Battery replacement in HP 8591A Spectrum Analyzer - Page 1
Forum section: electronics / repair

author: kawal
author_rank: Frequent Contributor
timestamp: April 30, 2020, 02:48:28 pm
content: I read in this forum about someone that has turned on a unit like this and to their surprise the constants where lost before they could back it up. So finally decided to change batteries.Hp was nice enough to provide the battery installation date on a sticker on back of the units. Both units have a Battery install date of 1991. I did check the battery dimensions inside the unit and they are 50mm long and 15mm diameter with Axial tabs. Original battery was made by Tadiran and the only marking I could see was AA.I did a little research for the right battery choice. First we need to know the current requirements. The HP 8591A uses Static RAM TC55257 which only provides max standby current range between 2uA to 30uA per chip. This is useless information. Next step would be to ignore the current usage since after all the battery lasted almost 30 years and still works.These are ~2Ah batteries (assuming all the capacity is used up by now ) so we can estimate that the average current to be no more than 7.8uA . In reality this is probably a lot less.So you want a battery with low current discharge and long life. The TL-4903 is rated for 40Years on shelf and is rated for ~55.000 hours (6.3Years) at 38uA discharge while producing 2.1Ah. This should be a very good battery for this application.Based on the data sheets I ordered TADIRAN TL-4903/P batteries..To get ready to replace batteries on 2 of mine HP 8591A spectrum analyzers we need to backup Correction constants that are stored in RAM.First step is to get the manual and print out the section 9-2..9.6 "Backing up Analyzer Correction Constants"In worst case these could be lost if something goes wrong so worth having a hard copy (or Soft copy) to avoid expensive calibration.Here are the calibration constants from the 2 units I have. Maybe someone can use this info.With this data I feel good about waiting for the battery. No more stress over lost Calibration .

author: kawal
author_rank: Frequent Contributor
timestamp: May 04, 2020, 11:57:58 pm
content: Batteries came in today. The packing on these was crazy .These where made in July 2019 so now they only have 39 years 2 months of rated life left.

author: kawal
author_rank: Frequent Contributor
timestamp: May 05, 2020, 01:27:57 am
content: After opening up the unit and sliding off the cover it was time to remove the back. Its not needed but easier to remove the GPIB card.The old battery is best to be cut away and new battery soldered in its place. There is a reserve capacitor that should be good for min 30min of backup but probably will last a lot longer. I also removed the sampler card to take some pictures but it is not needed.

author: kawal
author_rank: Frequent Contributor
timestamp: May 05, 2020, 01:29:39 am
content: Some more pictures. Unit powered on and after pressing the defaults button the display showed the usual noise floor.I connected the 300Mhz cal output to check if the calibration is still good and it is. I saved the parameters either way so no problem if they where lost.

author: kawal
author_rank: Frequent Contributor
timestamp: May 06, 2020, 03:02:41 pm
content: Did the battery replacement on the second unit. I tested both of old the batteries and they seemed to OK and showing 3.6V. I attached a 63.4K resistor to each of the old batteries to get an estimate on remaining capacity . The first unit is holding stable at 3.6V after 2 days but the second started at 3.59V and dropped to 2.8V within 1 h. After 12h the battery is at 2.74V so its about depleted.Obviously the load of the resistor is about 10x of the actual load but that is still close to losing data. I would estimate that a few more weeks and the battery would have died and taken the calibration data with it. I would say that I was lucky.The second unit has more options installed but also has some interesting bodges. There is a few small boards installed on the processor board that where not present in the newer unit.

author: kawal
author_rank: Frequent Contributor
timestamp: May 06, 2020, 03:19:53 pm
content: The closeups of the bodge boards (or options)

author: Bicurico
author_rank: Super Contributor
timestamp: May 06, 2020, 06:59:43 pm
content: I took pictures of all calibration screens - found that to be quicker than writing down the numbers.As a note: newer revisions will ha e a coin style battery so one shoild take a look before ordering the battery.Regards,Vitor

author: kawal
author_rank: Frequent Contributor
timestamp: May 06, 2020, 08:17:49 pm
content: Quote from: Bicurico on May 06, 2020, 06:59:43 pmThe calibration data depends on Firmware version . On both of my units each frequency is displayed separately. I want to try newer firmware i found online from 1995 and 1998. Mine are from 1990/10 and 1991/05 so the newer Firmware might change the way the calibration is displayed.

author: kawal
author_rank: Frequent Contributor
timestamp: May 18, 2020, 02:04:14 pm
content: Tried Eproms from 1998 and the unit will not boot. Will try older ones

author: kawal
author_rank: Frequent Contributor
timestamp: May 18, 2020, 03:23:09 pm
content: Tried 1994 Eproms that i found in Bin format and these did work. Now calibration data is shown in different format. After a short period of trying to restore the flatness data and getting 2.9Ghz in span setting (which is wrong) i decided to call it quits and reinstall original firmware. Flatness data went in not problem and the self calibrations pass not issue.

author: Rik
author_rank: Newbie
timestamp: March 29, 2023, 11:47:30 am
content: Did you solder the new battery leads to the stubs of the old battery, and not lift the board?

author: Rik
author_rank: Newbie
timestamp: March 30, 2023, 05:38:16 pm
content: I pulled the board, installed the new battery. Working fine now.

author: Greybeard
author_rank: Frequent Contributor
timestamp: May 16, 2024, 09:55:00 am
content: Can I useTADIRANSL760/P2200mAhorSL-360/P2400mAhinstead ofTL-4903/P2400mAhor originalTL-5104/P(2100mAh?) ?

author: Homer J Simpson
author_rank: Super Contributor
timestamp: May 16, 2024, 02:51:18 pm
content: Quote from: Rik on March 30, 2023, 05:38:16 pmHow did that board removal go in terms of difficulty ?Any pictures of that process ?Thanks for posting this. I have two 8594E's that also need batteries.-K

author: Greybeard
author_rank: Frequent Contributor
timestamp: May 16, 2024, 06:26:59 pm
content: Quote from: Homer J Simpson on May 16, 2024, 02:51:18 pmI found this for the 8594E:http://www.azurelectronics.com/Repair%20HP%208594E%20Spectrum%20Analyser.htmPS: I've seen this piggy-back board also in a '98 HP8591E

author: Homer J Simpson
author_rank: Super Contributor
timestamp: May 16, 2024, 06:39:43 pm
content: Quote from: Greybeard on May 16, 2024, 06:26:59 pmI appreciate that post but I did a quick visual inspection on my units and they have the older style axial battery as in the original post.-K

author: Greybeard
author_rank: Frequent Contributor
timestamp: May 16, 2024, 06:48:12 pm
content: External battery hook up for 8591Ewith 3,6V 14500 AA type lithium thionyl battery, found on youtube:

author: W3KW
author_rank: Contributor
timestamp: August 28, 2025, 07:00:24 pm
content: HMMM. I just opened my HP 8591EM to replace the battery. Aside from 5 missing screws on random circuit boards, that battery does not exist in mine. Well, sort of. My unit has three boards in the bottom corner of interest.. One board removed reveals another. The next one removed by releasing scary friction locks reveals...nothing. No battery on the board in every video and website, and thread I've seen, including this one. The battery is in fact on one of the removeable boards. It was located on the sandwiched board. It is a Panasonic CR2477 3.0v button style battery.Digikey to the rescue.Panasonic CR2477/HFN 3VYes. I recorded all my timesbase and flatness correction constants. Hopefully I won't need them.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: August 28, 2025, 07:14:39 pm
content: tl;drits highly recommended to bypass the battery at exchange with an lab psu + resistor. In contradiction to the SM promise, the wondercap beside the battery does't deliver the power to hold the data in SRAM due aging. So your Cal-Data are lost.

author: W3KW
author_rank: Contributor
timestamp: September 03, 2025, 10:14:57 pm
content: Well, the new battery arrived and I did the deep dive. I measured 2.8 volts on the "back-up capacitor" and couldn't find an easy place to solder a power back-up battery so I decided to trust the manual that I had eight hours to safely replace the battery using the capacitor as back-up power. In all, it took less than two minutes thanks to being organized, having a solid strategy, and a Hakko Desoldering gun (best restoration purchase I ever made). I had all the constants recorded just in case.I had the 8591EM back together in less than ten minutes, powered her up, and bam. All the constants were verified and she is 100% operational. The old battery was installed in September of 1995. 30 years ago! Remind me in 2055 to replace this battery, huh?WesW3KWWhat a beautiful set when matched with the Agilent 8648A (or HP version) Signal Generator.

author: Bicurico
author_rank: Super Contributor
timestamp: September 04, 2025, 09:57:03 am
content: Great success!

author: Miti
author_rank: Super Contributor
timestamp: September 04, 2025, 04:07:24 pm
content: No more battery in my HP8594E. Well, there is a small coin battery for RTC. Check this out:https://www.eevblog.com/forum/repair/has-anyone-replaced-hp8591(e)-sram-by-fram/new/#lastPost

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 04, 2025, 06:39:48 pm
content: Depending on owners age it could be wise to put an datecode badge of battery exchange on the rear of instrument below the printed battery information*.I did so in '17 and therefore a couple of days back replaced my battery again. Found a "Renata" at 2,96V and put also a Panasonic in. Advantage: the different diameters of holes on pcb are matching perfect to Panasonics footprint. To prevent the same disaster happened in past, i backed up the power by soldering tiny wires direct on the solder lugs of the wondercap with a series resistor of approx 3kOhm.* ( below the vertival EXT Ref connector bridge )