PDP 11/40 AC LO and M7235 (Status K5 Module)

B Degnan billdeg at buzz1.com
Sun Mar 8 16:55:43 CDT 2015

> > With the cable removed from the h742a connector, P9 says DC LO and AC 
LO > are 3.53v, which is lower than the expected +4.8v. This tells me there 
is Have you confirmed these are steady DC voltages Or could the signals be 
oscillating with that as the average value? There is a capacitor in the 
H742 that if it fails (and it does...) the ACLO and DCLO will oscillate at 
mains frequncy (or probably twice it). Certainly on the 11/45, this really 
confuses the CPU microcode. It's the obvious smoothing capacitor for the 
supply to the ACLO and DCLO circuit. -tony
 Yes, steady voltage, but I agree with your point.  My question though is, 
will the M7235 "go lower" if it detects inbound AC LO is too low and lower 
it some more, or is the additional lowering of voltage a symptom of a fault 
in the M7235 as well as the h742a.  hope that makes sense...I ask because 
it's exactly 1/2 the voltage when installed.  DC LO on the other hand is 
not changed with or without M7235 installed.
 AC LO =  1.76v - with M7235 installed (3.52 / 2 = 1/76)
 AC LO =  3.52v - with M7235 disconnected 

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