Howdy folks, it's been awhile. Time to regale you all with the progress of the Nova 1200 power supply replacement.
Forgotten Machines/firebirdta84 designed a passive interface board for the Nova's power supply for our development work. Nevets01 and I vetted his work, and each ordered a few for ourselves. My batch of them arrived mid-August.
Another good friend of mine designed a metal bracket in the spirit of the original in CAD for me, but modified the layout to accommodate modern power conditioning modules, and increase the opening for airflow. It was manufactured by sendcutsend using 0.74" mild steel. They also bent the part for me, as I lack the tools and experience to perform that type of fabrication.
Using abusing compu_85's battery tab welder, we spot welded some steel grating to cover the ventilation hole for safety. Then I gave it a coat of "prototype red" paint to make it very clear that this is not original. After that, I began installing Meanwell modules that would be providing power regulation, and wiring them up. An LRS-150-15 provides the 15V rail, and an RT-125A provides the 5V, -5V, and 12V rails. I added a pair of high performance industrial 12V Noctua cooling fans -- the highest airflow available for the 12V range. The 12V rail would be standing in for the rather vague Lamp Power rail that Data General documents indicate could be anywhere from 11V to 14.5V, so I figured it had sufficient amperage to run both a pair of cooling fans and the lamps without issue.
Surprisingly, I was able to get both of the 120mm fans in place using some brackets and a few pieces of meccano. It's a bit of a tight fit, but it fits! It was then installed in the chassis.
Bulb tests went well, voltages and ripple seemed well within spec, so it was time to reassemble and see how we did.
Initial tests seemed like everything was going okay, voltages were where we expected them, but nothing was functioning as expected, most notably the front panel. The 12V section of the supply seemed upset all the sudden, and was going into cutoff oscillation, and one of the bus signals was following it, albeit capped at 10V, when it should only be as high as 5V at best. Then we noticed it, at the expense of an oscilloscope probe:
Seems that we had gotten the labels on the top and bottom of the power interface board mirrored.
The main power signals (GND, 15V, 5V) are all present top and bottom on the power board. The other 8 contacts only exist on one side of the card edge connector. /Power Fail, MEM OK, 60Hz, Lamp Ground on the bottom, -15V, +5V OK, -5V, Lamp Voltage on the top. And we got them
backwards... none of us caught the mirrored signal labels, and I was the first to power on and find out the hard way. The good news is that most of those signals weren't going to cause an issues. 60Hz and -15V were options I don't have installed. MEM OK and +5V OK are opposite from one another, and both were tied high with pull-up resistors. /Power Fail has its own pull-up resistor on the CPU board, so I didn't touch it. -5V was sitting where 60Hz should be, and it only goes to one specific card that was not installed at the time of testing.
That leaves Lamp Voltage and Lamp Ground. With those two reversed, the 7407's on the front panel that drive the display lamps were getting backwards voltage. The good news is that it's an isolated ground, only connected on the card edge connector itself. During my testing, trying to figure out what was wrong, I connected my probe ground mostly to the signal ground used by most devices. The last time, it was connected to what I thought was Lamp Ground, resulting in the probe conducting what was effectively a ground loop, and it released the magic smoke (see the FLIR image to see how irregularly warm it got). Once I realized that, I swapped all of the mirrored signals on the card edge connector, but lamps were acting real funky. The 7407's that drove all but one lamp had all been zorched by the unexpected voltage, so those all needed replacement. Good news is I had a pile of them, and I started replacing them and installing sockets.
After that, it was a game of chasing any signals on the CPU board that might have backfed 12V (or rather 10V after the voltage drop provided by the 7407's). U2 (a 74LS74 that handles RUN/HALT control) and U22 (a 7404 for more status control lines) had both been destroyed, and were replaced with sockets and new chips. After that, signals like RUN, HALT, FETCH, etc. started working as intended, but things like the program counter would not.
Unfortunately, that was as much debugging as I could do prior to Vintage Computer Festival Midwest. I had to pack the system up in the car for the road trip, but we suspected that 4 or possibly 8 of the 8271's in the CPU had been damaged, but we weren't certain. I had no 8271's or 74179's to install in their place, so I went to war with the Nova I had, and a seemingly working power supply.
I've reached the image cap, so more will follow in the next post.