I am continuing to make progress with the power supply modules for the PDP 11/04.
This weekend I got a chance to test the modules.
One of the H7441 +5V supplies worked fine.
The output is a little high at 5.5v but I understand that these do not have great regulation when not under load so this will likely come down with 10 PC boards to power.
The other +5V supply didn't run.
The output went up to just over 1v as the AC comes up and then settles back to 0.5v. The DC voltage after the rectifier and across the main input capacitor looks OK on the meter.
After thinking about it overnight I took a small 25VAC plugpack out of the junk box to make comparative testing of the two 7441 supplies easier (i.e from the comfort of the house rather than the workshop with the 1 degree overnight temperatures we have been getting here).
When I tested the known working 7441 it worked fine with the plugpack.
When I tested the "faulty" 7441 it also worked fine.
Reconnecting the faulty 7441 to the main transformer and bringing the voltage up with the variac and it didn't work. The problem is the rate of voltage increase. If I bring up the variac output slowly (say 0-240v over 5 seconds) then the 7441 will hang with the output under 1 volt. If I increase the voltage quickly (within 1-2 seconds) then it powers up fine and shows a good 5v on the output.
I have had some discussions on this and it appears that this can be a common problem with switching supplies.
It was a case of being too conservative.
The H745 -15V supply works fine although the Power OK Bulb (not LED like the 7441) on the back has failed.
(The folded plastic bags between the connector pins are to provide some insulation during testing. The -15V module requires both 20VAC and 15VDC to operate so there are two jumper cables bring power to the module).
I will replace the Bulb with an LED + resistor like the H7441. These LED's are visible at the back of the power supply box so failed supply modules can be easily identified.
I think it is time to put the regulators back in the chassis and try powering up the back plane.
Next weekends project.