31 Aug
2011
31 Aug
'11
4:10 p.m.
On Wed, 31 Aug 2011 20:12:48 +0100 (BST), ard at p850ug1.demon.co.uk (Tony
Duell) wrote:
From what I rememberm the PSUs are actually Astec
units. The scheamtic
shown in the reference manual doesn't match the PSU in my Europluses, but
then again it's very psosibl that several versions
of the PSU were used
over the years.
Yes, most if not all are from Astec, as far as I can remember. I've never
had much trouble with them, other than caps going bad. It's very cheap to
buy a new set of caps thouugh, I did that with the last Europlus I worked
on.
When I wsa sortign out my (totally dead) HP9820, one
of the first checks
I did after ensuring all the PSU rails were correct was the clock
circuit. This starts with an 8MHz crystal oscillator. Checkign the output
of this circuit (a couple of TTL gates, some R's
and Cs, and the
(socketed) crystal) showed a very distoete waveform at aobut 20MHz. This
remained the smae when the crystal was unplugged, and was acutlaly just
the natural osciallation frequencyt of one of the gates with a resistor
linking output to input. Fitting a good crystal got it back at 8Mhz.
Intrigued, I carfully cut the can off the defective crystal to see that
the quartz (?) plate inside was acutally broken in half (there's a
picture of it in mu flickr account). I asuem this must have happend when
the machine was dropped or something.
In a way I'm lucky then. The crystal is on very long legs, hovering above
the board, and it was completely obvious that it wasn't attached as it
should be. Easier than an invisible fault.
Cheers,
Mike