New project and a mistake in the beginning

I’ve got a new 8 bit computer sold as non working: The Atari 800XL

It has a few marks and scratches and little broken plastic but it’s overall condition is quite good. It needs a good clean though.

Since I don’t have any peripherals for the Atari line of computers I’ve ordered a power supply first. The thought behind was to see if I can get it working again an then buy some other stuff.
The power supply came quite fast but then the whole project slowed down and a few weeks had gone until I made my first observations and my first mistake.

After a visual inspection I plugged in the Atari and measured the voltage at a few points. I was surprised about the measurements and I couldn’t explain the readings. Unfortunately it was rather simple: The power supply has ground and +5V mixed. So hopefully I didn’t destroyed my new Atari completely. 😥
I’ve ordered a new power supply and a SIO2SD from Lotharek in hope that I’ll can fix it anyway.

Now I wait for the parts and hope that at least all custom chips are still working 😳

Update:
Got the new power supply and everything is working. I’m a tiny bit disappointed since I wanted to repair something but just a tiny bit 🙂
Now only the cleaning is left.

Another C64 black screen repair (ASSY 250407)

Just a short note what caused the issue for me. I started with the usual suspects like the PLA and the TTL logic U13 and U25. Unfortunately the screen still remained black and even the Dead Test Cartridge revealed nothing.
After some further testing with the scope I noticed that the /HALT signal on pin 2 of the 6510 cpu was kept low all the time. Because the signal is active low (indicated by the /) the cpu is constantly on halt.

Looking at the schematics (http://personalpages.tds.net/~rcarlsen/cbm/c64/SCHEMATICS/250407/) the /HALT signal (or /RDY in the schematics) is driven by logic U27 (MOS 7712, Quad 2-Input AND Gates). MOS logic chips are known for their high failure rate and with the scope I could see that even when both inputs on pin 9 and 10 are high the output on pin 8 was low. Replacing the MOS 7712 by a 74LS08 solved that and I got at least the starting screen.

But the Dead Test Cartridge showed weird characters in different colors on the screen. Since I was on it I also replaced the MOS 7709 later on with a 74LS258 and that fixed that issue too.
Unfortunately I have to dig deeper since I still get random black screens. After a power cycle it seems to work as expected though further testing is required.