Minimum Boot (Amiga 500)
A good place to start if you are having issues, is a minimum boot.
The following components could all be removed if you have a system, you cannot repair and are unsure where the issue lies:
- All lower RAM
- Paula (Audio)
- Both CIA (Connector interfacing)
- Denise (Video Out)
Optionally you can leave any or all in if you like, but if they are faulty, they could interfere with the diagnostics.
With all the above removed, we want to check if the system has come out of reset. Do this by checking the following:
- 68k !RESET (pin 18) should be 5V DC.
- U37 OR gate pin 8, 11 and 12 should all be 5V DC.
Without Denise installed, we can monitor for typically the green screen boot (due to RAM being removed) by probing the bottom left pin D11 (pin 20) of the BIOS ROM during power on.
This should pulse randomly with 5V data signals, then constant 118khz square wave, which is the green screen.
If there are other problems before getting to green screen such as the data paths or Gary / 68k etc... then this might be a different output signal.
If you see something different, check the BIOS, 68k, Gary and traces between them.
Next up check if Agnus is operating correctly, as well as if the BIOS was successfully read.
The instant you apply power you should see HSYNC, VSYNC and CSYNC all pulsing. If Agnus is working, this is what it will do before anything (68k / Gary) tries to control it.
If you do not see the video signals at instant boot, it is very likely Agnus is bad. These signals are fully automatic and should be present so long as Agnus is receiving power and clock signals.
To test basic angus function, remove the 68k, keeping the BIOS ROM and Gary installed, and angus should output VSYNC, HSYNC and CSYNC constantly as it will not be getting any signals from the 68k to try and control it.
The video signals are at the bottom right when looking at the console face on as the chip is rotated 90 degrees clockwise.
If you monitor D11 (pin 20) on the BIOS ROM and it shows the obvious green screen boot pulses, but you do not see the Agnus HSYNC/VSYNC/CSYNC signals, check the Agnus and traces to it.
After a second or so from power on, if the Gary / 68k managed to successfully read the BIOS data and start running the code, Agnus should continue to pulse out these HSYNC, VSYNC and CSYNC pulses just as it did on immediate boot.
If the code fails to load, Agnus often then goes into fault mode and stops outputting any video signals.
This is a great way to detect if the BIOS has loaded successfully.
If you see something different, check the BIOS, 68k, Gary and traces between them.
