restoring a second OSI

[dave]

I'm getting more suspicious of U5E. Are the chips socketed?

[nama]

No, unfortunately these chips are not socketed. Some are, but most are not. Fortunately I have a hakko desolder station

[nama]

Ok...I have some great news, some good news, and some not so good news.

Firstly, between your vulcan logic brain and my persistence to track down a problem, we both guessed right. U5E was indeed the culprit. I desoldered the chip, dropped in a socket and a new 74LS163 that I bought today...the video board fired right up. I plugged in the RAM boards (after fixing a couple of broken wires for the bank selects), and the CPU board that I fixed previously and I was able to load and run Galaxia over serial perfectly. I then loaded my memory test program and that brings me to the next piece of good news...all RAM checks out ok. Surprising!

Now for the bad news. I plugged all the boards back into the machine. fixed a bunch of frayed wires, and fired it up again. This time it still showed the random characters...good! The bad news is that of the two power units that are installed in the machine only the rear one which is connected to the +5V on the backplane seems to be working. The one mounted closest to the front of the machine, and which connects to the +9V line on the backplane seems dead. I have tested the voltages coming out of the transformer and it seem to be at about 10.2VAC. I tested the voltage from either side of the large capacitors and it reads 13.3VDC. By comparison the working power unit reads 9.4VAC and 10VDC respectively. The output from the bad unit is 113millivolts. The voltages on the CPU board are also in the millivolt range. Unfortunately power is an area I know absolutely nothing about. If I can't stick a logic probe on it I'm lost.

In other bad news, the keyboard also seems dead. When I plug it in (when the boards were set up and working). The machine jumps right into the monitor prompt and is unresponsive to any key presses...something to fix after I figure out the power supply.

Step by step, getting closer...

Phil

[dave]

Fantastic! Seems like you had this one in the bag without any help.

As for the power supply-- To my knowledge, OSI bought all of those, and I am not sure if they always used the same vendors or models. I don't ever recall seeing the schematics in the manuals. Can you post some pictures of the bad power supply?

Is the second supply powering a RAM board?

Dave

[nama]

Here is a pic of the two power units:

Power1.jpg (78.55 KiB) Viewed 13903 times

The non working one is on the right.

Power2.jpg (53.64 KiB) Viewed 13903 times

Power3.jpg (47.57 KiB) Viewed 13903 times

I will temporarily disconnect the power line to the backplane so that I don't accidentally put a crazy amount of power through the boards.

[nama]

I think I have the problem with the power supply solved.
The units are made be a company called Deltron, and have as one of their components a part called CA723CE. See here:

http://www.datasheetcatalog.org/datashe ... 944_DS.pdf

As these parts are socketed it was a no brainer to try swapping one from the working unit into the non working unit. Now I get just over 5V...Yay!

I will need to order one of these parts from somewhere, as I don't know if they will be readily available in Akihabara.

EDIT: Actually there seems to be a lot of equivalent parts:
EA33X8469 149255 221-Z9020 276-1740 3100001 569-0320-831 569-0540-623 723 723C 723CJ AMX3548 CA723CE CA723E EA33X8469 ECG923D GEIC-260 HA17723G IC-20 IC-20(PHILCO) L123CB LM723CN MC1723CL MC1723CP ML723CM ML723CP NE550A NE550F NE550N NTE923D RC723D SA723CN SG723CN SK3165 SN52723N SN72723J SN72723N TCG923D TDB0723A TM923D UA723CA UA723CJ UA723CN UA723DC UA723DM UA723HM UA723ML UA723PC ULN-2723A ULS-2723A WEP2331 WEP2331/923D

As you can see from the photo in the post above, there are some wires where all the plastic insulation has fallen off, I will need to desolder them from the power switch and put some shrink wrap over them to ensure that they don't short out on anything.

Phil

[dave]

Nice progress! I think the LM723 is a pretty common part; should be easy to find. Good for you for looking up the part cross reference. I started a list of part numbers for replacements for various OSI parts, but it's pretty short at the moment. I will add this one to the list, since I suspect that others will see the same failure, and may not know to do the cross-ref.

I'm looking forward to following your progress. . .

Dave

[nama]

Got a new regulator today, and replaced the dead one. Unconnected the leads before powering up. Tested the voltages, and all seemed good. Reconnected the leads and powered up the machine. The 4P looks to be working perfectly except for the keyboard, which I'll get to later. For the time being I'm using the C4-2P's keyboard.
Next I pulled out the disk drive unit, and powered it up. It made a horrible humming noise. Powered down, and powered up again, this time the internal mechanism started spinning, sounded a little bit like someone using a rotary sander, but it was spinning. I ran a 5.25" disk cleaner disk to make sure the heads were clean. After about 2 mins the unit stopped spinning and made that horrible humming noise again. Powered down, and up again, and it started spinning again. I'm guessing it may need a little squirt of lubricant somewhere to loosen up the bearings...but where?
Unfortunately I don't have any OSI disks to test the machine with, so I won't know if it can actually load anything until I beg, steal, or borrow one.

Today was a good fix day, I feel satisfied.

[dave]

That's great about the power supply. The disk sounds painful. It looks like you are destined to have a look inside. . .

[nama]

Check it out:

Fixed.png (167.3 KiB) Viewed 13903 times

Floppy is still not tested, but it got quieter after a squirt of CRC lubricant and having been left on for a while.