[Coco] Troubleshooting CoCo 3

[Salvador Garcia]

I jumpered the said pins and the CoCo 3 powered up; however, with a black display.
I tested it without RAM and with the Triad 512K installed. Same result.
How can I be sure that the 6809 is bad? You mentioned the blank green screen, but I am not getting even that.
If possible, I would like to know if the CPU is bad before I go off removing that chip.
Thanks! Salvador

      From: Rietveld Rietveld <rietveldh at hotmail.com>
 To: Salvador Garcia via Coco <coco at maltedmedia.com> 
 Sent: Saturday, April 29, 2017 3:20 PM
 Subject: Re: [Coco] Troubleshooting CoCo 3
   
Hi.  If u jump pins ‎3 to 5 and 4 to 6 where the power button sits on the board u will simulate a power on at the secondary side of the PS.
If u can carefully re-join the legs of caps 65 and 66, which are cut cleanly, and re-install the 128k of ram then the computer will power on to a blank green screen. I suspect a bad cpu since the screen stays green with two different sets of 128k ram or the 512k board installed.    The chances of all three memory sets being bad seems too far fetched.  ‎

,
  Original Message
From: Salvador Garcia via Coco
Sent: Saturday, April 29, 2017 1:40 PM
To: CoCoList for Color Computer Enthusiasts
Reply To: CoCoList for Color Computer Enthusiasts
Cc: Salvador Garcia‎
Subject: [Coco] Troubleshooting CoCo 3


Hi all,
I finally got to start troubleshooting the CoCo 3 that HRietveld gave me. He suspected the power supply, so I started by following the flowchart that's on page 58 of the CoCo 3 tech manual. Almost everything checked out ok:
5v at tp1-13 at IC8 pin 15
I was not able to check the 8vdc at the output of IC36 because I was not able to locate any ICx larger than 19.
The CoCo came with 128K, but caps 65 and 66 were snipped, so I assumed that the previous owner at some time had a Triad 512K RAM expansion or similar product that required these caps to be snipped.
For testing I pulled the 128K and left the CoCo 3 "RAMless".
What sort of screen image should I get if I power up a RAMless CoCo 3?
If I pull the keyboard Mylar, what would the image on the screen look like?
Currently I am getting a "black" screen on power up.
Thanks! Salvador
PS: Full disclosure. The only tool I have (besides screw drivers and a soldering iron)  is a multimeter..

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