Help w/DEC H742 +15V...

Tony Duell ard at
Wed Apr 15 09:56:23 CDT 2009

> > And waht behaviour is that? Can you do anything from the front panel, for 
> > example?
> >
> >   
> Ah, sorry - got lost in earlier threads.  I get a few seemingly random 
> lights on the front panel -- always the same ones, though:
> The "Run," "Bus," "User," and "Console" status lights are always lit, as 
> are bits 17, 10, 8, and 7 of the address, and bit 14 of the data.
> The console is completely unresponsive, nothing has any effect.

What, if any, terminator do you have at the far end of the Unibus? If 
it's an M9302, pull it out for the moment.

> > I trick I commonly use if I don't have the right extender card is to pull 
> > the board I want to work on and solder lengths of wire to useful points 
> > on that PCB. Then put the board back in place, make sure the wires aren't 
> > shorting to anything and then power up. I can then connect my test gear 
> > to the free ends of the wires.
> >   
> If I was as good at this as you, then that might work for me :).  As it 

(1) I am not that good...

(2) The only way to get experience (which is really what you need) is to 
work on these old machines.

> is, I'm hoping that once I get some extender boards I can start 
> debugging with my logic analyzer and learn how to work on this machine 
> as I go along.

There's no doubt that exteneder boards are very useful (why else would I 
have a set :-)), but it is possible to manage without them. Don't let the 
lack of an extender board act as an excuse for not getting this amchine 

> > It's slower than using an extender card, of course, but typically I only 
> > need 2 or thre 'goes' (pull board, solder on half a dozen wires, put it 
> > back, test) to find the fault.
> >   
> Can I fly you out here to fix mine? :)

In principle yes :-). In practice, there is no way I would get on a plane 
while the current daft 'security measures' are in effect. I would only 
consider travelling if I could take my toolkit with me as hand luggage. 


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