Amiga 600 in reboot loop

Amiga 600 which was constantly rebooting.

I got this machine as a repair, someone before me tried to do a recap to the machine.. After that it was not working anymore.

First thing I tried was to replace the 555 Timer (U14) and the C612 Capacitor, because this is common problem on an Amiga 600. This was not resolving the problem.

Hooked the machine to a serial cable to see the output in putty.

Now I saw that the detected Chipmem is 44KB.. 44KB ? Hmm that should be 1024KB.. So there is a problem on the chipram busses..  So I started to check all the traces for both address- and data lines.. no broken traces.. signals seemed ok, but it was not good.. Started to replace U21 & U22 (74LS245) both transceivers which are used to make a bridge between the CPU and the chipram bus.. still no solution.. ok.. Maybe Agnus of Gayle ?

Placed both chips on PLCC84 sockets, so I could easily test with other (spare) chips..still no luck.. ok.. Traces oke, transceivers ok, Agnus & Gayle ok.. hmm.. CAS & RAS signals seemed ok.. so.. could it be the one or both dram chips (256K x 16bit) ? Lets test it, so I replaced them.. still no luck.. hmmm.. ok.. once again I checked the trace of the address- and data lines between all chips and all important traces that are needed for the chipram. everything looks ok.. once again I checked _BCASL(0), _BCASL(1), _BCASU(0) & _BCASU(1) on U27, looks good.. because there is signal.. but.. that is the only chip I did not replaced.. so.. lets do that..

Really ? what is that.. ? Solder that is making a short between pin 13 & 14.. I removed that and replaced the chip..
Lets see if it works..

YESSSSSSSS!!!!!! It works.. I won’t tell you how long it took to find this.. (8 hours)..

Started a memtest with Amiga Test Kit (Systest from Keir Fraser), and everything seems ok..

Other things I did to this board:

  • Fresh and good recap
  • Fixed audio circuit, distorted audio..
    Electrolyte leakage under the LF347.. so cleaned it and replace the opamp..
  • Replaced joystick port, had some broken pins

So in short: you can find the solution to a problem at a location you won’t expect it..