PDP 11/45 partial success.

Mike Cesari mcesari at comcast.net
Fri Mar 18 15:38:03 CST 2005

On Mar 18, 2005, at 1:42 PM, Jim Beacon wrote:

> Hi All,
> after a few days frustration, I have got a little further with my 
> 11/45.
> The power supplies checked out OK with respect to ripple and output, 
> but a
> couple have them have the odd VERY fast spike on them - is this 
> normal? (I'm
> using a 100MHz 'scope - I suspect they may not be visible on a slower 
> one).
> After reducing the machine to its basics, I found I could load an 
> address
> from the front panel, but not write or examine any location 
> successfully.
> The address and data lines became active, but the processor then goes 
> into a
> pause state. I found that the MSYN line was not being asserted, and it
> appears that one ic on the Unibus controller (M8106) is not providing 
> the
> correct ouput - grounding the scope probe will force the line, and the
> processor completes its bus cycle (Yes the inputs to the ic are 
> correct!).
> I have found a spare serviceable UBC, so I'll try that when I next get
> chance, and if that fixexs the fault, I'll repair mine.
> Jim.
> Please see our website the " Vintage Communication Pages" at 

If that fast spike looks like:

   5v _|___|____
       |   |
       |   |

   0v ________

... or similar, this can (will) cause intermittent problems when you 
load the system
with usage like installation, compiles, etc. Especially when the spikes 
bottom out at
less than 2.5v. We saw this cause very "odd" system crashes on 11/70's. 
Components on
the h744 regulators started to degrade, `cause DEC pushed these to the 
We replaced the two output electrolytic caps, the output (power) 
transistor, and the
two disc caps that should be at the mate-n-lock that should go from 5v 
to ground.
(These are an ECO from the late `70's.) If the spikes persisted, we 
also replaced the
remaining components on the heat sink, the big filter cap, and the 
bridge rectifier. This
procedure came from several months of test and experimentation. In the 
field, we (AT&T
self maint. org.) generally replaced h744's with h7441's.

(I can't believe I can remember this! But then again, I can remember 
the details of RP06
maintenance, too. Fun times!)


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