Repair methods (was Cromemco 3101/Beehive B150 score)

[Roger Holmes]

>
>> 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.

On my old machine, with 4500 separate PCBs, its out of the question.  
I have
to fault find down to where a signal fails, and then comes the  
interesting bit
with a wire-or machine, which of the OR inputs is dragging a signal down
to -6.3 volts? Or is it a short circuit to a supply rail? Sometimes  
insulating
individual signals helps, sometimes it easiest to swap one module at  
a time
until the problem goes away.

>
>> That's what I do, but my modules are probably logically rather
>> smaller. One printed
>> circuit board holds one 3 input not-J not-K flip-flop. Another type
>> has four And gates.
>
> But why not repair them? I can't believe they're that complicated  
> to test
> or debug. And I would have thought the components were a lot easier to
> find than complete modules.

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  
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.

1960s components which look right are not easy to get new. The  
resistors all have
two green bands where you would expect to find a gold or silver band.  
Electrolytics
capacitors are all orange coloured, and bigger than their modern  
equivalents.
I have not seen any GET872 transistors for sale for a long time, and  
the last time
I saw an OA5 diode in the RS catalogue they were a pound each.

So when I run out of modules I will be down to taking components of  
boards, testing
them and soldering them onto other boards. Several of the modules  
have multiple
circuits (e.g 4 AND gates) on them, so that makes it a lot easier.

Surprisingly I have had success lately getting big Thyratron valves  
on e-bay, though
they all seem to be on the American continent, which pushes up costs,  
what with
shipping charges, VAT(tax) and handling charges. The thyrotrons are  
inside the
magnetic tape decks were built by Ampex in the States, though the  
rest of the
machine is British, using British components and built like a  
battleship. Oh, except
the Card Punch, which is to an IBM design dating way back to the days  
when
BTM (British Tabulating Machine company) were the licensees for all IBM
technology throughout the British Empire. The basic mechanism has  
been seen
with 'Lease-Lend' stickers on it, though the electronics underneath  
has been
radically modified. I will not ramble on any more.


>
>> When I eventually run low on spare modules I will have to start
>> component level
>> repair, hopefully being able to make 3 good modules out of 4 bad  
>> ones.
>
> That is one reason why you should never throw away a defective PCB. It
> might well be repairable later, or you might be able to use parts  
> from it
> to fix another board.

Absolutely, though I have sold common ones on e-Bay as novelties/ 
souvenirs.

> OK, 'almost never' -- ISA cards with one ASIC and
> not much else don't count :-)

No, not much hope there.

Roger Holmes.
1962 ICT 1301 mainframe nut (and Apple software author)