[Coco] Troubleshooting CoCo 3

[Salvador Garcia]

Thanks. Looks like I am going to have to get an oscilloscope to test that. I haven't used one since my college days. Salvador



      From: Rob Rosenbrock <bester at adamswells.com>
 To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com> 
 Sent: Sunday, May 7, 2017 5:23 PM
 Subject: Re: [Coco] Troubleshooting CoCo 3
   
Sounds like there's no clock

Sent from my iPad

> On May 7, 2017, at 6:16 PM, Salvador Garcia via Coco <coco at maltedmedia.com> wrote:
> 
> I finally had time this weekend.
> 
> I checked the output of Q1 and it is a stable 5.11vdc
> 
> I also extracted and reseated the GIME chip.
> 
> Trying to determine whether it was just the video that's out or whether it is something deeper than that, I connected one of the audio cables (red/white) to the audio out, powered on and then blindly entered PLAY "A" <Enter>. No sound. Retried with the other audio cable, same results. Just to confirm, if the CoCo was operational I should have gotten a sound, right?
> I was thinking of getting a male RCA to female RF adapter to test the video out on the modulator, but at this time I am not so sure that it will yield any favorable results.
> Salvador
> 
>      From: camillus gmail <camillus.b.58 at gmail.com>
> To: Bill via Coco <coco at maltedmedia.com> 
> Sent: Saturday, April 29, 2017 11:14 PM
> Subject: Re: [Coco] Troubleshooting CoCo 3
> 
> 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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