HP 3478A has poor Cal RAM Battery Life [SOLVED] - Page 1
Forum section: electronics / projects

author: enut11
author_rank: Super Contributor
timestamp: January 12, 2024, 11:38:01 pm
content: The HP 3478A DMM uses a 2/3A Li cell to store the calibration constants (BT701).This cell can normally last up to 10 years but on my meter it goes flat in just a few months.I suspect a faulty Cal SRAM chip (U512) which is an NEC 5101 type, 256x4 bit (1Kbit).Is there any way to check if it is the chip causing the poor battery life before replacing it?EDIT: this thread now belongs in the Repair section. Does anyone know how to do this?

author: bdunham7
author_rank: Super Contributor
timestamp: January 13, 2024, 12:16:43 am
content: Your other suspects are C510 and U550, right? So measure the draw on the battery. IDK what it is supposed to be but if the battery dies that quickly I'm assuming it will be many µA. Then remove or lift R768 to rule out U550, if that is good pull C510.

author: Mikaelin
author_rank: Contributor
timestamp: January 13, 2024, 12:24:01 am
content: I would try to find the leak. Before, I would think about calibration data, to not loose information. Next step is to insert in the battery line a microamp meter to measure the current consumption in operating state and in power off state. Then, just isolate the pin 22 of the U512 from the PCB (I use a desoldering pump or a desoldering braid) and repeat the measurement in power off state. If the current draw is the same, then is not U512 draining the battery and you can easily resolder back the pin 22.

author: enut11
author_rank: Super Contributor
timestamp: January 13, 2024, 01:06:07 am
content: Thanks@bdunham7and@MikaelinWhen I lost all my cal constants I inserted a uA meter and 3v power supply. The current was not stable and the meter varied from about 150uA to 170uA (spec is 10uA).

author: srb1954
author_rank: Super Contributor
timestamp: January 13, 2024, 04:11:06 am
content: Quote from: enut11 on January 13, 2024, 01:06:07 amAlso check the logic levels on the control pins to the RAM chip. An indeterminate logic level could cause additional power drain; the chip enable CE2 has to be below 0.2V to obtain the specified 10uA standby current drain.

author: enut11
author_rank: Super Contributor
timestamp: January 13, 2024, 04:38:00 am
content: @srb1954, if the meter is powered up, would not the 5v rail prevent any current drain from the battery?Seems to me to be a power off problem and the CS lines must be zero?

author: enut11
author_rank: Super Contributor
timestamp: January 13, 2024, 07:17:00 am
content: After isolating pin 22 of U512 from pin 14 of U515, the bulk of the rogue current dropped close to zero.So, it appears that U515, the CAL Enable chip, is faulty.U515 is a Motorola 74HC20 chip. The local electronics shop only has 74LS20. Would the LS chip work?I really do not see any need for a high speed HC chip in this application.

author: srb1954
author_rank: Super Contributor
timestamp: January 13, 2024, 07:59:16 am
content: Quote from: enut11 on January 13, 2024, 07:17:00 amBefore removing this chip first check that the logic levels going into U515 are good, either close to GND or close to VCC, when running off the battery supply. Poor logic levels will cause a CMOS logic chip to draw excessive current.QuoteNo. The 74LS20 has a much higher static current drain than a 74HC20 and will drain your battery in hours. An HC chip is not really a higher speed version of an LS chip but one built with CMOS transistors rather than bipolar transistors.

author: enut11
author_rank: Super Contributor
timestamp: January 13, 2024, 09:24:36 pm
content: Thanks@srb1954. I was about to buy the LS chip.It is interesting that the Jaycar catalogue implies the opposite, ie that the LS is the low power chip.enut11

author: adinsen
author_rank: Regular Contributor
timestamp: January 13, 2024, 09:38:12 pm
content: A 74HCT20 should do the job, if you find that the original 74HC20 has failed and it's not the logic levels causing the problem, as srb1954 suggests.

author: enut11
author_rank: Super Contributor
timestamp: January 13, 2024, 10:40:14 pm
content: Wow. Talk about being caught out. I just revisited the circuit diagram and realised that I had not eliminated all possible causes of the xs current.There is a small beige 104 value capacitor (C519) at pin 14 of U515. I have never seen one of these fail. When I removed it, it disintegrated. Replaced it with a similar one below and the battery current is now down to a few uA.Life is full of lessons....

author: robert.rozee
author_rank: Frequent Contributor
timestamp: January 14, 2024, 04:07:26 am
content: enut11: congratulations on finding the fault - when i first saw this thread i thought of those 0.1uF capacitors as a possible source of problems.a bit off-topic, but has anyone ever though of simply reading the contents of the calibration RAM and writing it into an EPROM that can then be used as an exact dropped-in replacement. ie, no circuit changes or adapter boards required, and no longer any need for a battery, but with the down side that the calibration is no longer adjustable.it is just that, it occurs to me that the vast majority of these meters probably never get their calibration adjusted anyway, so in most cases an EPROM poses no disadvantage. and if youreallydo want to do an adjustment one day, swap back to the original RAM chip and then once adjusted write a new set of EPROM data.cheers,rob :-)

author: enut11
author_rank: Super Contributor
timestamp: January 14, 2024, 04:54:19 am
content: Hi@robert.rozeeSo, those little caps are not as reliable as I thought they were.I agree that the 3478A seems to go forever without a recal so your suggestion bears merit. However, beyond my capabilities at the moment.enut11

author: srb1954
author_rank: Super Contributor
timestamp: January 14, 2024, 07:39:41 am
content: Quote from: enut11 on January 13, 2024, 09:24:36 pm74LS is lower power when compared to other bipolar logic families e.g. 74xx, 74Sxx, 74Fxx, but it still has vastly higher standby power than any of the CMOS logic families e.g. 74Cxx, 74HCxx, 74ACxx etc.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2024, 09:07:16 am
content: For some of the HP dmms one can read the cal data via GPIB. So it should be possible to get a bachup of the data this way. Normally the CMOS ram and battery is quite reliable.For the EPROM idea one may have a hard time finding such a small one and the older small one often needed an extra negative supply.

author: tooki
author_rank: Super Contributor
timestamp: January 14, 2024, 01:23:12 pm
content: Quote from: enut11 on January 13, 2024, 09:24:36 pmThe names of the 7400-family logic series (and there are many) are fixed, and anything in those fixed names referring to the speed or power consumption isrelative to its predecessor series.*For example, LS is a lower-power version of the earlier “S” (Schottky) series. Later, the ALS (advanced low-power Schottky) series came out, and later still, AS (advanced Schottky). Each series has different characteristics: some optimized for clock speed, others for propagation delay, others for power savings, etc.The newest series, HCS, was introduced in 2019!!This is the best overview of the logic series I’ve found:https://www.ti.com/lit/sg/sdyu001ab/sdyu001ab.pdf*It reminds me of sizes in connector naming: something is “miniature” compared to what came before it, even if it’s gigantic by today’s standards. The D-sub connector is a perfect example. It’s definitely D-shaped, but while it may have been “subminiature” in 1952, it hardly is by today’s standards, but we don’t rename it because that’s its name, not a description. (And yes, it was followed by micro-D and nano-D eventually.)

author: m k
author_rank: Super Contributor
timestamp: January 15, 2024, 06:58:52 am
content: Quote from: robert.rozee on January 14, 2024, 04:07:26 amWhy not a small MCU module with USB?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2024, 08:20:12 am
content: Quote from: m k on January 15, 2024, 06:58:52 amIt looks like the SRAM is directly connected to the CPU bus. Though not very fast it would still need to be quite fast for a µC to repond.From some other meters I know of a way of sniffing the RAM on power up to get the content. With only the limited size (8 addresses and 4 bits) this may be possible for a logic analyzer / µC.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: January 15, 2024, 09:35:48 am
content: Quote from: m k on January 15, 2024, 06:58:52 amlooks like that may have already been done, see:https://www.dr-bosch.com/volker/umbau/index.html#HP3478Aor translated into english by google:https://www-dr--bosch-com.translate.goog/volker/umbau/index.html?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=sc#HP3478Ahe is using an atmega48 and a bit of assembly code to handle serving up the calibration data (cached in the processor's RAM) to the 3478A. this suggests one could equally well use an arduino nano (includes USB) or arduino mini (serial only).unfortunately, i could not find any further information about his source code.cheers,rob :-)

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2024, 10:44:21 am
content: The more important information in the links is that one can read and write the CAL RAM via GPIB. So one can relatively easy get a bach-up.

author: pqass
author_rank: Super Contributor
timestamp: January 15, 2024, 03:16:52 pm
content: The link above is likely referring tothis eevblog thread.Where the undocumented GPIB commands are: READ: W<addr> which returns a <value>, WRITE: X<addr><value>Where <addr> is a binary byte from 0..255, <value> is an ASCII char from@..O (lower nibble is the cal data, upper nibble is binary 4).Rather than replacing the cal SRAM with an EPROM or MCU, it is probably more useful and easier to make anArduino-based GPIB-to-USB(or RS232) interface(seethis supporting eevblog thread) and replace the soldered-in lithium primary battery with a battery holder (and optional battery measurement port out the back). Then periodically replace the battery but only while the unit is powered-up. Print out a copy of the cal data and stick it to the inside of the enclosurein casewhen you screwup.If you don't know how to script the cal reads and writes yourself and you're running Windows with an AR488 or Prologix interface, you can usethis program from fellow member lmester to do it for you.

author: Miti
author_rank: Super Contributor
timestamp: January 15, 2024, 07:10:33 pm
content: Take a look at this trail:https://www.eevblog.com/forum/projects/3478a-fram-modification/msg4878902/#msg4878902