3 ms·
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 blo
by formerly_proven 5y ago
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 5y ago> 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 5y agoI 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.
- tverbeure 5y agoI'm on it! https://twitter.com/tom_verbeure/status/1464332190892756994 https://twitter.com/tom_verbeure/status/1464332190892756994
- hvgk 5y agoThe 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 :)
- kevin_thibedeau 5y ago5. They can't use shrouded probes. Retractable shrouds were made at one time but are nearly unobtainium now.
- mastax 5y agoWas a good excuse for me to buy some unshrouded probemasters.
- metaphor 5y agoCheck out Probe Master 8018S test leads[1] or 8044S kit[2] in your choice of length. I've got a HP 3468A on the home bench and don't like futzing with lead compatibility when I want to switch to a 87V handheld. [1] https://probemaster.com/8000-series-test-leads-only/ https://probemaster.com/8000-series-test-leads-only/ [2] https://probemaster.com/8000-series-kits/ https://probemaster.com/8000-series-kits/
- hellbannedguy 5y agoYou know your electronics. Thanks for all your imput. Number 3 surprised me.