3B1 will not boot from hard disk or floppy disk

[Jonathan Peart]

Last week my 3B1 stopped booting. I power it on and see one square displayed on the monitor in the upper left. The floppy disk light comes on and stays on. I have tried putting a diagnostic floppy in the drive but it won't boot. Prior to this problem I was able to boot successfully from both a floppy disk and the hard disk. I also see that all four diagnostic LEDs are lit and do not turn off.

From what I can tell in the manual the fix for this is to replace the logic board. That is not an option for me as I don't have a spare.

Has anyone here had to deal with this problem? Any suggestions on troubleshooting? The only thing I can think of is to open it up and re-seat all the cable connections and logic chips that are removable.

Jonathan

[DoN. Nichols]

What I would suggest you do is open it up and take a look at the
power supply connector. IIRC, it is 18 pins in a row on the power
supply, and the connector from the system board is a very stiff ribbon
cable arching it up and diving down to the top of the power supply.

The connector is a semi-transparent orange plastic, with a black
cover strip over the back where the wire goes into the connector.

Pull the connector, and look closely at it. In particular, look
for areas which have turned brown -- light brown or nearly black,
depending.

Trace from this to the corresponding pin on the power supply.

Look closely at the solder around the pin -- and from the
underside of the power supply board as well. If it looks frosty, it is
currently a "cold solder" joint.

If you find such -- and have a solder sucker -- suck off the
existing solder from that pin, and flow fresh lead-tin solder -- *not*
the tin only solder used now for plumbing joints. The temperature of
the tin only is higher and will likely accelerate damage to the power
supply board.

When done, the solder should look nice and shiny, not frosted.

Note that the power from the supply comes in duplicates. For
example, 5V is going to the CPU (and likely the disk controller chips)
from one pin, to all the memory chips from another pin, and to the disk
drives from a third pin. +12V mostly goes to the disk drives. In later
versions of the 3B1, the power supply has a separate cable to carry
power to the hard disk drive, because the total current tends to cook
the power supply pins. Oh yes, the +12V is also used to power the
monitor in the top half of the case.

Depending on which pin it is, you can get different symptoms.
It sounds to me as though the CPU may be stopping part-way through the
power-up cycle.

I've seen the 5V to the disk drives with a cold-solder joint on
the power supply glitch things so the disk gets sectors damaged during
write -- the only real cure for this (after repairing the power supply)
is to re-format the disk -- though you can run the repair part of the
format to substitute undamaged sectors for the damaged ones. But the
re-format (with clean power) will actually repair the damages sectors.
(Of course, you lose whatever data you had on the disk.) IIRC, the
format of the disk saves the last sector of each track as a replacement
sector for disk repairs. If you get a lot of bad sectors in a single
track, you'll be spreading around through several tracks for replacement
sectors.

If the connector is only lightly browned on one or two pins, you
can likely continue to use it. If it is dark brown or black, you should
ideally remove the connector and install a new one. (The wires in the
ribbon cable press into notches on the connector -- so no soldering
needed there. But it is tricky to pull out of the old connector without
damaging the ribbon cable. I was lucky enough to find (and recognize) a
replacement connector at a hamfest. This does not mean that I remember
a brand. It may have been TRW.

Of course -- other things can cause problems, but this I find to
be the most frequent source of problems in the 7300 and 3B1.

> Jonathan

Good Luck,
DoN.

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[Jonathan Peart]

I checked the power supply connector and didn't see any black or browning coloration. I pulled out the power supply to check the bottom side. None of the pin joints looked frosted but I re-flowed them all anyway just in case. I put it back in the computer and powered it up with the same symptoms of the diagnostic LEDs all on and the floppy light on.

Next I removed everything down to the logic (mother) board. I re-seated all the chips that were removable including the CPU. Put it all back together and it booted up like it is supposed to.

Jonathan

[DoN. Nichols]

On 2020-10-01, Jonathan Peart <jonath...@gmail.com> wrote:
> On Monday, September 28, 2020 at 10:21:23 PM UTC-4, DoN. Nichols wrote:
>> On 2020-09-26, Jonathan Peart <jonath...@gmail.com> wrote:
>>
>> > Last week my 3B1 stopped booting. I power it on and see one square
>> > displayed on the monitor in the upper left. The floppy disk light comes
>> > on and stays on. I have tried putting a diagnostic floppy in the drive
>> > but it won't boot. Prior to this problem I was able to boot
>> > successfully from both a floppy disk and the hard disk. I also see that
>> > all four diagnostic LEDs are lit and do not turn off.
>>
>> > From what I can tell in the manual the fix for this is to replace the
>> > logic board. That is not an option for me as I don't have a spare.

[ ... ]

>> Of course -- other things can cause problems, but this I find to
>> be the most frequent source of problems in the 7300 and 3B1.
>>
>> > Jonathan
>>
>> Good Luck,
>> DoN.

[ ... ]

> I checked the power supply connector and didn't see any black or
> browning coloration. I pulled out the power supply to check the bottom
> side. None of the pin joints looked frosted but I re-flowed them all
> anyway just in case. I put it back in the computer and powered it up
> with the same symptoms of the diagnostic LEDs all on and the floppy
> light on.

O.K. It was worth a check -- and re-flowing the solder likely
makes that less likely to be a problem in the near future. If you have
Craig Labs "De-Oxit" (or if you still have any of their discontinued
"Cramolin") pull the power supply connector, spritz the pins and inside
the connector, and slide it up and down a few cycles.

> Next I removed everything down to the logic (mother) board. I
> re-seated all the chips that were removable including the CPU. Put it
> all back together and it booted up like it is supposed to.

Great. Sprizing those with the contact cleaners above before
reseating would have been good too -- but you got enough oxide off the
pins to make it work.