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

[John Robertson]

On 2019/03/18 11:39 p.m., David Shoemaker wrote:
> Have had this item for years and finally figured out what it is good for.
>
>
>
> 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
>

I found a device years ago called the Global Specialties 'Short Squeak' 
(manual in the FTP archives - see link below) which produces an audio 
tone that is inversely related to the resistance of the circuit. This is 
great for finding shorts or low resistance loading - mostly under around 
around 2 ohms. It looks like a regular probe but has a separate power 
supply wall wart. You hook the common lead to your common point (often 
ground, but can be Vcc, etc.) and then run the probe tip along various 
circuits listening to the pitch change, the pitch gets higher as the 
resistance lowers between the probe tip and the probe common.

ftp://ttl.arcadetech.org/TTL/Test_Equipment/Global%20Specialities/GlobalSpecialities_ShortSqueek_Model_SQ-1.pdf

I should make a video of the thing in action I guess, the HP Current 
Probe video referenced above, is not bad but I think the Short Squeak 
could be faster...should take it apart and copy out the circuit.

John :-#)#

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