Got a Amiga CD32 for repair. It doesn’t work, no video output, power LED does not fully illuminate on powering up the machine, so no reset on Power cycle.
First thing that came up in my mind was the voltage detector, replaced it and the machine boots again. But video output is totally mangled. Checked all voltages (5V, 12V, Reference voltages, +VID), everything looks fine. Replaced all capacitors responsible for the voltage supply of the video circuit, but without result. Checked input and output of U36 (74HC4066), that isn’t looking good, so replaced it. Video output still mangled, so replaced video encoder (U12 – CXA1145) without result. Replaced the DAC (U30 – ADV101). After that replacement we got a bit of video output, not the best quality but it’s a good starting point.
So now we got video output, but another problem occured.. the machine stops working after 30-60 seconds. Huh?! ok voltages are fine, let’s check reset state, that’s also fine. Clocks? Checked the 28MHz clock, and what do you think? Yes! Tthat’s the problem, the clock is stopping after 30-60 seconds. So where is that going wrong?
Let’s check U25 (74F00), and there is our problem, I discovered there is no clock output.. the clock is present on pin 9,8,5, but no output on pin 6. Replaced U25 and it works! Machine is passing 5-10 minutes, so that is fixed..
But now the video output. It looks like this:
After testing and probing all signals on the DAC, I can conclude one thing; DAC is fine, but what is wrong. Zender diode? D215 (LM385)? replaced it, but no change. Voltage on DAC pin 17 is 5V.. how can that be? there is a ceramic cap between +VID and R216.. hmmm.. let’s remove C216 and see what happens. Okay, display is looking good.. lot’s of interfrencing but it’s “stable”.. let’s meassure voltage on DAC pin 17, it’s 2.3V.. ok… now let’s place a new C216, and see what happens. IT WORKS! GREAT!!! So now video output is good, machine is now running for 1 hour and still ok.. so it’s time for a total recap.
Result so far..
But we aren’t done yet, the joystick down button doesn’t work on port 2.
Let’s check the components that are needed for the down button.. what?! They are missing?! Okay, let’s put some new components and let’s see.. It works!! so machine is ready!! phew..
Machine did some test runs between 1 and 2 hours, and working great!
- U14 – RH5VA43A (Voltage detector)
- U12 – CXA1145 (Video encoder)
- U30 – ADV101 (DAC)
- U25 – 74F00 (Quad 2-Input NAND Gate)
- U36 – 74HC4066 (analog switch)
- D215 – LM386 (Zener diode)
- Various ceramic capacitors and resistors.