[sdiy] OB-8 and..my chip checker..

[Bob Weigel]

Has anyone else had dealings with digital chip checkers?  I have one 
made by UEI...a PC-86 "Handy Tester".  It seemed to work great the first 
several times I used it then..the incident where it started telling me 
all the 74LS04's I think they were were bad in my OB-8....and as I 
replaced them with new GOOD measuring chips...functions degraded until I 
replaced the clock buffer which also measured 'bad' and..the whole 
machine wouldn't boot!  I scraped the 'bad' chips back out of the trash 
and reinserted them.  Powered up and it's worked with the relatively 
minor problems I started with ever since...which I just hadn't had time 
to sort out. 
      Last night I saw an OBX-a that needed repair. (I wasn't willing to 
bid a massive amount since I just don't need the trouble right now and 
it went for close to what I got my OB-8 for and probably in need of less 
work but..oh well. $455 seems just too cheap.  Crazy economy.  I also 
bid 1125 on a P5 that went for 1124!   Man..did everyone run out of coin 
all the sudden??  It's been a long time since I lost two in a row but my 
bids were prrrettty loooowwww.)   I decided to use it as motivation to 
get the OB-8 up again as it had dropped half the voices for some 
reason.  I trace the problem to a lack of response on the CV lines for 
that board.  They are all common to an op amp and...a 4051 chip.  I had 
already run the 4051's through my handy dandy (oops...they didn't say 
dandy did they? Hmm..maybe that's the problem!) tester.  All were "Good" 
according to it.  So..I swapped the op amp since outputs were at 10.some 
volts which ...might happen though another volt or two would be more 
likely.  But it made no difference as you might have guessed.  So....I 
put in a Harris 4051 in place of the Toshiba TC4051BP I had put in there 
previously when I had a chip that did measure bad I believe at first and 
it fixed a bad voice.  The Harris chip fixes it.  All voices work 
again.  Hmm.
     So..my question....does anyone know what a chip checker like this 
actually checks?  I'm sure it's probably a pic based thing but I wonder 
is the device having problems or are these things just 'iffy' by nature?  
     And also, has anyone else had trouble with Toshiba 4051's in this 
kind of application?  I used them also on the pot board and I've had 
strange sluggish/erratic response from some controls and I'm about to 
delve into that and try to finish this thing up finally.  -Bob