My latest Ebay purchase

[keli]

This morning, this Mega 4 arrived in the mail.

IMG_20170922_114402.jpg

I haven't had a chance to hook it up yet (have to go to the basement an scour for cables, etc.), but the seller said the floppy drive is not working. (Floppy LED lights up but it does not read any floppies.) I have a Gotek in my old 1040 STF, so I guess I'll be swapping some hardware components around.

I like the Mega ST form factor, as it fits nicely on my desk when not in use. The Mega extension bus is also exciting -- I see potential for playing around with extensions... The internal DMA port connector could also come handy.

I opened it up, and in addition to collected dust, I noticed this thing soldered on top of the CPU:

IMG_20170922_115207.jpg

Any ideas what it's for? The chip is a 74HTCLS74AN, which is a pair of d-type positive edge triggered flip-flops with asynchronous preset and clear inputs. It looks like it's soldered directly to pins 12-18 on the 68000 (if I've figured it out correctly), which means it's hooked to /BGACK, /BR, Vcc, CLK, GND, /HALT, and /RESET. Plus the yellow wire going to somewhere on the motherboard. Is this some form of overclocking or maybe a repair of some sort?

[keli]

I found it. It looks like a fix to synchronize the /BGACK coming from the blitter to the CPU clock.

[exxos]

It is the blitter patch, you probably do not need it.

http://www.exxosforum.co.uk/atari/last/MEGAST/index.htm

It was just another Atari bodge to cure noise on signals which were causing the letter to malfunction on some production runs.

[keli]

It also could explain what seems to be cut traces on the motherboard next to it.

IMG_20170922_115628.jpg

[exxos]

Looks like the solder pads for the blitter.