Date   : Sun, 21 Sep 2003 23:39:28 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Two BBC Micros need some help!

On Sep 21, 21:46, Andrew Hancock wrote:

> So I've got a non-working lazy power supply and a non-working Issue 4
> Motherboard.
>
> PSU - low voltage any pointers?
> Beeb - cassette motor sticking, long beep at power-on?

> Beeb No.1 Voltage 5.53v
> Beeb No.2 Voltage 4.74v
>
> Beeb No.2 voltage looks a little low, is this within tolerance?

4.74V is a little bit low.  Try pulling off the three sets of faston
tabs for the +5V/0V, tightening them up (squash gently with a pair of
pliers) and refitting.  Bad connections are the most common cause of
low voltage.  My personal rule-of-thumb for TTL circuits is 4.8V-5.2V.

If tightening the connectors doesn't work, it is possible to adjust the
output voltage by clipping out one of the resistors in the potential
divider that supplies the reference voltage.  On the diagram, you'll
see a group of resistors marked with asterisks (this is from memory, I
loaned my diagrams to someone and don't have them back yet).  IIRC ,
there are three in parallel on the lower leg of the divider, and two on
the upper; you want to disconnect one of the lower ones (again, IIRC).
 this however, is a last resort.

CAUTION:  if you intend to do any work on the PSU, disconnect it
completely from the mains, and wait several minutes for it to
discharge.  Do NOT touch any exposed metal parts in the high voltage
side or you may get a very unpleasant (at best) surprise -- the
capcitors can stay charged to several hundred volts for quite a while,
and several metal parts carry that high vooltage.

If only the -5V is way too low, don't worry about that -- it's only
used for the cassette interface and sound.  It won't stop anything else
working.  And the +12V is only used on the external auxiliary power
connector (eg for disk drives).

5.53V is rather high.  That's not good for the TTL.

I'm now slightly confused as to which board in which Beeb with which
PSU is giving which problems :-)   However, from what you've said, I
would guess that the 1770 chip is faulty (jamming the data bus or maybe
an address line, or perhaps it's jamming the NMI line low) and that's
what's stopping one of the Beebs from initialising, hence the
continuous tone (standard symptom of a Beeb that can't run the first
part of the code in the MOS).

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York