Intermittend boot problems Amiga 600

[ritzel]

A friend of mine used to own an Amiga 600. After nagging him for years to get the thing recapped he finally gave it to me. Of course all the caps were already leaking. I cleaned and recapped teha Amiga. Now for about 60% of all the cases the machine boots just fine, both from floppy as well as CF card. In abut 40 % of the cases it gets stuck with a white screen. If it succesfully boots, you can reboot using ctrl-A-A without issues. If the machine gets stuck the reset works but it get´s stuck again. The voltages on the board are in range. Reset signal at the 68000 works normal. The clocksignal is also good.

Any ideas how to proceed?

[Boing-ball]

Worth a look

https://www.amigalove.com/viewtopic.php?t=324

[ritzel]

What I get is dark grey, light grey and white. If the computer locks up, it´s with a white screen. In locked state when I press ctrl-A-A it goes through the cycle dark grey, light grey and white again and locks up again. When the machine is running youcan reset it via ctrl-A-A repeatedly without problems. 

[Boing-ball]

You might have an intermittent connection problem on the main board.
Have you tried reseating the KS ROM chip?
Which ROM version are you using? Anything else connected on the main board? Floppy disk drive connected still?
Are you using a HDD drive on the IDE header?

[ritzel]

I have tried different Kichstarts 2.05 and 3.1 as well as Diag-ROM. The behaviour is allways the same. Interestingly I now se subtle changes of behaviour. I now seem to get a black screen at failed booting. The rate of ailures on the other hand seems to decrease. The pins on the NE555 look dull even though the reset signal seems to be fine. I have to trace the signal further. A have tried the machine with both the CF-Card at the IDE header connected and disconnected. Same with the floppy or a Gotek. I don´t see a difference there.

[Castellen]

Measure what _KB_RESET is doing when the problem happens.  Under normal conditions, this line should be near 0V immediately at power on (holding the system in reset state), then go high to around 5V after about 100ms.  If it's staying low (or U14 pin 3 measures around 0.6V instead of close to 0V as it should normally be), the system will be forever stuck in a frozen state.  A common problem is C611 getting corroded by capacitor electrolyte and going resistive, making the power on reset circuit stay active.  Replace C611 - 100nF, 1206 package ceramic capacitor, if there's been any corrosion in that area, which it sounds as though there is.

If there are still problems, or the problem isn't related to _KB_RESET, i.e. it's around 5V but the system still isn't running; then use the serial output from DiagROM to at least see what's happening when the system tries to boot.