Rom Board. Video Roms

Rom Board

The rom boards are probably the easiest of the boards in the stack to diagnose. Generally these issues will show up as bad roms on the rom check.

If the rom board won’t boot, its likely something related with IC1-IC25. Given that these ICs are basically directly connected to the board connectors (and each other) connection issues are quite easy to test. For IC1-IC16, with a continuity meter go through each of the addressing pins and data pins for one of the ICs. with one lead of the meter on the pin CAREFULLY and lightly drag the other lead of the meter along each side of connectors CN2 and CN3. Your looking for a beep to indicate its connected. Don’t worry about what it’s connected to in those connectors just thats its connected to something. I’ve come across 2 corroded vias on a board just under one of the connectors, so had to run a wire around to connect them. For IC17-IC25 do the same thing but for CN4. I’ll probably do something more definitive for these as I’m basing a lot of this part on memory.

Now if it boots but some of the roms test as bad...

If possible swap the reported bad roms from another board. Having said that I’ve not yet encountered a rom that was actually bad, but I’ve only been messing around with these things for about 9 months now.

Video Roms

So the key with working out what’s going on with the video roms (IC26-IC41) is mapping out the physical locations on the board so you know which ICs reading the rom is dependant on.

Here’s a general guide:

Bad ICs	What to check
26, 28, 30, 32, 34, 36, 38 and 40	Check the outputs of the ACT574’s (IC49 and IC50). There are some vias near jumper JP13 which are useful for doing this. If any are stuck or floating, replace the IC.
27, 29, 31, 33, 35, 37, 39 and 41	Check the outputs of the ACT574’s (IC51 and IC52). There are some vias near IC48 which are useful for doing this. If any are stuck or floating, replace the IC.
26 and 27	Check the addressing pins on one of the roms, if any are stuck or floating use continuity tester to identify which of the ACT574’s is faulty (IC53, IC54s, IC55)
28 and 29	Check the addressing pins on one of the roms, if any are stuck or floating use continuity tester to identify which of the ACT574’s is faulty (IC56, IC57s, IC58)
30 and 31	Check the addressing pins on one of the roms, if any are stuck or floating use continuity tester to identify which of the ACT574’s is faulty (IC59, IC60s, IC61)
32 and 33	Check the addressing pins on one of the roms, if any are stuck or floating use continuity tester to identify which of the ACT574’s is faulty (IC62, IC63s, IC64)
34 and 35	Check the addressing pins on one of the roms, if any are stuck or floating use continuity tester to identify which of the ACT574’s is faulty (IC65, IC66s, IC67)
36 and 37	Check the addressing pins on one of the roms, if any are stuck or floating use continuity tester to identify which of the ACT574’s is faulty (IC68, IC69s, IC70)
38 and 39	Check the addressing pins on one of the roms, if any are stuck or floating use continuity tester to identify which of the ACT574’s is faulty (IC71, IC72s, IC73)
40 and 41	Check the addressing pins on one of the roms, if any are stuck or floating use continuity tester to identify which of the ACT574’s is faulty (IC74, IC75s, IC76)
Single rom only	Likely to be one of the two ACT 244 ICs on the underside of the board beneigh the chip. It’s quite tricky to test the outputs as you’ll need to test only when the output enable is on. It’s probably just easier to replace both chips.

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