Date   : Mon, 13 Apr 2009 22:19:32 +0100
From   : dm.hunt@... (David Hunt)
Subject: 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