Author Topic: Osborne 1 Repair (Read 4587 times)

willseward · « **on:** May 04, 2018, 11:47:52 pm »

Hello,

I bought an non-working Osborne off EBay in hopes of repairing it in my spare time, but I seem to be way in over my head in terms of debugging prowess. I'm curious if I could get a little bit of help.

The symptoms are as follows:
Won't boot
Beeps continuously
CRT shows boxes and random characters

I've been poking around with a scope and found a few oddities. I'm no electrical engineer, so I could be barking up the wrong tree...

So far I've replaced the Z80A, the '393s in A13 and B14, and the '175 in D14 to no avail. I chose to replace those on a hunch after looking around with my scope. I've included a few photos of the waveforms I think seem problematic.

Any way, any suggestions or tips would be appreciated!

Thanks!

P.S. I originally posted this on VCF, but I didn't get much help...

Image names and descriptions

IMG_0072.jpg
74LS393 in B14 : pin 12 (CLK)

IMG_0073.jpg
74LS393 in B14 : pin 11 (2QA)
Notice the "blip" that only lasts a few ns.

IMG_0074.jpg
MCM4116 : pin 2

IMG_0075.jpg
Z80 : pin 24 (WAIT)

IMG_0076.jpg
74LS175 in D14 : pin 7 (Q1)

IMG_0079.jpg
CRT

willseward · « **Reply #1 on:** May 05, 2018, 01:05:37 am »

Thanks, wilfred!

So about the RAM. Yes. There is one 4116 that is dedicated to the graphics. I have a few 4116s on-hand, but I haven't been able to locate any failed DIMs. I am curious to know the failure mode of the 4116 so that I could sniff out any lurking failures. I might suck it out and replace it to see what happens. It couldn't hurt; the 4116s are notoriously unreliable. And, yes. The dim portion of the CRT is an artifact of the photo.

The character ROM could very well be bad. However, if the ROM is removed, the vertical lines become solid. That and the fact that there are characters generated on the screen in some power cycles leads me to believe that the ROM is OK. You may be on to something about the vertical blank columns, though. I'll have a look around the graphics circuitry again to see if I can find anything suspect.

Re Z80. I agree. Now I have an extra Z80!

The rails are fine: two 5V and two 12V. The power supply is in poor condition, so I have it on bench supply for testing. While running the OCC supply, one of the RF caps blew in somewhat of a spectacular fashion, but I'm not too concerned about it.

I'll post some images of the PCB soon. Nothing is visibly wrong with the board, from what I can see, though: no blow holes or bad joints.

Any clue if the pulse in the scope capture is errant? It seems to be ringing throughout the circuitry around the 'LS393s and LS175. Maybe it's nothing.

willseward · « **Reply #2 on:** May 05, 2018, 01:19:29 am »

No. There are only two +5V and two +12V. There is an extra -10V on the PS, but it isn't connected through to the board. I'll measure the 4116 voltages tomorrow when I get some time to tinker with it.

willseward · « **Reply #3 on:** May 05, 2018, 01:28:12 am »

Here are some photos of the PCB.

Also I've included photos of the strange video terminator that came with it. It seems to be an after-market terminator with a composite connector. It's manufactured by Integrated Systems, Inc. in MI.

And I did happen to catch the "runt pulses" in the latest video. I'm curious if I have a misbehaving cap on the board. I'll investigate that, too.

tpowell1830 · « **Reply #4 on:** May 05, 2018, 02:06:36 am »

Something similar happens to my Osborne every time it sits for long periods. There are multiple connectors and connection points for the boards. My solution has been to clean all of the electrical connection points with eraser (ruby red) and then with alcohol. When I push the connection points back together, I do it several times to insure that any oxide layer is broken and makes good contact.

Hope this helps...

willseward · « **Reply #5 on:** May 05, 2018, 02:09:32 am »

Thanks for the tip tpowell.

I've replaced the 40-pin socket on the Z80 with a double wiper after finding a few pins not making connections. I'll try your method to see if it's worth replacing the other sockets, too.

willseward · « **Reply #6 on:** May 05, 2018, 02:10:37 am »

Also, the rails across the 4116 are correct: -5V, +5V, and +12V.

Chris56000 · « **Reply #7 on:** May 05, 2018, 06:45:21 pm »

Hi!

Documentation if anyone needs it. . .!!

https://www.dropbox.com/s/rguo3ca2hwevshu/2F00040-00_Service2ndEdition_1983.pdf?dl=0

Technical Manual:–

https://www.dropbox.com/s/evcr5h6r4n6uxzv/o1techm.pdf?dl=0

Chris Williams

tpowell1830 · « **Reply #8 on:** May 05, 2018, 10:36:51 pm »

I haven't fired up my Osborne in 10 years, thanks for posting this and reminding the world of the first portable computing system. I had my first exposure to spreadsheets with the Osborne, as well as computer games.

The last time I tried to start the Osborne, the disk or disk drive was faulty, and the PC that I had the Osborne OS backed up on is no more, so I need to find the correct floppy disks and the OS in order to try to start the Ossy again. Or, just donate the whole thing to a museum...

I have the original paper documents, but wanted to scan these into PDF's, now I don't have to thanks to Chris560000. thanks again Chris.

willseward · « **Reply #9 on:** May 06, 2018, 12:31:16 am »

tpowell,

I have CP/M, d-base, and Wordstar. Once I get it up and running, I'll make you some copies if you'd like.

james_s · « **Reply #10 on:** May 06, 2018, 01:20:06 am »

The Z80 is probably the last thing I would have suspected. Unfortunately with stuff like this a scope is not the most effective troubleshooting tool, what you really need is a logic analyzer. You might also look around and see if there's a test ROM available, my friend found one someobody had created for his Commodore PET when he was working on that. Using a logic probe checking the inputs and outputs of logic chips looking for something that has an active input but stuck output can be helpful. Also RAM followed by ROM are some of the most common failures I see in equipment of that era.

willseward · « **Reply #11 on:** May 06, 2018, 02:16:33 am »

Yeah, unfortunately I don't have access to a logic analyzer, so I'll have to continue debugging with my scope.

I have a EPROM reader on the way, so I'll be able to check out the ROM when I get that setup. As for a test ROM, I doubt that is available, but I'll take a look. Word on the street is that a diagnostic program shipped with a few early models, but mine is not an early model.

More RAM updates: I noticed that by piggy-backing the bottom 4 4116's from the right caused the data out pins on the entire columns to output data. Not sure if these are used in the boot process, but it's something at least. Do you think it's worth replacing those for good measure?

tpowell1830 · « **Reply #12 on:** May 06, 2018, 02:21:51 am »

Quote from: willseward on May 06, 2018, 12:31:16 am

tpowell,

I have CP/M, d-base, and Wordstar. Once I get it up and running, I'll make you some copies if you'd like.

Well, I don't have the correct floppies, I think they were 180k single sided, single density? Can you still get those?

Otherwise, I would love to get the old Ossy going again, so that would be great, if you want me to PM my info, PM me and I will give you my info. You don't happen to have the CP/M version of the Pascal compiler do you? Anyone?

james_s · « **Reply #13 on:** May 06, 2018, 05:14:51 am »

Quote from: willseward on May 06, 2018, 02:16:33 am

Yeah, unfortunately I don't have access to a logic analyzer, so I'll have to continue debugging with my scope.

I have a EPROM reader on the way, so I'll be able to check out the ROM when I get that setup. As for a test ROM, I doubt that is available, but I'll take a look. Word on the street is that a diagnostic program shipped with a few early models, but mine is not an early model.

More RAM updates: I noticed that by piggy-backing the bottom 4 4116's from the right caused the data out pins on the entire columns to output data. Not sure if these are used in the boot process, but it's something at least. Do you think it's worth replacing those for good measure?

Absolutely the RAM is needed for the boot process, if you have bad RAM it usually won't work at all. That's one of the more common failures in vintage computers.

willseward · « **Reply #14 on:** May 06, 2018, 11:31:36 pm »

Just replaced the 5 right-most, bottom 4116s. No change.

I'm beginning to believe that the DRAM is not the problem. After reviewing the schematics, I've isolated the "dirty" DRAM signal inputs to the '153 multiplexors (4 of them). What is strange to me is that all 4 are outputting runt pulses. Could that have something to do with the inputs? All the inputs seem clean...

james_s · « **Reply #15 on:** May 07, 2018, 12:52:06 am »

Possible, what are they driving? I recently replaced several 74153's on an Asteroids Deluxe board, they were all the same brand and several of them had failed, it was strange. Debugging with a scope can be tricky when you have tristate signals though, what looks like runt pulses can sometimes just be floating lines when nothing is on the bus at that moment.

willseward · « **Reply #16 on:** May 07, 2018, 01:11:08 am »

The 153s are multiplexing the address lines to the RAM. One drives the column strobe and row strobe.

willseward · « **Reply #17 on:** May 22, 2018, 12:37:32 am »

Well... The 153s were not the problem, so I'll have to keep investigating. My next step is to do a wholesale replacement of the RAM.

I've attached the ROM dumps for 1.43 and 1.3 if anyone should be interested.

Mark0x01 · « **Reply #18 on:** June 26, 2018, 08:02:17 am »

I'm also in the process of repairing an Osbourne 1 that hadn't been powered on in 20 years.

The PSU has received a full rebuild with new caps, and the monitor is working.

Same symptop if garbage on the screen and the continuous beep.

At the moment I'm tracing lower logic levels on some (4) of the data bus lines. These ones only reach max of about 3.5 volts on the scope, so may be the source of the problem.

Hopefully I don't end up with no chips left on the data bus and still have a problem

Mark.

willseward · « **Reply #19 on:** June 26, 2018, 11:52:27 pm »

FWIW, this project was discussed in another forum. It is a Z80 exerciser that can be used to sniff out problems in the board.

https://fjkraan.home.xs4all.nl/digaud/arduino/Z80exer/

Pretty neat project, I think.

FlyingHacker · « **Reply #20 on:** June 27, 2018, 12:12:07 am »

Are those little blue tantalum caps trustworthy in these systems?

james_s · « **Reply #21 on:** June 27, 2018, 12:29:58 am »

Quote from: willseward on June 26, 2018, 11:52:27 pm

FWIW, this project was discussed in another forum. It is a Z80 exerciser that can be used to sniff out problems in the board.

https://fjkraan.home.xs4all.nl/digaud/arduino/Z80exer/

Pretty neat project, I think.

That's totally cool, I've occasionally considered developing something like that but never got around to it. Now if only it had a 6502 mode too, add 6800 and 6809 and it would be a really useful tool for diagnosing vintage computers and arcade boards.

Mark0x01 · « **Reply #22 on:** June 27, 2018, 10:37:21 am »

Thanks for the tip.

It looks like the Arduino Uno that arrived today for a ram chip tester project might become multi purpose with a stack of hats.
I even have a suitable unused 40 pin header with ribbon cable in my parts collection so well on the way.
The prototype hat boards are so cheap now days that it becomes so much easier to build things.

Maybe a library of test routines for common systems will evolve.

I acquired a cheap clone logic analyser last week as well, and was able to borrow a 40pin clamp from my BBC PET to make it easier to use. Retro recycling

I also have a collection of S100 cards in the "fix me" queue, so these will certainly get a workout over time.

I certainly didn't have access to these sorts of cool ~~toys~~ tools when I was bench servicing Z80 systems back in their day...

Mark.

Mark0x01 · « **Reply #23 on:** July 30, 2018, 10:13:15 am »

I now have a z80exer built and running as my first arduino project, for about NZ$50, and it reveals that I have a stuck D5 somewhere on the board.

That would certainly stop it executing code

The data bus goes to at least a dozen chips, so it may be a process of elimination.

I'm starting with the ram and will slowly work through them and hopefully find the culprit.

I'll look at creating some Osborne specific routines once it is working, and should be able to use this on other 80pin CPU's since I created a patching hat to match the z80 pinout with a header for the IDC DIP 40pin Plug cable I have had for a long while.

The cost of a hat is under $10 so can be built match any 80 pin cpu required.


EEVblog Main Site	EEVblog on Youtube	EEVblog on Twitter	EEVblog on Facebook	EEVblog on Odysee

Author Topic: Osborne 1 Repair (Read 4587 times)

Share me