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Sick SBC6120

bobaboba

Experienced Member
Joined
Sep 7, 2013
Messages
400
Location
Scotland
Looking for something else in the workshop I came across a SBC6120. Well, well, forgotten all about it. I bought the bits soon after it was announced so I suppose around 2002-3 or so, built it, fiddled with it then put it away.

So, hooked it up, prepared an OS on CF and tried it. Not so good, the status lights flicked back and forth continuously between (supposedly) a RAM failure and running. Left it running like that while I browsed the manual for a bit and when I next looked it had settled down and identified the CF card. Tried to boot it, got the ‘.’ and OS/8 came up. Well well.

But it does this every time so something’s amiss. When switched on it errors for several minutes then settles down presumably after something’s warmed up, not that anything much gets very warm of course but maybe a single junction somewhere - who knows. Doesn’t seem to help it varying the supply voltage a little either. Once running it does seem OK except for a single oddity - Adventure seems to hang for some reason-???

Fished around the www for a bit and downloaded some more recent ROM images and burned some EPROMS to try. More bad news - with these there’s a hard error on boot which diagnoses a failed 6120. Odd - I don’t think the newer ROMs do a more extensive processor test, just add a few extra commands to the ‘bios’....

So I think I’m a bit stuck for the moment. So near and yet ... Don’t have any spare chips to try swapping for the processor or RAM so looks like a short pause while I consider what to do next.
 
So going back to the original ROM image still sort of works? After the settling time?
 
For what it's worth, I built a few of these AFTER I wire-wrapped one. On my wire-wrapped board (about the size of a sheet of US paper), I could not get it to start correctly until I put a capacitor somewhere (but I don't offhandly remember where). I would look at the crystal clock circuit and possibly tweek the values on the (10-30 pf?) caps connected between the crystal and ground. Also, if you can, ground the body of the clock crystal itself.
 
Thanks all.

Yes - still works (?) unchanged with the original ROMs
Oscillator seems fine on the ‘scope.
My first thought was to reflow the solder joints. Didn’t affect things though and nor does flexing the board.
Haven’t tried replacing or shunting any capacitors yet aside from a decoupler inboard of the polyfuse. Also tried shorting past the fuse (it drops quite a few hundred mV) but no change.

I’m a bit perturbed by (a) Adventure hanging and (b) the newer ROMs flagging a processor fault but of course it could still really be anything.
It doesn’t seem to operate over as wide a voltage range as I’d expect - should really work easily over 0.5-0.75V variation around 5V I’d have thought but it’s more sensitive than that which to me leans towards a chip issue.
 
I have two 'spare' 6120 processors here. They are spares for my SBC6120-RC in case I have a problem in the future. If you exhaust other lines of enquiry and want to test with another CPU, I am sure I can help out with a loan. If it fixes your problem, I am sure we could come to some arrangement.

Dave
 
Very kind Dave. It might come to that though I’m considering trying some of the basic 8e diagnostics to see if that points somewhere. Just need to make up some suitable images. Time, time .......
 
>>> 8e diagnostic...

That sounds like a good plan.

Let me know if I can help further. I may actually be in Scotland at some point in the near future. Edinburgh airport on the way to Kelso. Not sure when though.

Dave
 
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