C-Lab MK2 CA401332-002-REVC

[Steve]

Model:
CA401332-002-REVC
Y44902000604

Interesting SDMA bypass on this board.
Uses clock patch varient 1.2
- https://mikrosk.github.io/clockpatch/

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.

Under SCSI board -

• 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

Underside of motherboard -

• Pin 8 of U19 AM26LS31PC chip patched to pin 2 of J12 speaker connector

Note:
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:

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

- But it seems weird that a later produced Falcon motherboard such as this would require factory patches...

Extra note:
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: https://www.exxosforum.co.uk/forum/viewt ... =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 ;)

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[exxos]

Interesting SDMA bypass on this board.

What did they do ? looks like the track was cut and then jumped over ?

[Steve]

@exxos I don't quite understand it either, yeah the track is cut and then bridged further up.. not sure why. I don't think theres anything significant in between like a via or anything... I'll take a look underneath the board tomorrow and take photos.

My guess is perhaps Clab were playing with bypassing the motherboard sdma track entirely, cut the track, but then reversed the mod.. I don't know. The client tells me it has never had any work since it was new, so this is all factory stuff.

[Steve]

Also interestingly on the clock patch R216 which provides the FPU clock has had its resistor removed, but it hasn't been connected to the clock patch. So I'm not sure if the FPU would function like this, but there might be a patch on the rear, will find out later.

EDIT: Sorry mistake, R221 is for FPU and is connected. R216 for some reason has been cut on the board and they connected the expansion bus clock by the VIA instead.

[exxos]

My guess is perhaps Clab were playing with bypassing the motherboard sdma track entirely, cut the track, but then reversed the mod.. I don't know. The client tells me it has never had any work since it was new, so this is all factory stuff.

Possibly yeah. I've never looked into what C-lab did mod wise.

Also interestingly on the clock patch R216 which provides the FPU clock has had its resistor removed, but it hasn't been connected to the clock patch. So I'm not sure if the FPU would function like this, but there might be a patch on the rear, will find out later.

FPU will die without a clock :) Sounds like a bit of a bodge job so far.

[mikro]

This PCB is interesting in another way, too - it's a MK2, it's from the 09/1994 batch but still, the production number is Atari's. I would say there's a good chance that sometime during this month Falcon PCBs switched hands and started to put new production numbers. I have seen 09/1994 models with both Atari and C-Lab production numbers.

[Steve]

Just adding some photos of the rear and close ups of the rear audio section. Clab have replaced quite a few SMD caps and I also notice a 22uf tantalum cap that isn't usually there.

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[Steve]

@mikro I wrote down some details at the top of the post on my observations. Since you're a Falcon historical expert having sparrows and all I thought I'd bring it to your attention. I wonder if some of the mods under the scsi board are required specifically for the internal scsi or whether they're for something else. I really don't know why the patch wire on U19 was done either. I need to have a look at the pin-out of that chip to find out.

[mikro]

Thanks Steve, any Falcon investigation is always an interesting job. :)

Too bad C-Lab as a company has disappeared, it would be great to be able to ask the (then) CEO a few questions...

[thtna71]

R216 was removed on my C-lab PCB also. See my post from sept 2020.

Also interestingly on the clock patch R216 which provides the FPU clock has had its resistor removed, but it hasn't been connected to the clock patch. So I'm not sure if the FPU would function like this, but there might be a patch on the rear, will find out later.

EDIT: Sorry mistake, R221 is for FPU and is connected. R216 for some reason has been cut on the board and they connected the expansion bus clock by the VIA instead.