[pups] pdp11/05 questions.

John Holden johnh at psych.usyd.edu.au
Tue Jul 8 13:27:46 AEST 2008

On 08/07/2008, at 7:35 AM, Milo Velimirovic wrote:

> hi all,
> The recent flurry of activity in early pre-C UNIX for an 11 with a
> small memory got me back to working on my 11/05. So far I've
> identified two nasty problems with the data paths board, the M7260.
> One of the 8266 MUX chips looks like the plastic boiled and bubbled
> and circuit board is discolored underneath it. I'd welcome both
> sources for replacement chips and techniques for replacing it.

Tricky. The 8266 is rather odd in that it inverts one of the inputs
and I know of no direct replacement. I'll check if I have a spare  
M7260 card.

As for replacing chips, particularly on old cards. The only safe way  
is to:-

1)	with a fine pair of cutters, cut every leg of the chip and remove  
the body
2)	with needle nose pliers, grab each leg in turn, heat on the other  
	with a soldering iron and pull out the leg (use a light force and  
make sure
	the solder is molten on the top side, particularly if it has a track)
3)	with a solder sucker (or desoldering station), remove the solder  
from the holes

> Additionally there's a lifted and broken trace on the non-component
> side of the module near the F edge connector. Any sugestions for
> repairing a damaged trace would be welcome.

If the chip was toasted, then this track probably took all the  
current. You can
either glue it down and bridge the break with solder, or cut the track  
where it's
good, scrape off the PCB lacquer, then carefully solder in some wire.  
If it is
a short run, use some stiff tinned copper, then hot glue in place.

> Lastly, I'd just as soon use a DL11W in the 11/05 rather than go to
> the trouble of setting up an external clock to feed the on board UART.
> I can get both 9600 baud and RS232 from the DL11W instead of 2400 baud
> current loop from the built-in interface. I haven't yet found the
> jumpers to remove/install that would disable the built-in console
> interface. There's also the LTC.

You can disable the serial interface by removing W1 from the control  
logic /
microcode board M7261. It is not possible to disable the LTC, simply  
enable it on the DL11W. I have, in the distant past, modified a M7260  
9600 baud RS232, but it involves removing removing several components  
modifying the 9602 oscillator timing. Using a DL11 is a better choice.


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