Repair help: OSI 600

[ianoid]

Hi,

I have a Challenger 1P with a 600 motherboard inside. Initially it wouldn’t get to the select screen, but I swapped out the 6502 and now have it working up to the initial select screen. the unit does not seem to respond to anything other than break (which gets me to the DCWM screen). Any suggestions to get the keyboard working? I prefer just to swap out chips but I am a hack with a scope if necessary. I reseated all of the socketed chips already. I also put in two 8T28 chips (DS8T28N) that were missing, although I’ve since heard that those are not necessary.

This is my first experience with OSI computers. I hope to get this one working!

Thanks for any assistance you can provide.

-ian

[erazmus]

Do you have the caps-lock engaged? The Shift keys don't behave the way you think they should, and the prompt is expecting upper case characters, so you need the caps-lock key pushed in (it's a latching key).

[ianoid]

Thanks for the response.

I did try it with shift lock engaged and not engaged. And I tried it with combinations of either shift key in case one wasn't working. I tried exercising the keys as well, since that can be on thing on other vintage computers.

[Klyball]

The sams manual has the waveforms for trouble shooting , should be in the manuals section c1p

[dave]

The good news is that a lot needs to be working to get where you are.

I would have put my money on shift lock as well. In fact, just in case the switch is bad, you may want to try shoring the shiftlock pins just to make double sure that's not the issue. If not, then move on to the keyboard circuit.

The keyboard rows are driven by a pair of 74LS75 latches at U2 and U3. You can check to see if they are being strobed by looking for a pulse on the WKB (write keyboard) line at any of pin 4 or 13 of U2 or U3. If so, you can check to see if the data is getting properly latched, but using the WKB signal as a trigger for the scope, then looking at each pair of input and output for U2 and U3 (i/o pin pairs are 2->16, 3->15, 6->10, 7->9). See page 11 of the 600B schematics.

If everything looks good there, check the ~RKB (Read Keyboard, active low) line to U4 and U5. pins 1,4,10, and 13 of U4 and U5 are all tied to this signal, so you can probe any one of them. If you see pulses on that line, then you can trigger the scope from ~RKB, and check the inputs and outputs of the 'LS125's to see if they match up (pin pairs 2->3, 12->11, 5->6, and 9->8), again see page 11 of the schematic.

You also may want to check to see if the column lines are properly pulled up. I wouldn't expect otherwise, but it's always good to verify the basics, including power to each of the chips in a suspect circuit, etc.

I prefer to debug over swapping chips, but I certainly understand the appeal of chip swapping. If you do so, then you can do it smartly. For example, looking at the schematic, the SHIFTLOCK key is row 0, column 0. 'M' is R2, C2. So a bad U2 or U4 might prevent a shiftlock detection and/or detection of 'M', while a bad U3 might interfere with detecting 'W', and U5 would interfere with "D", "C", and "W". Since it's unlikely that all those chips are bad, U2 and U4 would be good targets for swapping out. If you don't have spare '75 or '125 chips, you caould try swapping U2&U3, then U4&U5 and see if you can at least get "M" to work (and don't forget about Shift Lock!)

Good luck,

Dave

[ianoid]

Ok so I did what evaluation I could using a multimeter and I believe it’s not the keyboard itself that is faulty. I was able to get voltage changes pressing most of the keys out of U2 and U4 (something like 4.3v drop to 3.9v). And specifically I found pin16 of U2 to be responsive to the shift lock. So I believe there is likely something wrong with the keyboard circuit. Looking at the schematic, Break doesn’t use the keyboard circuit, rather it goes straight to the CPU, so that might make sense.

I have an Analog Discovery 2 which is a USB interface for the Waveforms scope software that I could use to see if things pulse, although I’m an absolute hack with it, having used it only once for a project. I don’t know how to set up a trigger, which would be nice to figure out, although I’m really tempted to try swapping U2, U3 and U4 one at a time with new chips and seeing if I get anywhere.

This has so far been an interesting project, forcing me to look at the schematic in the servicing data, which outside my comfort zone.

I really appreciate your detailed assistance, which has come in handy. I will mess about some more and see if I get this one working. And then, when I have time, I have a second Superboard 600 to test out.

[ianoid]

So I swapped the chips at U2 U3 U4 and U5 with new chips, but there was no improvement.

None of the 4 keys responds. Really I can’t prove that any key on the keyboard works aside from Break, which seems to have it’s own line and doesn’t indicate much.

Any suggestions moving forward, assuming we’ve ruled out those 4 chips?

[ctalmeida]

Hello,

I had a problem similar to that.

Look viewtopic.php?f=3&t=589

Was the U17 that select the BASIC chips. Look those posts.

I removed it put a socket and a new one, all worked. Not saying that is the problem but can be something similar.

[ianoid]

Thanks for your ongoing comments and help.

Trying to return to the Superboard 600 which does not have keyboard functionality aside from the Break key. I can work on socketing and replacing U17 (74LS139) as in the previous post, presuming I can find a replacement chip, and even run through replacing out U16-U23, but it would be nice to have a better thought out approach. Trying to figure out where I got the idea in the service manual that those chips are involved in keyboard input. I did buy a logic probe as suggested but haven’t messed with it.

Also have a second Superboard 600 that also has issues, but it doesn’t get to the boot screen. the keys respond only by showing some change in the gibberish on the screen. This board has several chips the other one does not, but it was missing the 8T28’s at U6 and U7. Actually had some lying around so I put them in but it doesn’t change anything. I pulled them and set them aside. I tried swapping out the 6502 even though I didn’t expect that to help since I have some of those lying around, and it didn’t help. I plan to go through the schematic and check waveforms with the service manual and see if I can pick something up.

I guess I don’t understand which chips are non-essential and if there are any missing on either board that I need to add.

(Board that does not boot at all)

94C278D6-563D-4758-B0D7-E6759B207529.jpeg (3.25 MiB) Viewed 7709 times

(Board that does not boot at all)

6A1646C7-CE41-4DA7-9FF2-11110001ECA8.jpeg (3.42 MiB) Viewed 7709 times

(Board that does not boot at all)

6BDE516B-76FC-475C-AAD3-C04947EFF36D.jpeg (4.39 MiB) Viewed 7709 times

(Board with keyboard issues)

97617E34-3E43-44B9-B303-5B30AD30A4F8.jpeg (4.34 MiB) Viewed 7709 times

[Mark]

From your screen shots, it looks like the pictures were taken after the RESET has been hit. There is a partial screen clear, and the part that looks like this near the bottom left:screenshot

DCWM-lo.gif (1.29 KiB) Viewed 7683 times

is actually the D/C/W/M? prompt, but only the lower 4 bits The upper 4 bits are 0. Since other parts of the screen look OK ($20- space) it may just be a bad RAM chip causing the display issues, or some broken/bridged traces. The fact that you are getting these characters means a whole lot of circuitry is working correctly. Why is the screen clear incomplete? Is it a problem writing or reading? If you power on without reset, or without the 6502, do you get any characters displayed on the uncleared lines that have anything displayed greater than the 1st 16 characters in the OSI graphics set? If not, then I'd guess a bad 2114 in the upper 4 bits, since it doesn't affect all lines. Do any lines get random higher bit characters? (It may take a few quick power off & on to scramble the memory, or power off even longer to make memory fade. Its possible RAM only fades to 0's too which wont help.)

For keyboard problems, use the logic probe to see if the select lines are being accessed DF00 -> 74LS125 U4/U5 (pin 1/4/13/18) DF01 ->74LS75 U2/U3 (pin 4/13) if not then the CPU is perhaps crashed or off in some other code or address select is bad.

You definitely need 8T28 U24/U25 not sure what the extra proto area stuff is. Maybe bottom pictures too?

Anyway, some things to look at...