General Category > Lisa Troubleshooting and Repair
Help please for my Lisa 2/5
sigma7:
--- Quote from: pintoguy on August 22, 2023, 07:09:58 pm ---I noticed that +5B only reads 4.8V or so, when +VSTBY is 5.5V. On my "good" board, it's 5.3V. So I suspected that diode D9 was therefore bad, but after I changed it, I still have the problem.
--- End quote ---
Note that +5B2 is different from +5B, they both are derived from +5STBY, but through different diodes.
+5B2 is via D9, which is an ordinary silicon rectifier, so a 0.7V voltage drop is not surprising.
+5B is via D3, a Schottky diode, so the drop should be less than 0.7V, making +5B higher.
So if +5B is indeed 4.8V; D3 is the suspect.
But I'd guess it doesn't matter because digital. To me, it looks like +5B2 only exists as a separate circuit to decrease the load on the battery, which drives +5B when power is disconnected.
--- Quote ---chip 8T97 at U1F is very hot to the touch
--- End quote ---
U1F drives the long ribbon cable that goes to the floppy; try disconnecting the lite adapter and see if U1F runs cool. If so, then look for a problem with the Lite Adapter, floppy drive, or cabling.
U1F also drives /DTACK when the CPU accesses the FDC SRAM, so if the 8T97 is not working, the effects would line up with the symptoms you've reported.
pintoguy:
Thanks sigma7. Yes, I misspoke, it is +5B2 that shows the .7V drop. And thanks for explaining the nature of these +5B and +5B2 lines. I was scratching my head on this for many days, but it makes a lot of sense now. B would stand for "battery" perhaps ?
And as I was tinkering around U1F, the system died. Upon turning on, the switch light turns on for a second or so, before shutting off. I suspect a short perhaps ? Might be consistent with the hot U1F chip that gave up? I have spare 8T97's ordered, so will update hopefully in a few days.
Thanks again
sigma7:
--- Quote from: pintoguy on August 23, 2023, 06:14:49 pm ---Upon turning on, the switch light turns on for a second or so, before shutting off. I suspect a short perhaps ? Might be consistent with the hot U1F chip that gave up?
--- End quote ---
Removing U1F will allow you to confirm that it is the problem. With U1F missing, the signals driving the Lite Adapter/Floppy drive will be missing, as will /DTACK generated when the 68k accessed the FDC shared RAM. So startup should go normally until the point where the self-test checks the FDC, which won't respond, causing an I/O Board error.
The 74LS367 has the same function as the 8T97, but with less output current. If you have those on hand, you could try one in place of the 8T97 while waiting for those to arrive, leaving the Lite Adapter unplugged or bending out the floppy signal output legs of the chip so they don't make contact with the socket. The drivers of /DTACK elsewhere in the Lisa are not 8T97s, so the LS367 may be adequate for driving /DTACK here (but likely not for the floppy signals).
HTH, James
pintoguy:
Thanks James. The five 8T97 arrived today, so I'll report shortly, after I get a chance to go back to the bench.
pintoguy:
Bad news today. I removed the 8T97, and the "short" is still there. I call it "short" because I think the symptoms (turns on for ~1 second and switches off) are consistent with it. But is it ? Any input would be greatly appreciated. I think I remember reading about these same symptoms earlier, but cannot find the post
What I know now is that it's not the PS, nor the COP421, nor any of the other socketed chips.
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