Upgraded 1.04 Mega 4 shows two bombs on boot

[Retronaut]

Hi, I bought a Mega 4 from Germany some months back, and no surprise, but it had TOS 1.02 German language installed. As an English speaker, this was not good, so I decided to upgrade the machine to TOS 1.04 English. So, I have followed the online guides and I don't think I have missed anything.
but...

When booted, it shows a grey/white screen, and access is made to the floppy drive. The keyboard is plugged in and shows power. After about 3 seconds or so, the boot screen shows two bombs. This repeats if you hard power the machine, OR reset it.

In terms of the TOS replacement. I have installed 4 more sockets, and burnt TOS 1.04, and installed the ROMS. The 3 jumpers are set as they should be for six ROM mode. I believe that is it, in terms of hardware changes?

ONE oddity, in some guides I have seen it stated that the 4 spare ROM sockets need to have capacitors installed to their right hand side. Now, I have this revision of board... (NOTE: This is NOT my board)


And as far as I can see, it has the 4 caps installed from factory. Oddly in the place of one of the caps on an earlier revision board is L52 (an inductor?), maybe this was added because of issues with power in this area? To the LEFT of the same socket though, is C11, the Capacitor for it I assume.

On difference I have noted, and also some confusion in guides. Is the NEED (or not) to remove chip U12, when in 6 ROM mode. Some claim only the 3 resistors next to it have to be set correctly. Others say it must also be removed. Right now, mine is still in my board.

Is there an issue with using TOS 1.04 with this revision of logic board? Or is there some other obvious reason why the machine (literally) bombs like this?

[rubber_jonnie]

Hi, you may see one post has been deleted, for some reason it was duplicated, not sure why.

[rubber_jonnie]

The jumpers sound correct, but a few people have reported issues leaving U12 in place, so you may need to remove it, however I would check the jumpers are working correctly before doing that.

A photo of your own board would be more helpful than that of another board.

Mine still has U12 installed. The capacitors whilst not necessary, are recommended, they are decoupling capacitors for the 5v supply to the ROMs.

TOS 1.04 works just fine on the Mega's, I even have TOS 2.06 in some of mine with a decoder and they work perfectly.

One word about these mainboards though, they are not terribly robust and can be damaged by manual handling. I'd suggest you reseat all of the IC's in sockets and use IPA and contact cleaner whilst you're doing that.

2 bombs is a bus error asserted by it GLUE or CPU (IIRC) so you need to check everything very carefully.

[frank.lukas]

If your motherboard works with the two-chip TOS 1.02 and you want to upgrade to a six-chip version, you need to set the three jumpers correctly for six 256k chips. Additionally, completely desolder U12 or just pinch off pin12

rom_table.jpg (74.14 KiB) Viewed 68 times

U12.jpg (79.81 KiB) Viewed 68 times

[Retronaut]

frank.lukas Thanks, the removal of U12 was the trick. I decided to socket it, but removing it proved to not be trivial. Got there in the end and the 74L chip survived. I tested it again in 2 ROM mode, with it installed, to make sure I had not borked that side of things. Once that was shown to be OK, removed the chip again and swapped the 6 ROMs back in. And it booted, phew, that was tricky. Thanks for your help everyone