DEC H7140 (11/44) power supply revisions and general advice?

Josh Dersch derschjo at
Sun Oct 11 22:01:42 CDT 2015

On 10/11/15 6:19 PM, Josh Dersch wrote:
> On Sun, Oct 11, 2015 at 12:07 PM, Brent Hilpert <hilpert at 
> <mailto:hilpert at>> wrote:
>     On 2015-Oct-10, at 8:44 PM, Josh Dersch wrote:
>     . . .
>     Note that the "+13V" supply must also be present, and it's
>     generated and referenced on the Mains side of everything.
>     In summary, for the relay to activate:
>             1) The Mains is rectified and filtered to produce Mains
>     +300V via bridge LNS-CR1 and caps LNS-C1,C2.
>             2) MOB-D1,D2,R1,R2 form a voltage divider / balancing
>     network to produce Mains +150VDC.
>                 They also balance the voltage across the Mains filter
>     caps (LNS-C1,C2) when in 220VAC mode.
>     **      3) Mains +150V is regulated via MOB-R3, BIB-Q6, BIB-C4,
>     BIB-D11 to produce Mains +13V, referenced
>                 from MOB-D2 feeding ("START-UP DRIVE") the base of BIB-Q6.
>                 16V zener BIB-D12 additionally clamps this level.
>             4) Mains +13V supplies 555 oscillator BIB-E2, LM393
>     comparator BIB-E3b and associated circuitry. These form a SMPSU
>     with MOB-T1.
>             5) The output from MOB-T1 is half-wave rectified in both
>     polarities to produce BIB +12V (MOB-D6,C3) and BIB -15V
>     (MOB-R4,R5,R6,D4,C2,D5).
>             6) BIB +12V produces BIB +5V via regulator BIB-E1.
>             7) BIB-E3a forms a voltage level sensor with reference
>     zener BIB-D13 and voltage divider BIB-R3,R4,R27 that must detect
>     an adequate level on
>                  the Mains +300V to turn on the relay via BIB-Q11,
>     BIB-E5, BIB-E4a.
>     LNS  = Line Supply section      PDF.146
>     MOB = Motherboard                       PDF.162
>     BIB = Bias & Interface Board    PDF.166-168
>     ** There appears to be something inconsistent in the schematic
>     here in that the base of BIB-Q6 is fed by +10V from MOB-D2, which
>     would leave the Mains +13V at more like +8V.
>     Am I missing something? - is there anywhere in that doc that
>     explicitly declares the association between the 2-dozen
>     connectors, for example, that Line Supply P1 & P2 connect to
>     Motherboard J5 & J4, and that Bias and Interface Board J1 & J2
>     connect to Motherboard J7 & J6?
> Awesome, thanks for the detailed writeup.  Things make sense, or at 
> least more sense.
> I went to take a look at the +13V supply and of *course* once I did 
> things started working, mysteriously.  The relay clicks over and I get 
> +13V and it looks like the bias voltages are present as well.  All I 
> did was tack on a wire to the 10/L header pins on the bias/interface 
> board (+13V)...
> So that's... good, but I have no idea what the actual problem is/was.  
> Going to clean the connectors and look for cold solder joints (again 
> :)) just to rule that out.  I'll get a dummy load hooked up and see if 
> anything happens when I throw the front panel switch.
> Thanks again, all.
> - Josh

Re-inspection revealed no cold solder joints, but I reflowed the 
backplane connector pins on the Bias/Interface board pins L and 10 
anyway.  I reassembled everything in the chassis, hooked up a dummy load 
and everything came up fine.  Reinstalled the CPU boardset + terminator 
and I get the Console chevron prompt after a couple of seconds and the 
built-in diagnostic tests pass.  So, success, of a sort.  I feel like I 
cheated somehow :).  We'll see if it keeps working...

Now to figure out mass storage; I have an SMD controller and a drive -- 
let's find out if either of them work :).

Thanks again for the analysis and advice, I'm slowly figuring things out 
but I think I really need to sit down one of these days and really study 
power supply electronics.

- Josh

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