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BA23 Cooling and H9278 Power Supply Questions


Experienced Member
Apr 26, 2012
I asked this questions ten years ago at comp.os.vms.narkive.com, but there were no substantial answers:

I have several BA23 enclosures with differing cooling assemblies and
power supplies.
I'm not sure everything is still as it once was when it was sold.

Questions concerning the BA23 cooling and PSUs:

Where/when and in what combinations were the following cooling options

- With/without fan speed control (70-22643-01; regulates from 5.9 V
[cold] to 10.3 V [warm] with 12 V input)

- With/without front fan baffle/cowling (70-21506-01)

- Full/reduced fan voltage (4 possible H7864 settings; 12.2 V, 11.5 V,
10.8 V, 10.1 V)

What are the differences between the three PSU versions (besides the
known 12 V 6A/7A)?

Known facts:

A sticker under the PSU tells that the fan speed control is to be used
with the 12 V setting only
(which makes sense)


The front fan baffle/cowling accomplishes that most of the front fan
cooling effect is concentrated on the PSU and card cage; the cooling
of the two drive bays at the front of the BA23 will get worse because
of that.

A fully loaded card cage with power consuming modules might be a
reason to use the baffle.
Using an RD53 and a TK50 in the front drive bays might require more
intensive cooling directed to the bays by not employing the baffle.

Quotes on the baffle:

EK-186AA-MG-001 BA23 Enclosure Maintenance.pdf (10/88)

Page 3-24:
"On some systems, remove the fan baffle and baffle holder from the
front fan."

Page 3-31:
Figure 3-20: Accessing the Front Fan: "OLDER SYSTEMS ONLY" [relates to
the baffle removal]

EK-MIC11-SG-001 MicroPDP-11 Systems-2.pdf (09/85)

Page 8-32:
"Remove the fan cowling and cowling holder (if present)."

Three versions of PSU H7864:

H7864 (30-20444-00) 5V, 36A / 12V 6A
H7864-A (30-21749-01) 5V, 36A / 12V 7A
H7864-B ???

Quotes on the PSU versions and fan voltage settings:

EK-186AA-MG-001 BA23 Enclosure Maintenance.pdf (10/88)

Page 1-14:
"There are three variations of the power supply, depending on the
enclosure model number: H7864, H7864-A, or H7864-B."

"The power supply has also two +10 Vdc at 0.45 A fan outputs for the
front and rear dc fans. You can increase the fan voltages to 12 Vdc by
changing a power supply jumper. However, the KA630 CPU module's
thermal and acoustical specifications are based on the +10 volt

Page 3-2: Table 3-1 BA23 FRUs

??? Who can shed some light on this ???




Jul 1, 2014
Germany, near Göttingen
Moin Ulli,

the only thing i have been told by different people that there was an high power version of the PSU with 50-150 watts of extra power for systems fully equipped with drives and cards,
remember f.e. the FPF11 option which brings the normal PSUs to the limit and above.
That included also the demand for better cooling regulation and optimisation of the airflow.

I can try to do some phone calls, eventually i am able to get some further information...

Viele Gruesse (best regards :) )


Apr 16, 2020
I have all 3 versions of the power supply.

The H7864 is made by Astec, copyright date 1982
Astec part number: AA12130
DEC part number: 30-20444-01
Rated at 320W input, 242W output.
5V @ 36A
12V @ 6A
10V/12V @ 0.45A (fan output)
12V @ 0.45A (fan output)

The H7864-A is also made by Astec, copyright date 1983
Astec part number: AA12131
DEC part number: 30-21749-01
Rated at 345W input, 242W output.
5V @ 36A
12V @ 7A
10V/12V @ 0.5A (fan output)
12V @ 0.5A (fan output)
The design of this one is similar to the H7864 but it has several extra riser boards. ACDC Delay Module, OCP, OTP, OPP Module Pri Control Module and Sec Control Module.

The H7864-B is made by DEC and is a completely different design. There are very few markings on the board or the case so I don't know what date this was introduced however the BA23 it belongs to dates from about 1989. This is an rtVAX 1000.
DEC part number: ? (H7864-B is the only part number given on the case)
Rated at 382W input. The outputs are not specified.