[Coco] Weird CoCo3 problem - AAAARRRGGGGGHHHHH!!!!

[Bob Devries]

Brian,

Pin 36 (/SCS) connects to pin 9 of the 74LS138 (U9). You should check that 
pin also. I'm assuming you have the Coco3 schematic?
Pins 4,5 and 8 of U9 should be grounded. Pin 6 should show logic 1. Pins 1, 
2 & 3 should be in constant change as various outputs are selected, and 16 
is Vcc +5volts.

Only pins 9, 13, 14 and 15 on U9 are used as outputs.

HTH
Regards, Bob Devries
Goulburn, NSW, Australia

--
Besides a mathematical inclination, an exceptionally good mastery of one's 
native tongue is the most vital asset of a competent programmer.

Edsger W.Dijkstra, 18 June 1975

----- Original Message ----- 
From: "Brian Blake" <random_rodder at yahoo.com>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Monday, January 04, 2010 8:35 AM
Subject: Re: [Coco] Weird CoCo3 problem - AAAARRRGGGGGHHHHH!!!!


> The latest;
>
> Upon further testing, R22 was not doing anything. The solder pad on the 
> bottom of the mobo was broke, an the leg on the end that's supposed to 
> connect to C65 had been lifted. I took R22 completely out and replaced it 
> with a new 120-ohm resistor. Then I took a short piece (3/16 - 1/4") of 
> speaker wire and soldered it to the leg of R22 that is supposed to go to 
> pin 4 of the GIME, scraped away some green stuff and soldered the other 
> end directly to the trace. Continuity restored. Continuity from R23 was 
> fine. Cleaned up the solder pads from where I clipped out the 6809, 
> soldered in the socket, installed the 6309 and put everything back 
> together. Turn her on and was greeted with the 'Extended Color 
> Basic....yada, yada' greeting, okay that worked. Tried Tetris, S/S Pak and 
> O-90, all worked as expected. Still have the same problem with the FD-502; 
> POKE &HFF40,1 does not turn on the drive light, and any disk commands end 
> up with an I/O? error with no activity
> being present on the drives when the commands are issued... 
> AAAARRRGGGGGHHHHH!!!!
>
> I went to R/S today only to find out they no longer carry a logic probe, 
> so I'm stuck with using my DMM for now. When issuing a POKE &HFF40,1 in a 
> loop, IC9 pin 6 is 'HI' on the DMM in logic mode, but, in frequency mode 
> it shows up as a 192.5-kHz signal. Pin 36 of the 40 pin connector shows 
> the same results. Other than evidently ruining a perfectly good 6809, so 
> far the only thing I've accomplished is completing a 6309 swap on a CoCo3 
> with 'floppy phobia'... So, I'm still open for suggestions. Tomorrow I'll 
> be bringing home my 50-MHz scope so I'll at least be able to look at 
> waveforms. At this point,based on my limited knowledge and materials I'm 
> looking at, I'm leaning towards replacing IC9 to see if that helps, but, 
> I'd rather not start chip swapping just for the heck of it, so PLEASE, if 
> you guys who are far more knowledgeable than I have any suggestions, I'm 
> listening!!!
>
>
> Thanks,
>
> Brian
>
>
>
>
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