How I fixed my 20 bombs error OR How I messed up badly and recovered

Various tweaks and fixes from video ghosting to audio fixes etc.
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Elethiomel
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Joined: Mon Oct 30, 2017 9:11 pm

How I fixed my 20 bombs error OR How I messed up badly and recovered

Post by Elethiomel » Mon Oct 30, 2017 9:44 pm

Recently I decided to upgrade my 1040ST with as much Exxos kit as I could (and more). The aim was to build a system with the following

4MB RAM expansion (under the PSU)
TOS 1.2/1.4 switchable
16MHz Booster
Blitter
Large cap replacement kit
Exxos PSU recap for SR98
NetUSB Ethernet Adaptor
CosmosEX

The ROMs were socketed, but to remove the old RAM and caps meant I had to desolder 576 pins. I removed the old CPU and installed a socket which I placed the booster in. To my surprise, everything worked first time and I had a system capable of TOS 1.2 or 1.4, 1MB or 4MB memory and 16 MHz or 8MHz booster. Everything switchable.

I then decided to remove the CPU socket I had installed and solder the boosted CPU directly to the board. Here's where I messed up royally. I got sloppy because of my previous success and instead of taking things slow, I rushed through it and ended up damaging the board. These old boards can't take more than a few heat cycles and you need to be realllly careful. When I soldered the new CPU in, I got nothing on my monitor. Just an "Out of range error"

DRAT

After reflowing all the pins, I still had the same problem, but discovered that the system would show something on the screen in low res mode. 20 bombs, sometimes flickering.

OK. We know we have a CPU that's somewhat working. We know that the CPU busses are probably connected as it can read ROM and read/write to RAM. The shifter is working too as we're getting output on the screen. However, the system is going funny very early in boot, before it does a detect for monochrome mode.

The thing is, bombs mean something depending on the number of them. 2 or 3 is the norm meaning a bus error or an address error. 20 means something especially odd is going on. Strange exceptions/Interrupts are firing. With the help of a schematic I verified the address and data bus were connected everywhere they should. I then turned my attention to the pins that control interrupts. IPL1 and 2 are connected to the GLUE, but pin 0 is unused on the ST and is pulled high. With the CPU in place I couldn't find any connection from the IPL0 pin to any of the resistor packs (long red things with multiple legs). I should have been reading 4.7K from +v to IPL0, but was getting 12.7M. Something was definitely odd on this pin. I traced the rest of the pins on the resistor packs and found one not going anywhere. I couldn't see exactly where the trace was going as the CPU was in the way, but it looked like it routed to the IPL0 pin on the CPU.

Using a small length of wire, I patched the IPL0 pin on the CPU to the resistor pack, crossed my fingers and booted the machine.

SUCCESS!

TL;DR IPL0 was floating. Should have been connected to 5v via a 4.7k resistor in one of the resistor packs. Soldered in a small wire. Ran Gembench and Memory tester a few hundred times and it looks like we're back in business.

PHEW!

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exxos
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Re: How I fixed my 20 bombs error OR How I messed up badly and recovered

Post by exxos » Mon Oct 30, 2017 10:01 pm

:goodpost: :bravo:

Glad its fixed :)
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https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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