Defective MST board

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Captain Atari
Captain Atari
Posts: 472
Joined: Thu Nov 01, 2007 10:01 am

Defective MST board

Postby Arne » Fri Jun 01, 2018 5:26 am


I've got some MegaST boards for repair and one drives me crazy.
It's got additional sockets for CPU and PSG.
Blitter removed, both solder-bridges closed.
I checked the whole 4-IC chipset in a 1040STFM board. Works flawlessly.

I can see two problems that might be connected to each other somehow.
- /BERR is oscillating with ~60kHz
- /IPL2 is constant low

I removed the CPU from its socket and the symptoms are still the same.
So /BERR is not affected be the CPU. AFAIK /BERR can only be asserted by
CPU, GLUE, Blitter and MegaBus device. I've just got GLUE populated.
Originally /IPL2 is already low at GLUE, routed through the 74LS148 the 68000
sees /IPL2 low, too.

If I remove GLUE /IPL2 is high i.e. there is no short-circuit between /IPL2 and GND.

AFAIK a IPLevel of 4 equals a VBI so I think GLUE works correctly just some input(s)
correctly trigger its /IPL2 line.
/MFPINT is high and /VPA is high, too. So no auto-vector interrupt.

Any idea where to look further?

thanks, Arne

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