H7864-A fan voltage selection

[jmdhuse]

Hello All,

I'm assembling a PDP-11/53 in a BA23 chassis with the H7864-A power supply, which I understand has selectable Fan voltages. There are three posts on the fan power supply board marked as 1, 2, and 3, and a wire with a socket on the end that looks like it is meant to fit on one of these posts, I presume to select the different voltages. Does anyone have information regarding what the connections should be for which voltages?

Thanks, Jon.

 

[zamp]

I think that jumper is used to select the cooling fan voltage. Which changes the fan speed. See the thread https://forum.vcfed.org/index.php?threads/ba23-cooling-and-power-supply-questions.30952/ .

 

[jmdhuse]

I think that jumper is used to select the cooling fan voltage. Which changes the fan speed. See the thread https://forum.vcfed.org/index.php?threads/ba23-cooling-and-power-supply-questions.30952/ .

Yep, I got out my meter and checked. In fact there are 4 pins. The pin nearest the outside of the power supply sets it to 10.1V. Moving inward, the pins select 11.5V and 10.8V, respectively. The pin to the right (near the capacitor) selects 12V.

 

[Radix]

FYI, one of the posts just parks the jumper doing nothing, the other three select 1, 2 or 3 resistors in series, the lower the total value, the lower the voltage to the fans..

If you space the boards apart with bus grant cards and put the cpu in the second slot for much better cooling, you can drop the fan voltage by changing the resistors to make it less deafening - I tried one down to about 7V with 3 x 6K8 resistors and the CPU ran cooler than in the top slot with normal fan speed...

Don't try this unless you are willing to keep the boards spaced apart or you WILL cook your boards...