Ebay horror ...

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu Jun 14 15:20:15 2001

Yes, those odd-ball high-cost or many-years-obsolete devices certainly are
different from the popcorn $4, new 4 for $1, used serial boards. Of course, a
"real" serial board, with, say, a TCM 78808 instead of the garden variety uart
on it, well, that's not so easy to fix. I remember the DEC guy telling me that
it's cheaper to toss one of their 16-port boards than to have it fixed.

I never liked DEC ...


----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Thursday, June 14, 2001 12:03 PM
Subject: Re: Ebay horror ...

> > > > minutes it takes to swap them, and it costs 100x that much to fix the
> > > > board, at a minimum, it seems to make sense to swap the board. That
> > fix
> > >
> > > Are you seriously telling me it takes you 1000 minutes (or over 16 hours)
> > > to fix a serial board? The only time it could take me that long is if the
> > > 'serial board' was something like a DEC DMR11, and it had a nasty logic
> > > fault in the microcoded processor.
> > >
> > I'd never spend more than that 10 minutes I mentioned before on a serial
> Nor would I spend any longer than that, because it wouldn't take me any
> longer than that...
> > board, because they only cost 50 cents or a dollar at the thrift store. If
> You also have to consider the time taken to go to the thrift store, buy
> the item, and come home. In my case here that would be around 1 hour, I
> think. Hmm....
> > have a complicated serial board, i.e. one that doesn't talk the "normal"
> > asynchronous protocol, and that runs at a higher rate, e.g. 4.762 Gbps, and
> The DMR11 is not particularly fast (in fact it's slow by modern
> standards), but it does have a 'processor' on it (a lot of TTL parts and
> microcode PROMs). It's not the sort of thing you can replace with bits
> from the local PC shop, charity shops, etc. Oh, and it's a Unibus card
> for a PDP11
> [Before the DEC enthusiasts here leap on me, yes I know it's physically 2
> PCBs. I have enough of them, the printset, etc.]
> [...]
> > ones with socketed components seldom break. It's only going to break if the
> > part is soldered down.
> Isn't that the truth :-(
> [...]
> > > > True, but not if you don't fiddle with the cables. Moreover, even the
> > >
> > > Actually, a serial port meeting RS232 standards should be able to
> > > hot-swapping of the cables. Shorting any 2 pins together, or to ground
> > > do no damage.
> > >
> > Then why do they break? and why don't they socket the transmitters and
> Because half of them don't actually meet RS232 standards!. And dodgy
> grounding doesn't help. If the peripheral (or whatever) is not properly
> grounded, the 0V line can be floating at half mains voltage wrt local
> ground (due to the capacitors in the mains filter). That can zap the
> RS232 buffer chips.
> > receivers?
> Because sockets cost money, and most users wouldn't know how to replace
> chips anyway... I have seen devices where the I/O buffer chips are
> socketed (and almost nothing else is), presumably to make them easy to
> replace, though.
> > > Over here 'charity shops' (==thrift stores) don't generally sell
> > > computer parts. They may sell complete computers if you are lucky (and
> > > can find a volunteer to do electrical safety tests). But I've never seen
> > > a serial card or anything like it in one.
> > >
> > The common PC at a thrift shop has had the "interesting" boards removed,
> As I said, this is a country-specific thing.
> Over here, mains-operated electrical stuff has to be safety tested by a
> 'qualified person' before it can be sold. Since most charity shops are
> run by volunteers who have no knowledge of electrical stuff, most such
> shops can't sell second-hand computers, etc.
> And they certainly have no clue as to what 'interesting' boards are. I
> doubt they'd ever consider opening up the case.
> > PSU and fan costs $22. At the thrift store, you get a used fan with the
> > but it will get you through a few days until you can order and take delivery
> > a new case.
> TO be honest I'd rather by a _good quality_ fan from somewhere like RS
> components and install it in the original PSU. It'll cost more, but I
> won't be fixing the machine again in a year's time...
> > > I have to conclude that either you're soldering with an arc-welder or you
> > > are taking months to change a single chip.
> > >
> > It's not months, but electricity isn't that cheap here. My soldering iron
> I am still very suprised. Soldering irons are not exactly high-power
> devices...
> > butane, by the way, but still, it's much cheaper to replace the entire
> I must get a butane soldering iron sometime -- for 'field' repairs. When
> I'm at the workbench I prefer a temperature-controlled soldering iron.
> Period.
> > > For 'static' equipment (i.e. not stuff that's carried about, mounted in
> > > vehicles, etc), turned pin sockets (machined pin sockets) seem to be
> > > reliable. I always use them on my prototype boards, not because it makes
> > > it easier to replace chips, rather that it's easier for me to pull chips
> > > and force signals high or low when tracing design bugs.
> > >
> > Yes, but they cost like the devil, and the pins on the wire-wrap types are
> Sure, but they are relaible. I don't like wasting time tracing faults
> caused by poor-quality components. And if I'm demonstrating a prototype
> I've designed, I want it to work reliably.
> Having had a few problems from folded-contact sockets, I went over to
> turned pin ones exclusively. And never had any more problems...
> > long to suit me. For soldertails, I like the ones made by Burndy and Augat,
> You can always cut them dowu...
> > > Once I've got the design worked out, I socket (unless there are good
> > > reasons not to) :
> > >
> > The sockets take up considerable space perhaps better used for air flow.
> > a reason not to.
> If it's one of my own devices, I'll just use a slightly larger box and
> keep the airflow that way. If it's going in a cardcage, then yes, I might
> need to not use sokects to keep the airflow right.
> -tony
Received on Thu Jun 14 2001 - 15:20:15 BST

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