Hi, I'm new...

aliensrcooluk at yahoo.co.uk aliensrcooluk at yahoo.co.uk
Fri Aug 4 18:07:38 CDT 2006



 --- Tony Duell <ard at p850ug1.demon.co.uk> wrote: 
> > > > I figured (as you would) that my harddrive
> > > 
> > > Actually, I might not. My first reaction would
> be 
> > to
> > > stick a voltmeter on 
> > > the power lines. 
> > 
> > errr... no voltimeter here, nor do i have a clue
> > where to buy one from (or how to use one).
> 
> Note I said 'my first reaction'. I am something of
 a
> hardware type...
> 
> That said, I do feel that if you're going to run a
> classic computer (as 
> opposed to running the software on an emulator),
> then you are going to 
> have to learn a little bit about hardware and do
> your own repairs. I make 
> no secret of the fact that I totally object to
> making essentially random 
> changes (so-called 'board swapping') in the hope t
he
> machine will work 
> again. I feel the only way to keep one of these
> machines -- in fact any 
> machine -- working is to make measurements, figure
> out what the problem 
> is, and then correct it. The first 2 stages should
> take a lot longer than 
> the last. If you're spending most of your time
> changing parts, you're 
> probably going about it in the wrong way.
> 
> As regards getting a meter, Maplin Electronics, RS
> Components 
> (http://www.rswww.com) and Farnell
> (http://www.farnell.com) all sell 
> them. Personally, I find the best multimeters
> (combined 
> voltmeter/ammeter/ohmmeter) are made by Fluke, and
> that's what I would 
> buy. Problem is they don't come cheap.
> Realistically, you can probably 
> get away with a much cheaper (and less accurate)
> instrument for this sort 
> of work.
> 
> I have no idea what your background/knowledge is. 
A
> good book on general 
> electronics is 'The Art of Electronics' by Horrowi
tz
> and Hill, but that 
> might be rather advanced for you at this stage. Al
as
> I don't know any 
> more introductory books than that.
> 

As far as electronics go, it's practically zero.
I know how the CPU works and runs everything
, in theory, but I can't program in machine
language yet.
As for how the CPU and everything else works
physically, I don't really know anything, but
that is partly why I was keen to sign up
to this list.



> 
> > e
> > > that to pass too much 
> > > current, but from my memory of the A500 circui
t,
> I
> > > can't think of any 
> > > obvious candidate.
> > > 
> > 
> > Ahem, it's an A600 I own ;)
> 
> Yes, I know. I don't have an A600 circuit diagram.
 I
> am assuming the 
> basic design, at least around the PSU input
> circuitry, is going to be 
> similar. 
> 

Ahh, I see.


> 
> > 
> > 
> > > Have you tried running the machine with the
> cover
> > > off to see what is 
> > > getting hot? It might be something as simple a
s
> a
> > > leaking decoupling 
> > > capacitor.
> > > 
> > 
> > I was planning too, but I had concerns about
> > being electricuted (spelling?) as I had never
> 
> If the PSU is external (as I believe it is), then
> there are no high 
> voltages in the computer itself. The PSU outputs
> +5V, +12V, and -12V to 
> the computer. None of those voltages is high enoug
h
> to give you a shock. 
> 
> The most dangerous voltage you are likely to come
> across in classic 
> computing is the mains or a 350V DC voltage produc
ed
> by directly 
> rectifying the mains This turns up in what's calle
d
> 'Switch Mode Power 
> Supplies' (SMPSUs), and most computer supplies are
> of this type. This 
> voltage is lethal. It's also likely to appear on
> metal heatsinks, etc, in 
> such supplies. Don't work on one of those unless y
ou
> really know what you 
> are doing.
> 

Ok... no heat sinks in my A600 :)


> Monitors are often claimed to contain lethal
> voltages. Well, there's 
> mains (and most colour monitors use an SMPSU> circ
uit, so the hazard I've 
> just mentioned is there). But the even higher
> voltages to the CRT are 
> generally only able to supply low currents, and ar
e
> unlikely to be fatal. 
> Don't take risks, though, 25000V is darn unpleasan
t.
> 

No problems there either, as I don't use a
monitor. I plug my A600 into my TV via the
RF cable. The picture quality is good enough
for me.

>> snip <<


> 
> > 
> > What would the "leaking decoupling capacitor"
> > look like, if that was the cause?
> 
> It will look like any other capacitor. The 'leak' 
is
> an internal 
> electrical leak (a sort of weak short circuit).
> You'd have to find it by 
> doing electrical tests.
> 
> > 
> > Which is the capacitor? Anyone know?
> 
> The thing about decoupling capacitors is that they
> provide a local source 
> of energy for the various chips, thus avoiding the
> voltage drop due to 
> the resistance, and more particularly the
> inductance, of the supply 
> connections. Therefore there is one (or more) next
> to each chip. And 
> there's no way _anyone_ can tell you which has
> failed without doing more 
> tests.
> 

Ahhh, ok. 


> -tony
> > 
> > Probably best you didn't get the 16, as it's
> > inferior to the 12!
> 
> Come again? The 16 has a 68000 processor board (as
> well as the Z80) and 
> runs Xenix (along with TRS-DOS and CP/M). The mode
l
> 12 is a Z80 only, and 
> runs CP/M and TRS-DOS, although I belive the 68K
> board can be added later.
> 
> > The 12 was actually made later than the 16
> > and supports various modes making it
> > compatible with 16 and 4 (?) software
> > (and hardware?). Infact I think the Model 12
> 
> I would be very suprised if the Model 12 could use
> Model 4 hardware or 
> software. You certainly can't directly read M4 dis
ks
> in an M12 (due to 
> the fact the former uses 5.25" disks, the latter
> 8"), for example.
> 
> -tony


I'm relatively new to the TRS-80, and can
only go by what I have read in the issues
of 80 Microcomputing I have.
You, on the other hand, clearly are an expert 
compared to myself.


Regards,
Andrew B
aliensrcooluk at yahoo.co.uk




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