Non-booting TT030

[heckler]

I recently acquired a Rev B.1 TT030. Initially the machine booted up but no cartridge would work when I plugged it in (NetUSBee, Spectre GCR, or TT030 Diagnostic). Since I had success in improving the performance of my Mega STE after changing the data and address bus pullup resistors, I decided to see if the pullup change would help the cartridge port function on the TT. I changed the pullup resistor SIP’s for the address (RP104, RP105, RP108, and RP109) and data buses (RP102, RP103, RP106, and RP107) to 2.2K ohm. While I was at it, I also recapped the main board electrolytic capacitors.

I verified the changes both visually and electrically (with DVM) multiple times to make sure that part orientations were correct and no solder bridges were present. When I tried to boot the system after the changes, no video was available on the monitor (did not even detect an input). I also checked that power supply voltages were correct (power good = 4.89 V, +12 V = 11.777 V, -12 V = -11.939 V, -5 V = -5.0416 V, +5 V = 5.0333 V). Power supply had been previously recapped and the computer was booting prior to the rework.

I connected a logic analyzer to the system and found that a number of transactions between 0x0000 0000h and 0x00E0 0034h (Picture 1) followed by a couple of much longer duration accesses to 0x202E 02FEh and 0x202E 02FAh that are followed by an apparent hanging bus (Picture 2).

I then changed the resistors to values of 4.7K ohm. This resulted in the same bus transactions followed by the hang. I then backed out the changes (to 10K ohm) and proceeded to find the same logic analyzer results.

I then referred to the TT030 Service Manual and found:
1. CLK @ 68030 pin E1 = 32.2 MHz
2. STATUS @ 68080 pin J12 = momentarily high after reset but low otherwise
3. RESET @ 68030 pin F12 = momentarily low after power cycle/button push but otherwise high.
4. Swapped out 68030 with same logic analyzer result. Cannot eliminate double bus error.
5. HALT @ 68030 pin G2 always high.
6. BERR @ 68030 pin G1 always high.
7. Cartridge Port power is present.
8. Inspected board and found ST RAM expansion board with unsoldered pins on controller. Resoldered but no change in function. Nothing else obvious was found. I’ve tried booting without ST RAM board connected with no change.

I am not sure what device the last two transactions are targeting or if the system is expecting further responses from this device or from another. It seems that I may have a faulty integrated circuit but I am not sure which one it could be. I am also leery of starting to shotgun, as replacements parts are limited and expensive. At this point I am hoping that someone from the group could provide some recommendations/pointers. Any ideas would be welcome. Thank you.

TT030 Picture 1.jpeg

TT030 Picture 2.jpg

[Steve]

I think going from: not working cart port > replace all sil resistors is a bit drastic, did you not think to check the cart port fuse first?

It's very easy to accidentally break a trace when changing the resistors, so you'd want to confirm every trace from every SIL you changed.

[heckler]

Thank you very much for the reply Steve. Sorry that I did not include the verification of the cartridge port fuse in my prior post, but I did check the fuse prior to replacing the resistors and power was present at the output of the fuse as well as the cartridge port power pins. I will check the traces around each of the resistors to make sure that none of them have broken in the process. Thank you for your suggestions.

[mikro]

Indeed, not sure what kind of advice do you expect; it seems you have damaged the machine during the process (not sure why do you think there's a damaged IC, unless you were really careless when soldering the resistors / capacitors).

I guess you have to start checking short circuits, contacts and maybe even whether the new/re-inserted components are still working.

Btw the 0x202EXXXX addresses you're seeing are clearly bogus: that's TT-RAM >256 MB which is surely not in your machine. Looking at TOS 3.06 disassembly:

Code: Select all

_main:
[00e00030] 46fc 2700                 move.w    #$2700,sr
[00e00034] 31fc 0100 8606            move.w    #$0100,(DMA_CTRL).w
[00e0003a] 31fc 0000 8606            move.w    #$0000,(DMA_CTRL).w
[00e00040] 4e70                      reset
[00e00042] 0cb9 fa52 235f 00fa 0000  cmpi.l    #$FA52235F,cart_bas
[00e0004c] 660a                      bne.s     $00E00058
[00e0004e] 4dfa 0008                 lea.l     $00E00058(pc),a6
[00e00052] 4ef9 00fa 0004            jmp       $00FA0004

and TOS 3.05 disassembly:

Code: Select all

[00e00030] 46fc 2700                 move.w    #$2700,sr
[00e00034] 4e70                      reset

I'd say that there's a pretty good chance you're booting the latter (as DMA_CTRL is 0xffff8606), i.e. your board is failing to reset properly.

[heckler]

Appreciate your inputs, the both of you have been helpful. I am going to spend some more time digging in to this. Although I may have been in error in making the changes, I would very much like to get the machine back working again. At the very least, I’m sure that it will be a learning experience.

[heckler]

Measured resistance with DVM between CPU address/data pins and VCC and found unexpected values for D24 (381Kohm) and D30 (0.55Mohm). Both of these go to RP102. When compared to the disassembled code, the problem with D24 is shown also. Now I get to go damaged trace hunting. Thanks to both of you for getting me on a better path. BTW machine has 3.06 ROMS.

[mikro]

Ah, right. Now I see that your data bus snapshot also shows the values just before changing the DMA_CTRL. Good luck!

[heckler]

Success! You guys were so helpful and correct! I did not have the proper appreciation for how delicate the traces and annular rings around the SIL resistors were before I made the changes. Found broken annular rings/traces on the top layer for RP102 - D24, D25, and D30. Repaired them and now have my booting system back again! I appreciate you both taking the time to give some help and I am also thankful to have the forum to post to. - Don

[Steve]

Glad you fixed it :)

[mikro]

Glad to hear that, the feeling of a working machine is great, isn't it.