[Coco] Weird CoCo3 problem...

[Gene Heskett]

On Tuesday 05 January 2010, Brian Blake wrote:
>Hi Gene,
>
>Unless I accidently pushed the switch back to 1x, the probe should have
> been on 10x. It's a 50MHz Tek scope - seen a few years.
>
And historically, teks attenuator pads attached to the 1-2-5 range switches 
have NOT been stable with time.  Seen this many times, and no amount of 
fiddling with the probe compensator will alleviate it.  Tek of course will 
laugh at you when you query them for parts that old.  I can remember when, 
back in the 60's, tek had a lifetime warranty on some of that stuff, and 
honored it, NQA.  Now its ebay material.

So far (knock on wood) my now 20 year old Hitachi v-1065 is still good.  The 
worst thing was that all the case screws fell out into my clothes while it 
was packed, surrounded by clothes, in my biggest suitcase and flown to Iron 
Mountain MI and back a couple of years ago.  I checked the innards for more 
loose screws, tightened 6 or 7, and shook all the screws out of my clothes & 
put them back in and its ready to go for another 100k miles or 20 years, 
whichever comes first. :)

Tek, for about 30 years now, has been riding their brand name like a horse, 
but QC is not a high priority.  Their new stuff is nice, and if it works, it 
works pretty well, till parts age out of spec.  If, out of the box, it 
doesn't work, there is effectively no warranty and none of their so-called 
field engineers is capable of fixing it.  We bought a 2235 scope in '84, had 
the poorest CRT I have ever seen in it, coma & out of focus all over the 
place.  Tek refused to replace that CRT even though I hassled them almost 
daily for months, and I eventually ordered another and replaced it myself.  
And now its input attenuators are crap too.  Out of 4 each tek 1440's (an 
automatic transmitter corrector) only one of them ever worked right.  I 
undertook to fix/restore one of them that never worked, and when I got to the 
5th production mistake after almost a week, a piece of conductive elastomer 
riveted into a transistor socket as it was being installed on the board, and 
that still didn't bring it up, I put it back in the rack and bypassed it.  
Today I'd buy from anybody but tek.  And I think I just did, an Instek 
GPS-810TG spectrum analyzer for $3275 & shipping, brand new.  It is not by 
any means a $50k Rhode & Schwarz, but it will do the job we need done.

I'd like to retire, I really would...

>As I told Mike earlier, my second probe has grown legs and walked away. It
> hit me at 2am the phase shift I didn't see was probably the result of not
> being able to view both signals at the same time.

MPJones (google for them ) has the ones I've been using for about a $50 bill 
each, decent probes IMO and much more reliable than the $150/copy tek stuff.

>The scope was set to what the SM said: TP 1, 2, 4 & 5 were all set to
> 2v/Div and Horizontal was 0.2-usec/Div.

Ok, thanks.  But that also says the probe was set at 1x, not 10x, in which 
case it should have been set to .2v/Div.

>For all of these checks I referenced the scans on the Maltedmedia site.
>
>Should also be noted that the waveforms for the power supply section ALL
> looked exactly like the ones provided in the SM.

No high frequency stuff there unless the boards bypass caps have all 
developed 100 ohms of ESR, each.  Not impossible at these things ages though.

Individual caps could be due, but the tool to find those is about $225 these 
days, called a "Capacitor Wizard".  Nothing else like it.  It measures ESR at 
100khz, a characteristic of paramount importance in digital circuitry.  Its 
the ONLY thing that allowed me to keep 2 dozen Panasonic dvc-pro recorders 
going most of the time the last 4 years I worked sorta full time.  But the 
coco is slow enough its not nearly as critical as that stuff with 100+ mhz 
clocking. ;)

[...]

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)

	"Do you think there's a God?"
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