[Coco] Weird CoCo3 problem...

[Brian Blake]

Hi Louis,

Yes, the scope does have a cal signal and it looks fine. As I stated in a previous message, if you look at pages 67 & 68 of the CoCo3 service manual, the power supply waveforms produced by this CoCo3 are identical to those shown in the service manual. I don't think it's a scope issue. 

I'll run some comparison with a known good CoCo3 this evening. 

________________________________
From: Louis Ciotti <lciotti1 at gmail.com>
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Sent: Tue, January 5, 2010 8:40:01 AM
Subject: Re: [Coco] Weird CoCo3 problem...

A Logic Analyzer would be much more suited to observe timing, but you
have to use what you got on hand.  I agree with others that something
does not look right with the images. Does the scope have a calibration
signal you can connect the probe to to verify that is not the problem?

On Tue, Jan 5, 2010 at 8:16 AM, Brian Blake <random_rodder at yahoo.com> wrote:
>
>
> Hi Mike,
>
> 1. Like the picture in the CoCo3 SM.
>
> 2. I thought of this about 2am this morning. Unfortunately, my second probe has grow a set of legs and walked away. Gotta order a new one.
>
> 3. Will do this tonight.
>
> 4. Will look at this tonight as well.
>
>
>
> ________________________________
> From: Mike Pepe <lamune at doki-doki.net>
> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> Sent: Tue, January 5, 2010 12:04:24 AM
> Subject: Re: [Coco] Weird CoCo3 problem...
>
> Brian, looks good. I'd fire up my scope and show you some traces but it's currently sitting in pieces waiting for some parts. But, a couple of things to check:
>
> 1: what's your AC ripple look like on the 5V supply line?
>
> 2: You won't see the phase shift of the E and Q clock unless you're viewing both at the same time. The overshoot and ring apparent on the signals looks pretty bad but it could simply be the result of uncompensated probes.
>
> 3: run your poke loop and see what the SCS on the cartridge port looks like. It should be >2.5V most of the time with short blips <0.8V
>
> 4: if that looks normal, follow the schematic of the disk controller and see what the same signal looks like at pin 11 of the LS273 latch chip.
>
> I'm thinking now that it could very well be possible that the cartridge connector is not completely contacting the pad on the disk controller edge connector and the SCS isn't getting into the cart.
>