OBERHEIM SYNTH

Hi Martin, thanks for your suggestions.

The reason I suspect the EPROM is because of experience with a Textronix
468 digital scope. It was suggested that I replace the EPROMs. I was
sceptical, but did it anyway and it worked!
Nor sure if reading and reprogramming the chips would work. If the OB-Xa
can't read them would the EPROM reader/programmer be any different?
Like you say it would show a bad eprom, then I would need new bin files.
I checked all address and data lines with the scope. They all look ok.
I replaced the UPD780 CPU and 6116 CMOS ram - no change.
I replace the two 2114 RAM chips and there was a difference: there was
no flickering, some random LEDs were on, some buttons had an effect,
some not. Still no sound.

I've posted two photos of the board to the Photos section

Doug




     Posted by: "Martin Ator" cyllall@... cyllall
     Date: Wed Nov 28, 2012 7:31 am ((PST))

It's probably something to do with the Midi mod so have a look around 
for anything suspicious, wires/broken tracks etc.


It's very rare that you get problems with an Eprom, they are very tough. 
If you can read them with a programmer then UV erase them, then 
re-program them any problems will show up. Having said all this however, 
static is an enemy of us all. It depends how safe you are in this 
respect as to what possibilities may arise during troubleshooting.

It is much more likely that another IC is damaged/has been damaged by 
static or other reason/age etc, the main cause possibly being problems 
created during the installation of the Midi mod. EEproms (Eproms that 
aren't ceramic with a little erase window) can develop problems over 
time as they are erased/programmed with much lower voltages.I would not 
expect an OB to have these types installed.

I don't have any OB series synths. If there is a back-up memory system 
involved, then a big candidate for suspicion are the memory chips 
involved as any hint of static while checking the battery or replacing 
it is liable to damage the memory chips. I would look here first for 
problems if it was me.