Date : Tue, 23 Sep 2003 04:36:04 +0100
From : Mike Tomlinson <mike@...>
Subject: Re: Two BBC Micros need some help!
In article <6A4F555CEA3CD5118DBD0002E30AC0912C07DD@...
consultants.co.uk>, Andrew Hancock <Andrew.Hancock@...
consultants.co.uk> writes
>
>Okay no further on the power supply problem, so any help tracking power
>supply faults would be appreciated.
>
>All I can tell you is the supply 5v/12v/-5v supply looks rather low,
>unloaded disconnected from Beeb motherboard.
>
>Testing voltages when connected to the Beeb motherboard loaded, there seems
>very little voltage 1v possibly.
Be sure you have all the flying leads connected to the right places.
The PSU will make a quiet ticking sound if the outputs are shorted.
Change C9 (next to the switching transistor heatsink on the primary
side.) Carefully inspect all capacitors on the output side for signs of
swelling or leakage and replace any that look suspect. Use your nose to
see if any smell 'burnt'. If in doubt, replace. If you have an ESR
(equivalent series resistance) meter, it makes testing caps in circuit
much easier. Check the mains power lead has not fractured at the strain
relief grommet (caused by winding the lead around the Beeb for storage.)
And as Pete says, be *very* careful working on PSUs - they can hold a
lethal charge for quite some time after being unplugged. There is a
bleed resistor in the circuit, but it can and does go open circuit
(common on the Master PSU.)
>I've found if I remove the WDC1770 chip from the upgrade daughter board,
>that the Beeb power's up fine.
Sounds like a duff 1770. Unusual. Might have been killed by plugging
in a disc drive with the power on.
>I've not got enough logic at present to install the original 8271 kit that
>was removed, I must have used the 4013/4020 elsewhere!
Cheap CMOS logic, easy to get replacements. www.cpc.co.uk,
www.maplin.co.uk, http://rswww.com, etc.
>Is it a good test to try and install the original 8271 interface?
Yes, especially if you already have the 8271 chip itself and most of the
supporting logic. Get the 8271 installation guide from one of the doc
sites (try www.bbcdocs.com.)
The 8271 is as rare as hen's teeth now - it was already obsolete when
Acorn designed the Beeb; I suspect the designers just cut'n'pasted in
the working disc control circuitry from the original System 1/3/Atom
(the Beeb's predecessors) without considering the availability of the
chip.
I always wondered why Acorn didn't use the newer 8272 (I think it's pin
compatible with the '71) which would have offered double density. It
still lives on now in the uPD765 chip which is the basis of all PC
floppy controllers.
--
A. Top posters.
Q. What's the most annoying thing on Usenet?