Hello,
I successfully repair my HP3456A with information and contact from this newsgroup. It's my duty to help others.
You will find a lot of useful information on BAMA (htttp://bama.sbc.edu)operation and service manual.
The HP3456A has a flaw on the ROM chip, the Mostek MK36*** dies prematurely. The 3 chip are A3U5,A3U7,A3U8. You can replace the 3 ROM with old Motorola EPROM MC68766. This part is a direct (pin to pin) replacement. But you need to program them first. If your Mostek are dead you can't retrieve the program from them. You can buy another voltmeter and read the chip, or find the 3 program on internet, what i did.
I was lucky and find Pierre-francois Monet who repair a HP3456 a few years ago and he open his voltmeter just to send me the 3 files.
I'm ready to share those 3 files (bin or hex)with the community. Send me a note on this address snipped-for-privacy@spam.imapmail.org(remove spam).
Those Mostek where use on several other HP equipment, i have files for the HP3456A only.
If you read this on the archive and the address is not working you can find me in the address book of ecole polytechnique of montreal.
Maxime Thibault electrical technician
P.S. this is not a easy task and require a some equipment and skill. If you don't have access to a universal programmer, a signature analyzer or already remove chip from a Print Circuit board this is not a job for you, find help. If you are close to Montreal Canada contact me.