Repair methods (was Cromemco 3101/Beehive B150 score)

[Tony Duell]

> 
> 
> >
> >> An interesting question.
> >>
> >> If you have a module of the same age, I don't see any problem with a
> >> module swap.
> >
> > My main objection, actually, is to _random_ module-swapping, as  
> > suggested
> > by certain HP calculator service manuals. How you can _know_ the fault
> > has been cured if you don't know what/where it is is beyond me.
> 
> With a modern machine with just three or four modules (say power supply,
> logic board and one RAM module) then you take an informed guess and can
> swap modules until you guess right, unless of course a module you did  
> not
> swap causes failure of another module. Then it gets expensive.

That is _exactly_ what I object to. Fault finding is not (or rather 
should not) be bbased on guesswork. Unless you've found the fault 
logically, you can't know you've fixed it.

I've got stories of where a problem in one module appeared to be cured by 
replacing a different one that was, say, more tolerant of timing errors 
on one of the signals. Of course the problem came back as the timing 
error (which was the original fault) got worse.

And this is one reason I don't have a modern computer. I don't have (and 
can't afford) the necessary test gear to be able to _prove_ which module 
is at fault.

> I have scrapped an almost complete machine, so I have many of the  
> commonest
> modules. I understand digital electronics but I find it hard to get  
> my head around
> the analogue electronics on the boards. Testing bare components is  

'Digital circuits are built from Analogue parts' (one of Vonada's lawas 
IIRC). 

I haev never understood how you can understand digitial electronics 
properly and not understand analogue electronics. I certaimly couldn't 
understnad digital stuff until I understood things like transmision 
lines, termination, etc.


> hard enough
> but testing them in circuit is tricky, especially if they require  
> half a dozen different
> supply rails at weird voltages such as -18.0 -17.1, -12.6, -6.3,  
> -4.6, -2 and +12.6.

I think if I had a machine like that to maintain, I'd make up a test rig, 
with a power supply giving those voltages.


> 
> 1960s components which look right are not easy to get new. The

To be honest, I don't care too much about the appearance of the 
components, provided they are electrically correct. There are certainly a 
few obvious modern replacements in my 1968 HP9100B.

  
-tony