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

B Degnan billdeg at buzz1.com
Sun Mar 8 11:05:55 CDT 2015

Working on PDP 11/40, issues with DC LO and AC LO
 The front panel confirms a DC LO issue, CONS and RUN lights are off, BUS 
and PROC lights are on.  The panel is unresponsive.
 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 
a fault in the h742a.  I hate to remove the 742a power control board but I 
finally have the machine in my basement so I can work on it for an extended 
period.  Voltages of the other regulators are OK.  I think it's conclusive 
that the h742a has a fault to be repaired.
 Next, I re-connected the cable to P9.  The DC LO and AC LO wires lead to 
J18, which feeds DC LO and AC LO to the backplane on my system.  Reading 
from J18 the DC LO value remains at 3.53v, but the AC LO drops further to 
1.72v.  I get similar readings from the backplane pins themselves.
 When I remove the Status Module card (M7235) from the backplane the AC LO 
on J18 jumps up to 3.53v - i.e. AC LO jumps back to origination levels.
 Question - Is there is *also* a potential fault in the M7235 circuit 
related to AC LO, -OR- is it normal for the M7235 to detect and pull down 
AC LO further forcing the system to halt?  I am working to understand the 
circuits from page 120 of the KD11-A Maint manual 
 My plan is to repair the h742a first and see how it goes, but I'd like a 
2nd opinion if anyone has one, regarding the M7235....thanks!

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