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

Josh Dersch derschjo at
Sun Oct 11 20:19:39 CDT 2015

On Sun, Oct 11, 2015 at 12:07 PM, Brent Hilpert <hilpert at> wrote:

> On 2015-Oct-10, at 8:44 PM, Josh Dersch wrote:
> . . .
> > Thanks for the explanation.  I tacked on some wires to the bias board (to
> > bring them out for safe probing); the wires are connected to the 5V logic
> > supply and the +12V bias supply and I get nothing at all out of them when
> > the H7140 is turned on.
> >
> > I'm slowly coming to terms with the schematic but I'm not quite clear
> where
> > these two voltages are being generated.  I note from the block diagram
> that
> > the +15 and +12 are rectified on the motherboard (backplane) -- which
> > should make it GREAT fun to probe.  So it looks like some careful
> > disassembly is now required (unless there's a trick to probing things on
> > the backplane...)
> 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

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