[Coco] Weird CoCo3 problem...

[Arthur Flexser]

This may be a little simple-minded compared to analyzing waveforms,
but have you checked the resistance (continuity) between something
that connects to the SCS pad of the disk controller edge connector and
something that connects to the same pad of the CoCo's cartridge
connector, in line with Mike's final paragraph?  That is, plug the
disk controller in with its cover removed to the CoCo with its cover
removed and check continuity along the SCS path (with power off).

Art

On Tue, Jan 5, 2010 at 8:10 PM, Brian Blake <random_rodder at yahoo.com> wrote:
> Okay, I spent some time cleaning the probe and other things this evening. I'm happy to say that made an improvement in the waveforms - all waveforms between the two systems are almost identical.
>
> The one are I have concern is the /SCS:
>
> On the CoCo3 in which the FD-502 does work on, that is in the neighborhood of 4.25v with the blips Mike was talking about.
>
> On the 'Bad CoCo' there is a constant 2.24vdc with no discernible blips.
>
> Any other areas to check in order to confirm this?
>
>
> ----- Original Message ----
> 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.
>
>> -----Original Message-----
>> From: coco-bounces at maltedmedia.com [mailto:coco-
>> bounces at maltedmedia.com] On Behalf Of Brian Blake
>> Sent: Monday, January 04, 2010 8:23 PM
>> To: CoCoList for Color Computer Enthusiasts
>> Subject: Re: [Coco] Weird CoCo3 problem...
>>
>> Okay guys, got the o'scope, schematic and CoCo3 service manual, along
>> with Frank's 'Tandy's Little Wonder' out for good measure and started
>> reading and testing. The results have been posted to my blog at the
>> following address so I could post pictures of the actual scope traces.
>>
>> http://cocom3repack.blogspot.com/2010/01/troubleshooting-perplexing-
>> problem.html
>>
>> I'm not sure if these will be much help or not, as the test results
>> are... interesting. I've got the exact same signal from the GIME to
>> places I wouldn't think they should be (i.e.: GIME output to TP2, 3, 4
>> & 5 appears to be identical, and NOT what the service manual says they
>> should be). I freely admit there are folks here who have forgotten more
>> than I know about the CoCo family, so I hope these images help.
>>
>>
>> Thanks guys,
>>
>> Brian
>>
>>
>
>
>
>
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