On Wed, 20 May 2009 09:32:14 +0100, "Arfa Daily"
put finger to keyboard and composed:


"Franc Zabkar" wrote in message
news
On Tue, 19 May 2009 08:15:46 +0100, "N_Cook" put
finger to keyboard and composed:

Any tips for single ended signal injection and tracing for high resolution
tracing , floppy/HD head as searchcoil ?

I've repaired a lot of multilayer PCBs, but I've always had circuit
diagrams or a knowledge of the board.

What about a VCR head attached to a ballpoint pen tube? I would think
that as you rotated the pen, the max/min signal amplitudes would
indicate the direction of current flow.

You could also experiment with ribbon cable. That would give you an
indication of sensitivity to crosstalk, and the insulation thickness
could simulate a layer of PCB.

- Franc Zabkar

It's a thought Franc, but you would need either a damned strong signal
source (see my earlier thoughts about crosstalk in prallel track bunches),
or a very high gain and very low noise amp on the end of the video head. I
say this because from what I've seen of video heads over the years, they
have very few wire turns on them, relying for their output on the fact that
they are rotating at very high speed relative to the tape, and in extremely
tight physical contact with it.

Wouldn't it be easier in the end, to just use your DMM on its lowest ohms
range with a continuity beeper, to hook onto one end of a track where you do
know that it finds its way to the top or bottom surface, then go hunting for
the next place where it appears by initially listening for the beep, and
then checking for the near-zero ohms reading that you will be getting, if
you have found a direct connection ?

Either way, a very tedious process, and definitely not commercially viable
unless, as someone else commented, it is an expensive board for which there
is no service info available, and that someone wants to contract you to mend
a large number of.

Arfa

The 15" x 15" multilayer boards I used to repair typically had 300 TTL
chips.

ISTR that someone once suggested to place a sheet of aluminium foil
over one half of the board to test for continuity and then narrow down
the precise location with successive binary searches. Alternatively
you could make a chip-sized metal pad and probe an entire chip at a
time.

I agree that looking for a small signal may be fruitless. My first
thought was to adapt the method used in some digitizing tablets, but
these use a relatively high current pulse.

As you suggest, I do all my reverse engineering with a DMM on the low
ohms range. I might try the Al foil method next time, though.

- Franc Zabkar
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