Scoping the Murphy’s Law CoCo

by | Nov 22, 2025 | Hardware, Repair

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Let's scope this out

When working on the CoCo 3 I got from Brian Wieseler, I really needed to take some wave-form measurements. However, My Tektronix scope was just too damned big for my desktop at home. I’ve had my eyes on a smaller portable scope for a while, but couldn’t justify the purchase since I really had nothing to work on at the moment.

Brian doesn’t know how much he helped me justify this expenditure wink cool. With that said, I really discovered the problem with the CoCo 3 before the scope even arrived. So what to do? Well, I stll needed to find a working 63C09 to put in, and I did that during the CoCo Nation Show today. So before closing things up, I decided to test a few wave-forms from the CoCo 3 Service manual.

CVIDEO & HSYC – pg 64 of SM

My settings were a little off on this. Ch1 should’ve been .5v/div and Ch2 should’ve been 2v/div – time/div (or horiz on this scope) was set right at 10 usec. Being the first time using it, and the documentation basically sucks, I guess it’s acceptable. I definately like the screenshot feature.

Colorburst & HSYNC- pg 64 of SM

Learned a little about the scope settings with this one. 

I tried to get the RGB & HSYNC readings, but ran onto dificulties. I’m not sure if this scope isn’t sensitive enough or if the RGB circuit has issues, but the G & B signals really were unreadable.

E-CLK & Q-CLK – pg 66 of SM

E-CLK & RAS* – pg 66 of SM

E-CLK & CAS* – pg 66 of SM

Summary: The scope I ended up buying was the FNIRSI dpos350p – this isn’t an endorsement, just stating the device I used for this. It’s a decent scope for an entry level hobbiest, and has a number of attractive features:

  1. 350MHz
  2. 1 G/s sampling
  3. Signal generator
  4. Spectrum analyzer
  5. Screenshot

…and much more.

If you’re looking for a scope, this might fit your needs unless you’re an uber-engineer, then you’ll probably find this device lacking. For a $280 scope, it works for me, and takes up much less space on my desk than my ancient Tektronix.

At any rate, I hope these wave-forms help someone out in the future. The images in the service manual aren’t all that eye friendly. More importantly, I hope the wave-forms here are correct. They look right to me, and I still need to look at the RGB signals a little closer.

 

Happy CoCoing!