Osborne 1 troubleshooting - Help!

[tezza]

I'd like to try to fix my Osborne 1, but I need some ideas of where to start. One or two ppl on this forum made a few suggestions as to what the problem might be when I first mentioned this a few months ago, but I'm hoping to get a little more information with this post.

Basically, the machine won't boot. All I get is a screenful of garbage characters. Sometimes a colum or two of these will be oscollating between 2 different characters. Pressing reset either does nothing or sometimes causes the B (not the A) drive to come on momentarily??

I've scoured the Internet for help but to no avail. I've downloaded the service and technical manuals which are a help with disassembly and schematics, but what I really need is a troubleshooting guide, or as least a pointer as to where I should start looking. What would cause this kind of symptom (I'm guessing lots of things but I'm hoping that's not so). RAM? The PSU? The Z80 chip? What?

Does anyone have any ideas, or know of a troubleshooting guide which describes the above condition?

Some more info.....

This machine hadn't been powered up for years. When I first turned it on, the computer showed the garbage characters. Hmmm.. I turned it off, waited a few seconds, then turned it on again. It then started normally (No garbage). "Whew!" thought I! Prematurely it turns out because after about 2 minutes the computer blew some capacitors in the power suppy (magic smoke and all that)!

I replaced the capacitors but now it doesn't go at all? The machine worked for about 3 seconds after I replaced the chips (the OSBORNE Splash Screen appeared), then the screen suddenly showed these garbage characters (Drives also started randomly spinning). They have been with me ever since!

I'm confident they are the right type of replacement capacitors. They are just RF filters so I wouldn't have expected them to damage any other components when they blew. However my hamfisted attemps at repairing the power suppy may have. But on the other hand, it showed this symptom the very first time I booted?

Any thoughts would be most welcome.

 

[Yzzerdd]

Now I am no expert on the Osbourne. In fact, I have never seen one let alone used one. Garbage characters? Sounds like you have a bad RAM chip or two. Maybe a bad character generator? As for the power supply, does it use a standard ATX supply?

--Ryan

 

[nige the hippy]

I'm with the RAM fault too, I had the same fault on my ozzie 1.

First of all checked the roms, they checksummed ok, swapped the processor because it was socketed too, again no difference, and thought "common point on all this is the 6821" I had a couple & replaced it, but no different there either.

I Didn't have any spare ram so couldn't check that, but at the time I never thought about checking the ram with my finger, which does appear to be the first thing I should have done. You have to be careful you don't end up with "d11A" branded on the end of your finger when you find the faulty one though!

If none are running hot, you can piggy back fresh 4116s on the top of the original ones (no solder required) one or 2 at a time, and see if the fault goes away.

 

[DoctorJ]

Hi,

I found the following service manual which may be of use to you in fixing your Osborne.

https://www.google.com/url?sa=t&rct...0WYk_FWbe6hDnoFXjHzt2Aw&bvm=bv.53217764,d.dmg