Date : Mon, 13 Apr 2009 23:48:50 +0100
From : bbcmailinglist@... (Ian Wolstenholme)
Subject: Easter Disaster
I was measuring somewhere on the motherboard. I know I did the Teletext
chip at one point because a screenful of random Teletext characters appeared.
Apart from that I don't remember anything unusual happening. I've measured
the power connector voltages since the disaster and they are giving the correct
readings. I've also measured most of the chips and they all seemed to be giving
5V or thereabouts, with the exception of the 26LS30 which was giving me about
9V.
There aren't any socketed chips so all I have been able to remove are the MOS
ROM and the keyboard, which I have tested in another machine and they are
working.
I left it on for a few minutes with the speaker unplugged and some of the chips
were getting warm like the vidproc and Teletext chip, but some others, which
I can't just recall offhand, were stone cold. Which is good - warm or cold?
Best wishes,
Ian
----- Original Message -----
From: David Hunt [mailto:dm.hunt@...]
To: bbc-micro@...
Sent: Mon, 13 Apr 2009 22:19:32 +0100
Subject: Re: [BBC-Micro] Easter Disaster
> -----Original Message-----
> From: bbc-micro-bounces+dm.hunt=ntlworld.com@...
> [mailto:bbc-micro-bounces+dm.hunt=ntlworld.com@...] On
> Behalf Of Phill Harvey-Smith
> Jules Richardson wrote:
> > It seems unlikely that hooking up a meter would have killed it, unless
> you
> > accidentally shorted something out (and even then ICs can be pretty
> forgiving
> > at times).
>
> Dammit, broken list config strikes again!
>
> All IIR my Physics O level correctly....
>
> If he tried connecting a multimeter across the psu on a current range
> that will have shorted it, you should always measure current inline with
> the thing you are measuring, as ameters are very low resistance. Better
> still measure the voltage drop through a known low resistance, and use
> ohm's law to calculate the current.
>
> Voltage you measure accros, as voltmeters are high resistance.
>
> Cheers.
>
> Phill.
It depends on where you were measuring the current. If you measured it
across the PSU, it'll survive. However, if you measured the current across
the external bus, or somewhere on the motherboard, the outlook isn't so
rosy.
Put the meter on volts and check the PSU rails and see if you're getting
+5V, it sounds like you're getting -5V as the speaker is working. If you
shorted out the 1MHz bus there is a custom IC that will need replacing, some
have sockets on earlier revisions, e.g. I have an Issue 1 with half the ICs
in sockets and an Issue 3 board with only one IC in a socket (excluding ROMs
of course!)
There is an informative section in the Master 128 Service Manual, see
section 7 the article is at;
<http://bbc.nvg.org/doc/Master128ServiceManual.zip>
Good luck with the repair ;)
Dave
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