also for bad contacts in old devices (or for the removable
| battery in my eBike), MG Chemicals conductive grease works
| wonders. And you can keep everything "original". For zebra
| conductors I would try applying a very thin layer of the
| grease to the copper pads (after cleaning them with a pencil
| eraser) to assure only conduction along the desired path.
| formerly_proven wrote:
| If the author is reading this here, I'd urge you to replace those
| RIFA metall-paper caps near the mains switch. At worst they're a
| fire hazard, at best they blow their nasty guts all over their
| general vicinity.
|
| The 3478A is a pretty decent and versatile meter. It has three
| weak points though:
|
| 1. The aforementioned RIFA caps
|
| 2. A lot on the analog side is done by a hybrid, including
| driving relays, input switching and a bunch of trimmed reference
| resistors (current source gain and parts of the input divider
| iirc). Also, the mounting of the hybrid is also what holds the
| front/back input switch in place. Stuff's flexing a fair bit when
| using that switch! If that hybrid is shot, the whole thing is
| pretty dead.
|
| 3. The calibration is stored in a battery-backed SRAM on the
| outguard side of things. I don't think you can extract the SRAM
| contents, so battery swaps need to be done live. If you send one
| of these in for calibration, it's going to be about as expensive
| as another 3478A (n.b. the circuit allows you to do stupid things
| like e.g. shorting the battery for a short time, because the SRAM
| is fed by the battery and the power supply through or diodes, and
| there's a 1uF or something like that tantalum near the SRAM. I
| don't know how long it survives off of that cap)
|
| 4. Don't feed mains into it when in ohms mode.
| tverbeure wrote:
| > I don't think you can extract the SRAM contents, so battery
| swaps need to be done live.
|
| You can read and write the SRAM contents over GPIB. See the
| references section at the bottom of the article.
| mastax wrote:
| I bought a cheap GPIB-USB adapter (LQ UGPlus) which didn't
| work because the Windows software couldn't handle the binary
| memory addresses.
|
| I should see if I can talk to it directly, it's probably HID
| or CDC.
| kevin_thibedeau wrote:
| 5. They can't use shrouded probes. Retractable shrouds were
| made at one time but are nearly unobtainium now.
| mastax wrote:
| Was a good excuse for me to buy some unshrouded probemasters.
| hvgk wrote:
| The battery swaps don't need to be done with the instrument
| powered. You can solder a "surrogate" pack with a series
| current limiting resistor across the or diode and ground while
| you swap the battery out.
|
| You can also recalibrate it from the front panel easily if you
| have another meter to compare it to with similar or better
| resolution.
|
| Guess who screwed one up :)
| hellbannedguy wrote:
| You know your electronics. Thanks for all your imput. Number 3
| surprised me.
| tverbeure wrote:
| I'm on it!
|
| https://twitter.com/tom_verbeure/status/1464332190892756994
| ananonymoususer wrote:
| I've also done a few successful mods with a hacksaw.
|
| One time I used a hacksaw to remove part of a 4-lane PCIe
| connector on a server motherboard so I could plug in a 32-lane
| PCIe video card. It worked great.
|
| Another time I wanted to plug in a full-height USB 2.0 PCI card
| into a (Soekris) embedded SBC low-profile slot (limited by the
| enclosure), so I sawed off the upper two USB jacks. I had to do
| some minor rework to the card (reconnecting some of the broken
| traces with mod wires), but it worked great afterward.
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