grbrady
Experienced Member
I have a CBM 2040 drive that I'm trying to repair. I've made a little progress but am now stuck.
The logic board came with DOS 2 ROMS (901468-12 in L1, 901468-11 in J1, 901468-13 in H1 and a 6530-034 RRIOT in K3.) When I first got the drive all the lights would illuminate and not go off and one or the other spindle motors would run. No blink codes. I pulled the three ROMs in L1, J1, and H1 and read them in my programmer and they all seemed bad, although the failure mode gave me pause, the second 2K was all FF while the first 2K was what was expected. I burned 2532s and replaced them and was able to get a single blink out of them which indicates a failure of zero page in the two 6532 RIOTs. I swapped those and got 5 blinks, which indicates a problem with the 6530 or 6504. I tried some new 6504s of questionable provenance and also a 6502 with a socket adapter I made and got a similar result. I also built a 6530 replacement board using a 6532 and the design and Eagle files from Ruud's web page here: http://www.baltissen.org/newhtm/6530repl.htm. For the ROM I used a 2716 with the 901466-04 image from Zimmer's burned into it twice. Still the same 5 blinks behavior. Doing some probing, the 6504 comes out of reset and runs for about maybe a second and then crashes. I hooked up the logic analyzer to the data bus and looked for the value for the reset vector from the 6530 to come up and did not see it. However, if I prevent the 6502 from running by pulling it or its ROMs I do see the correct reset vector address come up on the data bus and the 6504 runs more or less indefinitely (no blink codes here because the 6502 is disabled.) There seems to be a bus conflict on the data bus that is killing the 6504 right out of reset.
Next, I read that another common cause for the 5 blinks problem is bad 2114 SRAMs, particularly in C4 and C5. I desoldered all of the 2114s and tested them using an Arduino and the method and code here: https://github.com/Danjovic/2114-Tester. I did indeed find two bad chips and they were the ones from C4 and C5 (also found a few bad ones in my stock of 2114s.) I put in sockets and repopulated the board with tested SRAMs. I still get the 5 blinks. I then considered that maybe the 74LS157 multiplexers at C3, D3, E3 and F4 could cause a bus conflict, although they looked ok when I probed them with the scope. I desoldered and socketed them and tested them with a TL866-II and they all test ok, and I also tried new ones in the board and still get the 5 blinks. I also pulled the 6522 it seems to work perfectly in my PET. On occasion when I'm running the machine without the main 6502 running one of the stepping motors will randomly turn a few steps, so it seems like something happens occasionally with the 6522 and 6504. I also tested the main 6502 in my PET with success. So now I'm stuck. With the exception of some ancillary logic, I've tested or swapped pretty much all of the parts on the data bus and address buses. Superficially all of the signal goings to the other logic look sensible, so I'm not sure where I should look next. The 5V and 12 V rails all look good and don't have much ripple.
Any thoughts on what to try next to debug this? Anything common that I should look at?
Greg
The logic board came with DOS 2 ROMS (901468-12 in L1, 901468-11 in J1, 901468-13 in H1 and a 6530-034 RRIOT in K3.) When I first got the drive all the lights would illuminate and not go off and one or the other spindle motors would run. No blink codes. I pulled the three ROMs in L1, J1, and H1 and read them in my programmer and they all seemed bad, although the failure mode gave me pause, the second 2K was all FF while the first 2K was what was expected. I burned 2532s and replaced them and was able to get a single blink out of them which indicates a failure of zero page in the two 6532 RIOTs. I swapped those and got 5 blinks, which indicates a problem with the 6530 or 6504. I tried some new 6504s of questionable provenance and also a 6502 with a socket adapter I made and got a similar result. I also built a 6530 replacement board using a 6532 and the design and Eagle files from Ruud's web page here: http://www.baltissen.org/newhtm/6530repl.htm. For the ROM I used a 2716 with the 901466-04 image from Zimmer's burned into it twice. Still the same 5 blinks behavior. Doing some probing, the 6504 comes out of reset and runs for about maybe a second and then crashes. I hooked up the logic analyzer to the data bus and looked for the value for the reset vector from the 6530 to come up and did not see it. However, if I prevent the 6502 from running by pulling it or its ROMs I do see the correct reset vector address come up on the data bus and the 6504 runs more or less indefinitely (no blink codes here because the 6502 is disabled.) There seems to be a bus conflict on the data bus that is killing the 6504 right out of reset.
Next, I read that another common cause for the 5 blinks problem is bad 2114 SRAMs, particularly in C4 and C5. I desoldered all of the 2114s and tested them using an Arduino and the method and code here: https://github.com/Danjovic/2114-Tester. I did indeed find two bad chips and they were the ones from C4 and C5 (also found a few bad ones in my stock of 2114s.) I put in sockets and repopulated the board with tested SRAMs. I still get the 5 blinks. I then considered that maybe the 74LS157 multiplexers at C3, D3, E3 and F4 could cause a bus conflict, although they looked ok when I probed them with the scope. I desoldered and socketed them and tested them with a TL866-II and they all test ok, and I also tried new ones in the board and still get the 5 blinks. I also pulled the 6522 it seems to work perfectly in my PET. On occasion when I'm running the machine without the main 6502 running one of the stepping motors will randomly turn a few steps, so it seems like something happens occasionally with the 6522 and 6504. I also tested the main 6502 in my PET with success. So now I'm stuck. With the exception of some ancillary logic, I've tested or swapped pretty much all of the parts on the data bus and address buses. Superficially all of the signal goings to the other logic look sensible, so I'm not sure where I should look next. The 5V and 12 V rails all look good and don't have much ripple.
Any thoughts on what to try next to debug this? Anything common that I should look at?
Greg