[Coco] Troubleshooting CoCo 3

[camillus gmail]

Salvador, you tested the power supply, but did you test the 5V from the transistor ( the one with the big cooling ), you should have about +12 Volt in and +5 Volt out. If that is not the case then the SALT chip is failing. To test this you can temporary take out the transistor and take out also the connection to the transformer. Apply +5 Volt to the 2 Watt resistor next to the salt chip ( a big resistor ) at the side where it connects to the capacitor, underneath. The coco should boot now, regardless of the power switch, and you will see right away if the screens boots normal.
If something is not clear then pls feel free to send me a mail, 
camillus.b.58<@>gmail.com. Take out the <> for my real email.
 
I can offer you the service to take out the cpu and insert a socket. I can do this without killing the cpu. You do have to send me the board though. And I do this completely free, as I like to do this kind of repairs.

cb
On 4/29/2017 4:10:10 PM, Salvador Garcia via Coco <coco at maltedmedia.com> wrote:
Thanks Tom. Okay, that's a good idea. Salvador

From: Tom Seagrove
To: CoCoList for Color Computer Enthusiasts
Sent: Saturday, April 29, 2017 1:50 PM
Subject: Re: [Coco] Troubleshooting CoCo 3

Try reseating the GIME Chip? Cleaning its contacts? Just another thing to try...

Tom

On Apr 29, 2017, at 1:39 PM, Salvador Garcia via Coco wrote:

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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