Interesting SDMA bypass on this board.
Uses clock patch varient 1.2
Notes on C-Lab work:
Topside of motherboard -
- 222K Ceramic capacitor soldered between negative voltage legs of both C121 & C141
- 222K Ceramic capacitor soldered between negative leg of C92 & pin 1 of P5 SIL resistor
- Noise/signal filtering on mic/headphone outputs with 2 x 102K Ceramic caps on the GND and positive outputs, fed through a ferrite ring on both connectors.
- 222K Ceramic Cap soldered between middle base leg of 78L09 transistor and pin 36 of U7 Logic L53C80JC SCSI controller.
- 2N3906 transistor collector leg desoldered, lifted & patched to pin 26 of the floppy controller U20 Atari AJAX chip C302096-001A.
- SCSI drive voltage cable soldered to anode leg of D6 diode
- Pin 8 of U19 AM26LS31PC chip patched to pin 2 of J12 speaker connector
Other notable differences is that all the PAL circuitry has been retrofitted by Clab on the front and rear. I can tell as it has all been hand soldered around the modulator area and the front section near the fan. I am thinking that Atari had excess NTSC inventory and this required Clab to change these parts for the EU region.
Possible explanation for the mod wire on transistor / ajax:
- But it seems weird that a later produced Falcon motherboard such as this would require factory patches...Dal » Thu Oct 15, 2015 12:15 am
The red jumper wires look like factory fixes to me.
In your 3rd picture, the red wire is going from the ground pin of the transistor to the ground pin on U20 (AJAX chip). So I'm guessing the PCB was missing the ground line to Pin26 on U20.
I would wager the red wire in your first pic is also a factory fix too. It appears to be connecting pin 8 of U19 to Pin23 of the TOS ROM. According to the data sheet, this is ground also. So looks like a trace was missing during PCB fabrication
I have a theory that this board was transplanted from an C-Lab MK-X into a regular C-Lab Falcon MKII Shell. As the ferrite rings are not present on the following MKII uploaded by derkom: viewtopic.php?f=59&t=2142 but seem to be present on MK-X Falcons.
I also found an Atari forum post with photos of MK-X internals that look identical & interestingly they have also performed a SCSI PCB patch/fix like I have performed on the board, so this must have been a known problem. The rear base shielding of this Falcon isn't completelty straight and you can tell it has been handled/bent in places. Giving me more reason to believe it was transplanted from an MK-X. The underside of the headphone / microphone connectors also have a very large blob of solder with remnants of wire still stuck inside, as if cables that went to the MK-X RCA jacks were chopped off rather than de-soldered. I feel like a vintage Falcon detective now