I have a couple of the SG503's, both older and newer versions. I'll stick them in a TM500 and TM5000 mainframe and take some measurements in the next few days.
I had one that had a faulty crystal which meant the display didn't work (The crystal only runs the display, the output is generated separately with analogue RLC networks etc). I replaced it with this part and all is well again.
Also, if your unit has Texas Instruments IC sockets, you need to replace them. They are notorious for going bad. I replace them on sight with good quality circular pin types.
Jared.
Jared Cabot