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

Brent Hilpert hilpert at
Sun Oct 11 14:07:30 CDT 2015

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?

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