Quick HP 3456A ROM Mod - Page 1
Forum section: electronics / testgear

author: dr.diesel
author_rank: Super Contributor
timestamp: August 18, 2014, 01:43:57 pm
content: The 3456A can be had pretty cheap on eBay, mine was less than $100. Many of these found today are dead due to Mostek ROMs with severe alzheimer's.Lots of infohere.Basically, buy an 27256 EPROM (still available), yank old ROMs, flash with your programmer, buy a cheap GQ-4X/etc, mod like the below. Note pins 1-2/27-28 are hanging over the socket, old Mostek ROMs are 24-Pin, the 27256 is a 28-Pin.Mod works by utilizing the existing ROM chip select pins to address the 27256.Sorry for the crappy pict, but enough to get the job done. Thanks goes to Tom from the hp_aligent_equipment mailing list.27C256.zip (Checksum:00469BE9), rename to .bin

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: August 18, 2014, 06:42:44 pm
content: kudosJedec socket hack! My favorite easy kind of hack. I used to replace the character generator ROM on Hercules graphics (monochrome) cards with a larger type for additional character sets. Controlled the upper order address lines with parallel port output pins. Back in the day all the replacement video card makers copied IBM's monochrome card inclusion of a parallel port for comparability reasons.

author: dr.diesel
author_rank: Super Contributor
timestamp: August 18, 2014, 07:20:40 pm
content: May be really old, but the ROM hack is super easy, plus gets you cheap offset compensated resistance,

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2014, 04:28:06 am
content: 2364 to 2764 pinout adapters exist. I was going to use them to convert the Mostek ROMs in a Tektronix 7845 to 2764s until I discovered that the later 7854s had already stopped using the Mostek ROMs and changed to the 2764 pinout.http://store.go4retro.com/2364-adapter/

author: dr.diesel
author_rank: Super Contributor
timestamp: August 20, 2014, 11:28:03 am
content: One additional note, the 3456A doesn't use the now standard 0.75" banana plug spacing, it's just slightly larger.I duplicated robrenz's workhere, with a set of his ultra nice mini-pinchers for my K2010. Which I'm not able to use with my 3456a,Robrenz,please, I need a couple more sets,

author: saturation
author_rank: Super Contributor
timestamp: August 21, 2014, 03:34:13 pm
content: What was the 1st four digits of your 3456a that required repair? Not all the versions of these DVM have had reported ROM failures.Quote from: dr.diesel on August 18, 2014, 01:43:57 pm

author: dr.diesel
author_rank: Super Contributor
timestamp: August 21, 2014, 03:50:00 pm
content: My serial is 2015A04303, which had the (3) Mostek ROMs. AFAIK, there were a total of three main versions; first having 5 ROMs, then 3, with the final utilizing a single.

author: saturation
author_rank: Super Contributor
timestamp: August 21, 2014, 06:55:49 pm
content: Thanks Dr,d so far only 2015a have had ROM problems, its a comforting note as one can likely avoid the ROM issues by not getting a 3456a with that prefix serial number.http://www.febo.com/pipermail/volt-nuts/2010-November/000626.htmlQuote from: dr.diesel on August 21, 2014, 03:50:00 pm

author: guido
author_rank: Regular Contributor
timestamp: August 21, 2014, 09:56:15 pm
content: i have a 2201a which had ROM problems... Mmm, maybe i swapped the pcb with my second one. Can't rememberThere were three ROMs in there. I put in62532's (TI) as the pcb can hold both options and i had them around. Just some rewiring of a few jumper wires.Unfortunately the main analog pcb is not ok; bad reference, bad custom resistor and some fet troubles. Hopefully some day i find a parts unit to match.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 21, 2014, 11:47:27 pm
content: At work there are still a dozen or so of these old timers still plugin away and the only issues have been the usual power supply ones. ** @2023/4 also failures of the lm339 comparators on the analogue PCB causing weird ranging/ autozero problems, check those are working properly if you have strange function / measurement issues ! **Though most of the units are the 'fan less' version (ie 2201 ser. prefix).I do have an old 2015 serial prefix with the fan. No issues with the ROM's so far.Does your unit have the fan?.Any way nice 'mod' to refresh the old meters grey cells. and thanks for the link, That info will go into the manuals for further reference. ** And future arrived at 2024/5**BTWHave you seen the other thread with some HP service notes attached, might be applicable for your vintage 3456https://www.eevblog.com/forum/testgear/hp3456a-service-notes-for-owners-info!/msg144820/#msg144820

author: jkloepping
author_rank: Newbie
timestamp: February 01, 2017, 05:06:51 am
content: I thought I'd post an image of my HP 3456a ROM fix that uses the Retro Innovations 2364 ROM adapter with a 27L256 as suggested in this thread.http://store.go4retro.com/2364-adapter/Retro Innovations also sells blank ROMs for $2, so a total of $7 for the mod.The mod is simple, just pull the old ROMs, and install the above adapter into U5. Then remove U6 and jumpers 4-5 and 7-8. Install a new jumper between U6 pin 4 to U6 pin 10 (ROM select line goes straight to U5), then jump A13 from the adapter to the now empty hole 4 where the 4-5 jumper was, and then jump A14 to the now empty hole 8 where the 7-8 jumper was.And done. I've attached a picture. Note that I installed the empty socket before trying this solution and I didn't want to desolder it. Otherwise, it's a very clean install and works great.

author: lowimpedance
author_rank: Super Contributor
timestamp: February 10, 2025, 10:00:24 pm
content: OK this thread is well.... old, however it is still the best place for some additional related info.I just revived a laternonfan version of this model that uses a later version of the CPU PCB with only 3 rom component positions, not the usual 6 footprint one associated with the earlierfanmodel.So the picture shows the same mod effectively applied to anonfan revision using the same technique and rom image as the OP.Usually this mod has been only needed on the earlier 'fan' versions. Until now!I have also successfully used the OP mod 2 times on earlier fan versions.