Atari TT White Screen, but sometimes he boots

[Roberto]

Hi everyone,
I managed to buy an Atari TT at a good price here in Italy, not far from where I live. It came without a hard disk, only with the floppy drive.
The first time I powered it on, I got a white screen (no memory test or diagnostics visible), but after that it continued the boot process normally and I eventually reached the Desktop.
I tried burning 4 new TOS ROMs and after installing them I started seeing the Atari logo at startup, the memory test, and then the Desktop. The floppy drive was also working correctly.
It worked fine for a few days, then suddenly, when powering it on, the monitor reported no video signal, even though the computer seemed to be powered on and the floppy drive LED did not light up.
Sometimes it would boot, but most of the time it remained with a black screen. Eventually, after pressing the reset button several times, it started normally again.

I replaced the reset switch with a new one (luckily I had a spare new one), but now the situation is different:
When I power on the computer, the screen is white.
If I press the reset button, the screen turns black. When I release the reset button, the screen goes back to white again.
So I think the reset switch itself is working correctly.

I tested the PSU from my Mega STE (which is known to be good) on the TT, but the problem remained, so I believe the PSU is not the cause.
The CPU becomes warm immediately after power-on (I assume this is normal), but I noticed that the PAL U703 also gets quite hot.
I checked some CPU signals:
RESET pin: when the computer is powered on, it is at +5V. When I press the reset button, it goes to 0V. This seems correct.
HALT pin: +5V, so I assume this is OK (I know HALT is active low).
BERR pin: +5V.
STATUS pin: stays permanently at 0V (I read somewhere that it should normally be +5V).

There is a clock signal present on the CPU CLK pin, but I cannot see any activity on the CPU address or data bus pins.
I noticed something strange:
If I leave the oscilloscope probe connected to the CPU RESET (RST) pin while the computer is powered on, and then press the reset button, I see a square wave on the pin (see attached picture) and the screen remains black.

However, if I remove the oscilloscope probe from the RESET pin and press RESET again, the screen goes back to white. Measuring the RESET pin again afterwards shows a stable +5V level.

I performed all tests without TT RAM installed, using only the ST RAM.
The diagnostic cartridge gives no output at all.
Any suggestion or idea would be greatly appreciated.

Thank you very much!

Update:
The time to write all this and I went back to turning on my Atari TT: now it doesn't give any video signal (black screen) but the CPU is not in HALT.

IMG_7702 - dimensioni grandi.jpeg

IMG_7703 - dimensioni grandi.jpeg

IMG_7704 - dimensioni grandi.jpeg

IMG_7705 - dimensioni grandi.jpeg

[stephen_usher]

The reset shouldn't be a square wave like that, it should be continuously low when you hold the reset button in.

[Roberto]

The reset shouldn't be a square wave like that, it should be continuously low when you hold the reset button in.

Yes, in fact this is very strange, but this only happens when the probe of the oscilloscope is in contact with the RST pin of the CPU.
If I remove the tip it behaves as expected. I can't explain it to myself.

[Steve]

Before getting into any in-depth diagnostics, I would firstly remove anything unnecessary from the board, like the ST-RAM board (The motherboard has 2MB already) Then remove any chips in sockets, deoxit and re-insert. (Quite a few sockets on that board) I literally watched Adrian's digital basement spend hours on a motherboard the other day, turns out he just needed to deoxit some chips lol :)

[frank.lukas]

Check XRESET and XPOR at the outputs of U102 (part A,B,C,D,E) ...

Bildschirmfoto vom 2026-06-24 12-47-00.png

The Signal at CR101 came from the PSU, it is the PG (powergood) Signal and has +5V permanent.

Beneath the CPU daughterboard, there is a two-pin connector that supplies the master 32 MHz clock signal. Naturally, it must be connected and plugged in the correct way around (GND).

The PAL chips get very hot, but this is normal.




"Wer viel Misst, Misst oft Misst ..."

[Roberto]

Hi @Steve, As you advised me, I removed the additional card of the ST RAM and removed all the possible chips from the respective hooves, cleaning them well with isopropyl alcohol. But unfortunately the problem remained, that is black screen (at the beginning it was always white).


Hi @frank.lukas I'll answer you on Atari-Forum; I realize that I was probably wrong to open the same topic on two different forums.