Dead TT030 Repair

[grunwald]

I purchased three dead TT030s about eight years ago and have managed to repair two of them. The last one is turning out to be the most difficult. If I boot without a diagnosis cartridge, I get a pulse on the BERR line and no activity on the Bus. The Halt line however still stays high. I also get the video shown below (white screen, blue bars) consistently. If I insert the diagnosis cartridge, I get a black screen with continuing activity across both data and address busses. I do not get anything through the serial port. To date, I have changed the CPU (came to me with damaged pins), MCU, DMAC on the serial side, TTVIDEO, and both GALS. I have probed all bus lines and have eliminated all shorts and open lines to the processor. I have also recapped the power supply and verified outputs. Has anyone seen the display below?

20240505_234014.jpg

20240505_234055.jpg

[frank.lukas]

... my guess is a problem with the onboard 2MB ST RAM.

Use the Test cart and a null modem cable to a other computer with a serial terminal running 9600baud8N1

[grunwald]

I have the unit connected to a PC running a terminal prog. I get nothing coming across with the cartridge installed. I validated the hook up with another TT just to be sure. I get the full diagnostic menu on that one. Would a memory issue stop the data on the RS232 port?

[frank.lukas]

Work with the schematics and measure all Ram Data and Address lines and RAS, CAS, WE ...

https://forum.atari-home.de/index.php/t ... #msg268418

... use the Google Website Translator.

[tomster]

Had pretty much the same screen when I had issues with my ST RAM extension card. In my case it just wasn't properly inserted.

[grunwald]

I went back and remeasured the entire bus last night and all lines are good at 10k. The lower data bus however, is being pulled high when I boot with the cartridge. At about midnight, I discovered that the data lines D1-D7, on funnel U209, are displaying a signal. while the same lines on funnel U205 are being pulled high. Since they are wired in parallel, I assume that U205 is bad. I plan to pull and replace it tonight with a new spare. More to come....

[grunwald]

Changing the funnels has had no effect. I hooked the entire bus to my logic analyzer and found the following. When booting with the diagnostic cartridge, D0-D15 go active, then immediately are pulled high. D16-D31 stay fully active. A0-A15 are fully active. A17, A19, A20, A21, A22, A23 are all held high. The rest are fully active. I have been searching the schematics for a chip that would effect only these bus lines, with no success. When powered down, all buss lines are at 10K. I have new dram ordered along with a ram tester. Most of the major chips are now socketed. I may pull and replace the remaining dmac, dcu, and glue to complete that process....

[grunwald]

I ended up adding sockets and swapping out the remaining dmac, dcu and glue chips. I already had the spares and figured one less thing to worry about. That did not affect anything. Yesterday I received replacement drams so spent a couple hours adding sockets. Progress! A little at least. With the cartridge in, the system begins the initial system check, very slowly one character at a time and stops at the MFP timer. Without the cartridge l, it display the Atari logo and stops. I am heading out camping for the weekend and will jump back in next week.

20240524_074814.jpg

20240523_224023.jpg

20240524_003830.jpg

20240524_074843.jpg

[grunwald]

Made a bit more progress last night with the memory check now completing. I also found that the 2.457 clock is active on the inverter at U606B (74HCT04N) Pin3, but not on Pin4. I pulled/socketed the chip and will have a new one by Friday. Hoping that is causing the T0 MFP error.

20240527_003601.jpg

[Steve]

Good progress, thanks for documenting it! Perhaps when it's over you could summarise the faults & fixes for future reference.