HP no longer supports the 3456A and parts are no longer available.

The MK36XXX ROMS in this meter have been known to die of old age.
This used to be a common pinout. Equivalents are Signetics 2664,
AMI S68364, Harris HM6388, NS DM87S228, NS DM77S228. These ROMS
can be read on an EPROM programmer as a motorola MCM68766C35 aka
MC68766, which is a pin compatible EPROM used for prototyping.

To replace these ROMS with ordinary 2764 EPROMS you will need to
make a socket adapter with the following pin connection changes:
   EPROM-Vpp-pin-1 connects to +5V
   EPROM-PGM-pin-27 connects to +5V
   EPROM-/CE-pin-20 connects to GND
   EPROM-/OE-pin-22 connects to ROM-/CE-pin-20

         2764
    +----._.----+                  MK36xxx
 1 -|Vpp     Vcc|- 28               2364
 2 -|A12    /pgm|- 27          +----._.----+
 3 -|A7       nc|- 26       A7 | 1       24| Vcc
 4 -|A6       A8|- 25       A6 | 2       23| A8 
 5 -|A5       A9|- 24       A5 | 3       22| A9 
 6 -|A4      A11|- 23       A4 | 4       21| A12
 7 -|A3      /OE|- 22       A3 | 5       20| /CS
 8 -|A2      A10|- 21       A2 | 6       19| A10
 9 -|A1      /CE|- 20       A1 | 7       18| A11
10 -|A0       D7|- 19       A0 | 8       17| D7 
11 -|D0       D6|- 18       D0 | 9       16| D6 
12 -|D1       D5|- 17       D1 |10       15| D5 
13 -|D2       D4|- 16       D2 |11       14| D4 
14 -|Vss      D3|- 15      Vss |12       13| D3 
    +-----------+              +-----------+

        2564
     +----o----+
 Vpp | 1     28| Vcc   Pin 26 and 28 are connected
/CS1 | 2     27| /CS2
  A7 | 3     26| Vcc   internally only one is needed
  A6 | 4     25| A8
  A5 | 5     24| A9
  A4 | 6     23| A12
  A3 | 7     22| PD/PGM
  A2 | 8     21| A10
  A1 | 9     20| A11
  A0 |10     19| Q8
  Q1 |11     18| Q7
  Q2 |12     17| Q6
  Q3 |13     16| Q5
 Vss |14     15| Q4
     +---------+

Meters with serial numbers HIGHER-THAN 2015A03070
contain a NEW revision of the ROMS with part number:
   U5=1818-1629  U7=1818-1630  U8=1818-1631

The meter contains three 8Kx8 ROMS. These ROMS can
be read on an EPROM programmer as an MC68766 part,
as long as the programer strobes CS or OE when reading
each consecutive address. Most will do that.

They can be directly replaced with pin-compatible EPROM
MC68766 or MCM68766C35 but these are obsolete though
easily available as surplus parts for about 10 dollars
each. The SCM90448C might also be a direct replacement.

Modern EPROM 27HC641 aka M27HC641 can be used and is pin
compatible, as long as you cook the data before burning,
because A10 and A12 (if I remember correctly) are swapped.

Three of EPROM 2764 can be used but a socket adapter
needs to be made to compensate for a few differences
in pinout. This is very very easy using wire-wrap sockets.

The data from the three ROMS can also be packed into a
single EPROM 27256 but a socket adapter must be made,
and a couple of address lines must be tied to the EPROM.

The data can also be split among 6 of EPROM 2732A if
you solder in the missing sockets and change around the
jumpering feeding the chip select decoding logic.

...Stepan