• Please review our updated Terms and Rules here

H786 Power Supply Schematic Wanted

NF6X

NF6X

Veteran Member
Joined
Sep 9, 2013
Messages
1,569
Location
Riverside, CA, USA
I have a new-to-me PDP-11V03-L system that's popping the breaker on the CPU cabinet when turned on. It does it with or without the backplane connected to the supply. I'm having trouble finding a component-level schematic diagram for the H786 power supply that I'll need to debug. Can anybody help me find one?
 
I haven't found a schematic diagram yet, but I started poking at my bad power supply on the test bench last weekend anyway. I immediately found that one of the large input filtering caps is shorted. New caps are on the way from Digi-Key, and I'm crossing my fingers that there aren't any other problems in the system. Maybe I'll get it booted up this weekend?
 
NF6X said:
I haven't found a schematic diagram yet, but I started poking at my bad power supply on the test bench last weekend anyway. I immediately found that one of the large input filtering caps is shorted. New caps are on the way from Digi-Key, and I'm crossing my fingers that there aren't any other problems in the system. Maybe I'll get it booted up this weekend?
Click to expand...

If this is like the supply for my 8m, you'd best check the input diodes (bridge on mine; might be discretes with yours). If the input cap is shorted it's quite likely to have blown one or more of the diodes. That was the case with mine.

Mike
 
Thank you, I'll check out the diodes. Mine has a bridge with a metal heat sink and a thermal cutout switch on top. I hope that the breaker popped fast enough to protect the diodes.

I sure wish that rack was just 1U taller, so I could add an RL02 underneath the RX02. It appears to have 5U of unused space, and an RL02 is 6U tall.
 
I received my new filter caps yesterday, and installed them last night after I got home from work. The rectifier bridge tested out ok. With high hopes, I reassembled the chassis and powered it up… Fans spun, no smoke, but also no voltage out and no DC OK lamp. Drat! Even with the shorted filter cap, I saw the LEDs on the bus terminator board light up briefly before the breaker popped. Now it's even deader.

Today I started debugging further. I didn't find 115 VAC between any pins on the 9-pin power supply input connector… just around 18 VAC here and there. I decided that it was time to get inside the power entry module at the back of the chassis, look for problems in there, and trace out its circuit to determine what all of those pins on the 9- and 12-pin AC power connectors do. When I got it apart, I found one of the pins slipping out of the connector as I pulled it away from the housing to start tracing wires. Hmm, maybe I just knocked it loose while plugging in the stiff mating MATE-N-LOK connector, and that caused my problem? That style of connector can be a bit uncooperative at times. I went ahead and traced the circuit, and checked the fuse and EMI filter for opens.

After reassembling it all once again, pausing to vacuum more dust out in the process, I tried powering up the system again… SUCCESS! The voltages were in spec, but I touched them up while I had the power supply hinged open just for good measure.

When I boot the headless machine, the RUN lamp turns on and I hear one of the RX02 drives click. The next steps will be to get a terminal hooked up, dust off the floppy disks that came in the drives, and try booting RT-11. I have a feeling that it'll be up and running today.

I still would not mind having a power supply schematic in my records, but it looks like I managed to find a simple failure without needing to get too deep in the power supply. Hopefully I won't eat my words later after finding an intermittent problem to explain why there was a spare fuse and a blurry photocopy of a portion of schematic stuffed inside the supply the first time I hinged it open...
 
IT LIVES! One of the serial cables that came with it happened to have the correct pinout and gender to plug in to my VT-131. I booted it from each of the two floppies that came with it. One of the floppies was dated 1987, and the other 1985. So this is now my first working PDP-11. I'll write up a blog post on my web page soon.
 
You must log in or register to reply here.
Back
Top