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SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Mon Aug 21, 2017 8:06 am
by ClassicHasClass
I was antsy about the 2.5" SCSI in my ThinkPad 800 (the original Type 6020 which was never, apparently, sold), so based on reports that it works fine with the TP850, I bought a "PowerBook" SCSI2SD of the same type I currently use in my PowerBook 540c, configured it and installed it. I made sure to insert the pins the same orientation into the carrier board. When I fired the ThinkPad up, it promptly failed to POST with error 0370830. I replaced it with the original drive and got the same error. At this point I may have scared the cat.

Some digging around indicates that this error means the "SCSI circuit breaker is open" ( http://ps-2.kev009.com/eprmhtml/eprm1/h2583.htm ), at least on the TP850. However, the recommended tasks of letting it sit for 15 minutes didn't work, nor did unplugging all the SCSI cables and trying again. Even with no SCSI devices on board (no CD-ROM, no hard disk) it still does this. As usual the IBM service guidance at this point would be to replace the system board, which isn't going to happen.

I disassembled the unit to see if it was actually a fuse (given that this is an earlier model) that opened rather than a circuit breaker, which will be interesting to get back together, but anyway. Once I got the Faraday cage and the video card off I found a couple candidate surface-mount fuses marked F1 and F3, but they're both 2A (which seemed a little high for a SCSI breaker) and continuity testing checks out on both. I can't find any others. F3 seemed like the best candidate since it was nearest to the CD-ROM SCSI port, but it looks like both fuses are still closed. Some of the system board is still covered by the PCMCIA cage and the sound daughtercard, however, so it's possible there are other fuses to find.

Before I make an even bigger mess by trying to get the board completely free and loose from the chassis, anyone got experience with blowing SCSI fuses? Is this something internal to the controller chip itself (which by the way I haven't even identified yet, though I have a couple guesses), or is it usually on the board? What should I be looking for?

I'm also rather alarmed about the SCSI2SD doing that. Could it be that it's pulling too much power from the termination lines?

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Sun Aug 27, 2017 7:03 pm
by ibmfiles
Would you be able to take a photo of the area in question?
Maybe it's a diode or something.

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Mon Aug 28, 2017 7:19 pm
by ClassicHasClass
I'm pretty sure both are truly fuses, they're marked as such and have "2A" on the top (F1 even has "125V 2A" on the board next to it). It doesn't have the one-way behaviour of a diode and it looks too big for that.

I haven't gotten up the nerve to try to wiggle the other boards off yet.

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Mon Oct 09, 2017 6:05 pm
by ClassicHasClass
I disassembled down to the logic board and could not find any bad fuses. Since I need to get the corpse off the dining room table, I reassembled it and it does power up ... but it still gives the same SCSI error.

The next step is to pull the entire logic board out, I guess, and see if there are any components on the underside. That will have to wait for awhile.

Moral of the story: don't use a SCSI2SD with a rare computer, and I would be very leery of using it with any PowerPC ThinkPad in general. There's no guarantee I'll be able to recover this unit.

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Tue Oct 10, 2017 6:46 am
by Shiunbird
ClassicHasClass wrote:There's no guarantee I'll be able to recover this unit.


That's one of the saddest things I've EVER heard.
I keep trying to get my hands dirty with such machine, but whenever a offer pops up somewhere, it always costs more than I'm willing to pay.

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Wed Oct 11, 2017 6:28 am
by ibmfiles
ClassicHasClass wrote:I disassembled down to the logic board and could not find any bad fuses.

So apparently SCSI devices do use fuses, it makes sense though in retrospect. I just never encountered any problems with SCSI fuses before.

How did you test the fuses?

http://www.mpc-forums.com/viewtopic.php?f=42&t=179923
richie wrote:The way I test fuses is switching the volt meter to ohm, if I get basically the same as what I get when I touch both leads together, then I pretty much know that the fuse is gone.


I'm kind of curious why the SCSI adapter caused the fuse to close in the first place. I mean, you could always desolder the fuse bu then that could damage whatever SCSI IC is being used, and that would be more irritating to replace.

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Wed Oct 11, 2017 8:25 am
by ClassicHasClass
No, it caused the fuse to open. What I'd like to do is, at least initially, bypass it and make sure the system works with the original devices, and then desolder and replace the fuse. The fuses were tested with an ohmmeter, like the quote you gave. There must be a fuse that's still open I haven't found yet.

The IBM service docs for the 850 say it's a circuit breaker, not a fuse, but this is an 800 and it's not guaranteed that's the case here. This is the first time I've ever opened one myself, but I think this is mostly due to the fact most SCSI devices aren't crap and stay within the specified power draw. My current theory is that the SCSI2SD's terminator power usage is too much for this machine.

I talked to Michael McMaster and I'm going to test a few things on the SCSI2SD in question for him when I get back from my business trip. It is entirely possible it was defective (which is even sadder).

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Thu Oct 26, 2017 11:01 am
by astouffer
ClassicHasClass wrote:The IBM service docs for the 850 say it's a circuit breaker, not a fuse.


It probably uses something like this http://www.ti.com/lit/ds/symlink/ucc3916.pdf

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Thu Oct 26, 2017 9:43 pm
by ClassicHasClass
From Unitrode, a division of Initrode? </officespace>

Maybe, though that particular part seems too new to be in this unit. I've seen two-pin SCSI breakers on some unrelated systems. I haven't gotten a round tuit on the SCSI2SD yet.

Re: SCSI fuse advice (or, how to kill a ThinkPad 800 with a SCSI2SD)

Posted: Sun Nov 19, 2017 10:30 pm
by ajw99uk
Once had a SCSI card start doing strange things and that was a blown fuse - though given the vintage it was easy to find next to the termination resistors. The manufacturer did say it would most likely have been caused by over-termination on the bus, I am sure (since then I remove device termination as far as possible, and terminate the cable separately).

Can't help re T800, sorry, other than to suggest you look at where the SCSI circuitry has its terminator (if you can find that).