Repair or Replace? [Was: Repairing Timex] - test-drb@ccmp.vtda.org

18 Jul 2000

...
  Basically they will tell you if a given point is
'high' 
 (above a certain 
 threshold voltage), 'low' (below a different threshold voltage), 
 illegal/floating (between the 2 thresholds). They also indicate if a 
 point is changing state ('clocking'), or if there's a narrow 
 pulse on a particular point. 
Right, so I need to know which pins of various chips do what then. Dammit -
that's more info than I have time to find ATM. Pity there isn't a 'Logic
Probing for Dummies' book :o) Once the museum room's finished and I get a
proper work area I can maybe kick back and start getting down to that sort
of level.

...
  The cheaper ones have fixed thresholds for TTL chips
(which 
 is all you 
 really need) and maybe 4000-series CMOS. More advanced (and 
 expensive!)  
Maplins appear to do 2, one just over a tenner (ukp10) and the other at
ukp17.

...
  Now, as to how it's used. You can use it to find a
stuck (or 
 not driven 
 at all) line on a data or address bus. To check if buffer 
 chips are being 
 enabled. To see if the output of a gate is stuck high or low, 
 and what 
 the inputs are doing. To see if a CPU is getting a clock signal, etc. 
Definitely too much info I need to learn for now I think....

...
  If you want to preserve both the board and the chip,
then 
 first suck off 
 the solder with a temperature-controlled iron (you should 
 really be using 
 one of these for all work...) and a solder sucker. If a hole  
My gas one is temperature controlled, plus our erstwhile engineer down in
head office is retiring, so I might be able to buy some of their test kit
off them since he was the last one who could do those sorts of repairs. An
entire electronic workshop will be vacant!