Anyone have/know of Persci drives, parts

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Jul 27 17:01:35 2004

> > socketed. This has not stopped me repairing it. Actually, I am not sure
> > what benefits socketed ICs provide for troubleshooting/repair --
> > desoldering and replacing the faulty IC is not a major job, and you
> > shouldn't need to remvoe ICs other than the faulty one (in other words
> > troubleshooting should not consist of replacing ICs until the thing
> > starts working).
>
> Say, for example, you have multiple open collector outputs connected to
> eachother, and for some reason the line is stuck at a certain level. Of

Presumably stuck low for TTL.

> course there are other possible ways, but I think it's quite handy to
> remove all connected ICs and find out which one keeps the line at said
> level.

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.

>
> Or for example, the PSU prevents the supply voltage from coming up
> because the circuit draws too much current. A quick inspection by
> removing all ICs and reinserting them one by one is a convinient way to
> find out where the problem lies.

Actually, in my experience, hard power line shorts are often due to
decoupling capacitors.

>
> If you know better ways than this I would really love to hear about it.

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
from a current limited supply and see where the current is flowing with
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
Received on Tue Jul 27 2004 - 17:01:35 BST

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