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Happy 2024 to all learned HP experts, I am humbly asking for any advice you may be able to give.

i acquired a 3478A DMM recently - it is in good mechanical condition and was relatively clean and nothing obviously missing.
It was completely dead - no display .

Only the occasional brief flicker of a few segments on power on/off shows any life in the display.

The LCD voltage divider chain is good and the actual display was exchanged with another same display part from a 3488A (useful to know)...

Checking power rails showed they are all good and the internal battery is just over 3V, also good.
There is clock activity and the processor seems to be active but halted.
I have a download of the manual and fortunately a 5006A signature analyser which is needed for diagnostics on these meters.
Following the instructions/guidance all the codes appeared correct until the EPROM tests which appears to be suspect.
Presumably prom code rot is the culprit
The original device is a mask programmed Mostek chip and was soldered in - I removed it and fitted a socket for future ease.
I’d like to read the contents out if I can.

This is the current state and here comes the questions...?

Reading one of the repair forum pages suggests it can be replaced with a standard eraseable UV EPROM - does anyone know which type?
i have a Dataman S4 chip reader/programmer but unsure of if it is capable of reading the old Mostek device as there are no parameters listed
which describe the chip type and I am reluctant to just try to read it by force - fearing damage.

This is mainly just to see if any stored code is viable or damaged.
The real question is what to fit as a replacement - I am thinking a 2732 would be ideal and have downloaded the Rom dump from a forum page.

But I am looking for any further advice on what device to use and it’s compatibility...?

I fully expect more problems in the analogue circuitry but for now I just want to get it to come to life.
it looks like a good meter and mentioned by many people - so I am keen to try to fix it if possible,

Any help/advice is welcomed.
Regards?
Nigel?

> There is clock activity and the processor seems to be active but halted.

To state the obvious - have you scoped the supply rails and the Reset signal to make sure there's no intermittent fault there ? The firmware does a kind of external watchdog thing where it pings a reset IC periodically. If that process fails it would fail to run (can't remember if it would even display anything).

> signature analysis

With at least my ROM (same as the "dc118.bin" you probably downloaded) the signatures do not match the service manual.

> Reading one of the repair forum pages suggests it can be replaced with a standard eraseable UV EPROM - does anyone know which type?

Pretty much anything with a 5V tolerant, 2764 pinout will do. You mentioned 2732, but the firmware dump is definitely 8kB. Unless your machine is an oddball with 4kB ? I don't know how they would have pulled that off while keeping the functionality...

I put in an AT28C64 (modern EEPROM, way more convenient than UVEPROM) to develop my firmware patches. Since you already have a programmer, just make sure to get something it can handle...

Good luck !

Some of the older mask ROMS do not have EPROM (pin compatible) equivalents. E.g. I found 1 kByte mask ROMs in my HP5342A counter that have 3 chip select pins which (to complicate matters) can be made active high or active low depending on the actual mask contents.

So, I think you will have to check carefully both pinouts as well as chip selects. There are conversion PCBs out there, courtesy of arcade game restores, that allow an EPROM to be fitted in modules designed for mask ROMs.

Wilko

Couldn’t the chip select lines be connected to address lines instead for the extras?

is it possible by being labeled as chip selects it discouraged people trying to source a generic part?