[Coco] Weird CoCo3 problem...

[Bob Devries]

Brian,

after having checked out the schematic of the NTSC Coco3, I realised that 
the capacitors C65 and C66 should be removed if a 512k expansion board is 
installed. I believe also that R23 was affected, but if I remember 
correctly, it was only short circuited with a 1MB expansion (from Disto). 
These components are in the /CAS and /RAS part of the circuitry near the 
GIME.

--
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: Saturday, January 02, 2010 6:05 AM
Subject: [Coco] Weird CoCo3 problem...


>
>
> All of these new developments on the CoCo front finally lit the fire under 
> my arse to set up real hardware again. Besides my repack project, I have 3 
> other CoCo3's, two with 512k upgrades (one C9 and one PBJ) and one 128k. 
> The one with the C9 upgrade works fine - can load and play Sockmaster's 
> Donkey Kong.
>
> The one with the PBJ upgrade, not so... At boot, it comes up as it should 
> with the proper display. However, any floppy commands are met with no 
> response from the floppy drive (with an FD-502 controller) at all. Almost 
> like the drive is off. The drive responds as it should in the other two 
> machines. Upon removing the top case and k/b, I saw two wires, each had 
> been soldered to opposite sides of R23. Now, under the keyboard, is 
> another wire with alligator clips on each end, and the clips are holding 
> on to the bare end of the wires soldered to R23, effectively bypassing the 
> resistor. I'm looking for the PBJ instructions, pretty sure I have them, 
> to find out if this is called for. I can also see where C65 & C66 have 
> been cut and re-soldered. I have reseated the GIME and the PBJ memory 
> chips, and cleaned the 40 pin connector.
>
> Any other suggestions? My 100MHz o'scope bit the dust so I don't have that 
> tool any more (anybody have a CRT for a Leader 3 channel 100MHz scope??)
>
>
> Thanks,
>
> Brian
>
>
>
>
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