1 Sep
2011
1 Sep
'11
5:30 a.m.
Well, another step forward. Thanks to Tony's advice, I continued
investigating the video output, and found another dead IC (74LS51 this
time). I'm beginning to wonder how a machine could end up with so many
faulty ICs; that's two in just the video circuitry. Maybe the original
PSU short to ground did something horrible. I now get a clean video
display (which rolls on the Monitor ///, but that's not a big surprise).
At this point though, I really need to replace the crystal. Holding it
so the broken leg makes contact with one hand while powering the machine
up with the other really isn't a good way of going about things; this is
why the picture's still rolling, I can't adjust the hold on the monitor
and keep the crystal steady enough with the other hand!
Silly question of the day: I'm guessing no-one would have any idea where
I'd find a 14.25045MHz crystal in New Zealand...
On 1/09/2011 7:12 a.m., Tony Duell wrote:
> ARGH!.
>
> 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.
>
I'm actually wondering if I could do something like this to get the
machine going in the short term, while I wait for a new crystal to be
shipped. Reckon it might be possible to trim the bottom off the
crystal's can and remove it, so that I have access to the crystal
itself, and then solder a lead to the crystal side of the base? As it
stands, the original lead is broken off flush with the base, so trying
to re-attach anything to that is virtually impossible. I obviously still
have a lot of work I need to do on this machine (now that I can see the
video output, it's the typical dead-Apple mess of characters), I don't
want to put it aside for a couple of weeks while I wait for parts.
Cheers,
Mike