Stanford Research Systems SR650

[Magiciaen]

Hi all,

Earlier this year I scored a broken Stanford Research Systems SR650 bandpass filter for $100 shipped. I rebuilt the whole thing, a blown tantalum cap had caused a cascade failure in the instrument. I need two of these for my experiments, so still on the lookout for one more.

I cleaned the switches during the repair, and although the unit works great, the switch contacts bounce like crazy. I don't have a schematic of the instrument, but I do have one of the SR560, which is fairly similar in design. There is no hardware debouncing of the switches at all, and so I assume it's done in software. The switches are organised as rows and columns for demultiplexing, and the columns are tied to a NOR gate which signals an interrupt on the Z80 microprocessor if any switch is pressed.

My SR650 seems to be quite an early one, serial number 3036. If there is any software debouncing at all, it doesn't seem to be very good. I was thinking of modifying the hardware to add a physical debouncing circuit to the column lines before they reach the NOR gate, but thought I'd ask here if anyone knows if there was a firmware update, or even better, if anyone has the firmware from a recently produced unit. My hope being it was a firmware issue they eventually fixed. The new SR650 I used in my lab at university definitely didn't have this problem, but I no longer have access to it.