Presumably stuck low for TTL. The problem with doing this is that all (common?) open-collector ICs are multi-section, so if you pull a chip you disable other parts of the circuit too -- parts that might affect an IC that's still in place and which also drives the common wire-AND line. Yes, I have seen circuits where this would be a problem You can get some (very expensive) adaptors with a test pin for each IC pin and a tiny 3 position switch that does the following : (a) test pin, IC pin, and socket all connected; (b) test pin connected to IC pin, isolated from socket; (c) test pin connected to socket, isolated from IC pin. But last time I looked, I couldn't afford a full set of such adaptors. Actually, in my experience, hard power line shorts are often due to decoupling capacitors. The 'correct' tool for this is a current tracer, which will indicate current flow along the PCB tracks. In the second case you power the board the tracer (assuming there aren't internal power and ground planes!). In the first case you can actually see which branch of the common wire-AND line is taking the current. Without a current tracer you're pretty much reduced to cutting tracks/IC pins and then soldering wires (not just a blob of solder, please) to repair them. This is actually recomended in some service manuals. However, to return to your Persci drive, does it (a) draw excessive supply current, or (b) contain significant numbers of open-collector ICs other than those that drive the cable (and which can therefore be isolated from ohter ICs by pulling said cable)? If not, then there's no real problem in troubleshooting the digital side. -tony