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Centris 650 external SCSI interface troubleshooting

I have a Centris 650 with seems to be a faulty external SCSI interface. The internal interface works perfectly fine for hard drives, CD ROM drives, or SCSI2SD. However, the moment I connect an external SCSI device to the DB25 connector on the back of the case, e.g. Apple CD-ROM drive or external hard drive, the machine will invariably freeze on boot at a grey screen without a Happy Mac. I tried a wide range of id assignments to make sure there isn't some unexpected conflict going on to no avail.

Does anyone have advice on how to start troubleshooting this issue?

The Centris is pretty much in stock configuration with 100% tantalum caps from the factory and no damage from battery leakage. I have been unable to find any obvious physical damage on traces or other components on the PCB. The PSU itself is not recapped, but seems to have stable 5V and 12V rails.
 
Thanks for the advice. I do have a SCSI Terminator at the end of the chain, so that is not the problem here I believe.
Now, this is embarrassing, but while reconnecting the devices it struck me to soak the connectors in DeoxIt contact cleaner and work in into both ends on the back of the machine. Color me surprised that both my external SCSI devices came right up after that!
So it must have been bad connection / oxidation on the contacts after all! Sorry for the false alarm.
 

trag

Well-known member
Excellent that you solved the issue.

I think that oxidized contacts are going to increasingly be an issue in this hobby.

I'm programming some Flash chips that have been in tubes for over 15 years, and I've found that if I rub the legs with a Pink Pearl eraser and then clean off the residue with 91% isopropyl that they are much more reliable to program. The chip programmer is over 20 years old too, so it might have drive strength issues. I did replace all the caps in the programmer, pink pearl'ed legs of the socketed chips, and replace the AC adapter.

I need to try that Deoxit cleaner. It might be a lot easier than the eraser method.
 

joshc

Well-known member
Well this is weird. I've just had the exact same problem with a Quadra 610. External SCSI was working absolutely fine and then decided to stop and cause the same behaviour you were seeing (blank grey screen at boot, no Happy Mac). I did the same suggested here, contact cleaner in the DB25 port on the Mac side, and now it works again!
 
[...]

I need to try that Deoxit cleaner. It might be a lot easier than the eraser method.
DeoxIt works wonders on IC legs and sockets too; highly recommended, and as you say, kind of mandatory for our hobby at this point!

Well this is weird. I've just had the exact same problem with a Quadra 610. External SCSI was working absolutely fine and then decided to stop and cause the same behaviour you were seeing (blank grey screen at boot, no Happy Mac). I did the same suggested here, contact cleaner in the DB25 port on the Mac side, and now it works again!
Glad to hear that this was helpful to you! There must have been enough connection for the system to detect the device being connected, but perhaps some critical responses that the system expected weren't coming back due to oxidation.
 
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