[Techtoolslist] Used HP 547A to find a pin to trace short

[David Shoemaker]

Have had this item for years and finally figured out what it is good for.  

 

Repair Log: Shadow Dancer

Sega System 18 PCB

Problem 1: Bad sprites in player and other screen objects

Reseated Rom board: No change

Checked all roms: Matched mame

Probed around with logic probe and didn't see anything odd.  Saw a clutch
hitter on ebay for cheap so picked it up to do some comparisons with as I
didn't have any System 18 in my stocks.

Tried rom board on CH board. No sprite problems.  Went back to original
board and still had corruption.  

Started looking at the board closer, and found a nick in the traces.  jumped
4 traces with Kynar wire.  Problem solved

 

Problem 2: No sound

No sounds at all, but the CPU seemed to be running.  Sounds fine if putting
the rom board on the CH main.  Verified the CPU to be good and the Sound PAL
was also good.

Started probing around the 74LS244 & 245 and found pin 5 on IC276 ('244) was
looking bad.  Grabbed my HP Logic comparator and clipped it on.  Hit test
and Pin 5 bad.

 

As there are no schematics for the System 18 I did some continuity checking
to try to figure out where that pin goes, found it goes to pin 2 on the
sound PAL.  And up to the interconnect for the Rom board.

 

But due to a slipped probe I noticed something else.  I was also getting
continuity at GND (about 175ohm).  And at the +5 rail (3 ohms).  

Bad 244, shorted internally so I clipped pin 5 and checked continuity again
for GND and +5.  Same thing.  BUGGER.

With the trace damage I had seen I thought there was probably a pin touching
a trace somewhere.  But after more than an hour with a tiny screwdriver and
a magnifier I wasn't any better.

 

Did some YouTube research on tracking down shorts.  Really didn't like the
idea of dropping a few amps down the +5 rail grounded through the pal pin to
try and find something warm.  I don't have a milliohm meter and they run a
couple hundred $.  I do have an ESR meter which is a small resistance meter
but it wasn't high enough resolution to find it by probing around the board.

Found a video on the HP 547A current tracer and using it to track down
shorts: https://www.youtube.com/watch?v=Verow5aGL0w

I just so happen to have one of these I bought off ebay years ago thinking
it might be useful in tracking down vector board problems.  But I have never
taken it out of the box to date.

Ran my logic pulsar output into the +5 rail with the ground of it tied to
the sound pal pin 2.  Then I was able to adjust the light on the 547A to
just showing when touching that pin.  Started tracing around the board and
picked up the signal near the connector to the rom board.  Ran along that
area until I found a trace it was running up and followed it a couple inches
then BOOM no more signal.  There was a chip pin leg right the spot it
vanished.  Used my little screw driver to lift the pin and no more signal on
the trace I was following and nothing at the pal pin 2.

Put the rom board back on and sound fixed.

 

Total cost of repair:  < $.50 in kynar wire (not counting the Clutch Hitter
PCB which I will keep as a ref board for the future)

Total time spent: about 4 hours