Atari 400 Some letters not right

[Schleven]

Hello,recently bought an Atari 400 off an auction site.

It powers on and works. Came with centipede and that works however I am having an issue with some of the characters being wrong.

The T displays as a U

The R displays as an S

Most are right, but the SPACE character displays as an exclamation point so the background is filled with those instead of being blank.

Any idea what would cause this?

I have done board level repairs on other things just never on one of these.

Not sure if there would be an easier possible fix.

Thanks

[ClausB]

Those character codes differ by 1 from the correct ones, so something is wrong with the low bit of the data bus, D0. It could be a bad chip but more likely it's a bad internal connection. All the chips are in sockets which can corrode over time. Reseating the chips in their sockets and the boards in their slots can help.

 

 

[ClausB]

Since it's running and only the display is corrupted, the main suspects are the ANTIC chip, A302 on the CPU board, or the 4116 chip Z512 on the RAM board.

[+Stephen]

Had a 400 that did something similar, it ended up being a defective ANTIC.

[Beeblebrox]

Yup, I agree. Looks like Antic. 

 

It might be worth lifting the antic chip a little and resetting it in it's socket. Could be decades of sitting in it's socket has built up some oxidation on the legs. Lifting slightly and resetting can break any oxidation built up. 

 

If you find the chip is failing then you can source an a replacement PAL or NTSC flavour Antic chip (I can't tell where you are based), it's easily swapped out on the cpu board. 

 

Just make sure :

 

A) you get the right Antic, (PAL or NTSC) 

B)  you make sure the chip goes in the right orientation with pin1 

 

 

[phaeron]

The ANTIC chip appears to be reading the display list and font data properly, which implies that it can't be a simple failure on the D0 line.

 

[Rybags]

Weird one - an early thought was error from sequential memory accesses but it's only the character set accesses that are faulty.

Maybe A3 to the E Rom?

But you'd have to think that if that address line was stuck, there'd be no way the OS would properly work.

[Schleven]

I'll open it up and see if resetting the chip works

[ClausB]

8 hours ago, phaeron said:

The ANTIC chip appears to be reading the display list and font data properly, which implies that it can't be a simple failure on the D0 line.

 

Yes, good point. So is the internal ANTIC character buffer suspect?