At Home with Josh, Part 3: Power Supply Testing and Initial LMI Lambda Power-up

Last time around I went through the cleaning and inspection of the Lambda. Overall, apart from a few errant screws and a faint musty odor, things looked pretty good. We’re inching closer to the point where we can power this thing on and see what it does, but there are a few things left to go over before we can get there.

Power Supplies!

We haven’t yet looked at the power supplies in the Lambda beyond verifying that mice haven’t eaten all the wires away. LMI made this task pretty easy, and I want to thank the person who designed the chassis: the whole power supply assembly is on rack-mount slides and just pulls right out of the rear of the cabinet like so:

The LMI Lambda Power tray, inside the cabinet.

Disconnect the two cables and slide it right out. Magic!

The fan tray in the rear is normally situated right below the card cage, and serves to keep the logic well-ventilated. There are two power supplies mounted in the front of the tray. The narrow ACDC Electronics supply on the left provides +/-5V and +/-12V to the backplane, and the large blue LH Research supply on the right provides +5V at 150 Amps. That’s a lot of power, and it’s used to run the majority of the logic in the system. The smaller supply provides power to run the ECL components for the high-resolution terminal interface, the RS-232 drivers for the console ports, and other odds and ends.

The screw is a tightening atrocity.

As with the card cage inspection, it’s important to go through both of these supplies with a fine-toothed comb looking for damage and for things that don’t belong inside power supplies. For example, this screw that fell out of the smaller supply as I was opening it up to take a closer look.

And like our misplaced screw from the last entry I have no idea how it ended up in here, but there it was. Had this supply been powered up with that screw in place it could have shorted something out and done serious damage.

What I usually look for in a visual inspection of a power supply are obviously bad parts: bulging electrolytic capacitors, charred tantalum capacitors or transistors, burned traces, things broken off, etc. Passing a visual inspection by no means indicates the supply will work — many parts can (and often do) fail invisibly. But visibly broken parts obviously won’t work and so it’s a good starting point.

Unfortunately, I did my power supply inspection just before I decided to start thoroughly documenting the restoration process, so I don’t have any detailed photos of the insides of these supplies as I was examining them and testing them (and it’s a sufficient amount of work to remove them again that I’m not taking them back out to take pictures now. I apologize for my laziness.) Suffice it to say, apart from that screw, nothing out of the ordinary was found and everything looked much cleaner than I expected — no corrosion or signs of damage of any kind.

It’s at this point where I usually debate with myself whether to just preemptively replace the electrolytic capacitors in the supplies. Shotgun replacement of caps isn’t always a good idea (and I suspect there are engineers out there who will take umbrage with even suggesting such an approach) but for a supply of this age, and for one that’s sat in sub-optimal conditions (cold, dry Pennsylvania winters, hot humid summers for 20+ years) there’s a good chance that the capacitors have dried out and gone out of spec. At LCM+L we typically go one step further than capacitor replacement: Since our goal is to run many of our systems 24×7 (or at least during museum hours) we will often bypass the original supplies and retrofit more efficient and reliable modern supplies (this is usually done alongside the originals so that the system can be returned to its original configuration if need be). I don’t have the budget for that option (150A 5V supplies are expensive), so I’m sticking with the original supplies.

I also figured, what the heck, let’s test the supplies with the original capacitors and see what happens. This is done by hooking up a “dummy” load to the power supply — switching supplies don’t like being powered up without a load to power — and measuring voltages and testing the supplies for ripple. Ripple is a deviation from a nice, flat DC voltage and an excessive amount of it (more than 50-100mV typically) indicates trouble in the power supply: bad smoothing capacitors or dead rectifiers or transistors typically. The exact effects and causes differ depending on the type of power supply but it’s never a good thing to have.

Again I lament my lack of foresight as far as taking pictures of this portion of the restoration. On the positive side: everything tested out fine. All voltages were present and working under load within specifications. I let the supplies run for an hour or so. No funny smells were emitted, and the magic smoke remained safely ensconced in the supplies. I may still end up replacing the capacitors in the supplies at some point in the future, but for the time being I’m leaving well-enough alone.

Fans!

Dirty, filty, ugly, naughty fan…

The importance of cooling in your average computer system cannot be overstated, and thus it is vital to ensure that all the fans are spinning freely and actually moving air around. A closeup shot of one of the fans in the fan tray pre-restoration is to the right. You can see how much crud and rust has accumulated on it over the years. Of the six fans in the tray, two of them spin freely, and the others make a noise not unlike a kazoo when given a spin. However, these are well-made fans and the three exposed screws on the underside there indicate that they were probably made to be serviced — it should be possible to disassemble, clean, and lubricate them.

Sure enough, they come right apart. The major thing to keep track of is the Circlip that holds the fan blade rotor onto the shaft, as well as the numerous washers involved. Cleaning the bearing shaft off and applying some light machine oil to it and to the felt washers is all that’s required to make one of these spin freely again; I also took the time to clean the fan blades as thoroughly as possible. They’re never going to look like new again, but at least they’re not dirty anymore.

eBay photo of the replacement fan

After reassembly, I applied power to the fans and four out of the six worked just fine — they made no appreciable noise and they spun at the right speed, moving a lot of air. The other two spun up very slowly even with help and never reached the proper speed. I suspect that the windings in these motors have been damaged (possibly while in service years ago). These two fans will need to be replaced. I was able to find an exact replacement on eBay, new-old stock. You can find just about anything on eBay.

First Power-up

The power supplies are tested and seem to be working, and enough of the fans are spinning so as to keep things cool at least for a little while — let’s power this sucker up and see what happens.

As discussed in the last write-up, the System Diagnostic Unit (SDU) is the nexus of the Lambda: it bridges the two buses in the backplane and is responsible for booting the operating system. It also provides a diagnostic console over its RS-232 serial interface, which is what I’ll be talking to a lot in the coming weeks. For the initial power-up the only board I will have installed in the backplane is the SDU. This will confirm the functionality of the power supplies, wiring, the backplane and hopefully the SDU itself.

I pulled the other boards out of the backplane, leaving them in the slots but pulled out so they are disconnected, and wired up my trusty Qume dumb-terminal to the serial port marked “Remote” on the rear bulkhead and configured it to 9600 baud, 8 bits, 1 stop bit, no parity. I plugged the power cable into the wall, crossed my fingers, and flipped The Switch.

The Switch.

Serial cabling on the rear bulkhead panel

Fans spun, the LEDs on the front panel and the SDU itself came on. No smoke! But also nothing on the terminal. And all three lights on the front panel were on. This indicates a fault — under normal operation the LEDs should progress through a pattern and then end after a few seconds with just the RUN light on (and probably the SET UP light as well, this indicates that the battery-backed up settings have been erased — expected since I’d pulled the long-since-dead battery out.) Per consulting with Daniel, if all three lights are stuck on, this means that the SDU isn’t passing its initial round of self-tests. This could be caused by any number of things — bad RAM or EPROM, a clobbered CPU bus, or the RESET signal to the CPU being stuck on.

It’s difficult to make out in this picture,
but all three LEDs are stuck on. Not what I wanted to see.

I rechecked power supply voltages and they measured fine. I pulled the SDU out and re-examined the pins on all of the socketed chips and found that a few pins on the 8088 CPU were still pretty grungy (sloppy work on my part during my earlier cleaning/inspection pass, I suppose) so I went over them again.

Powered up the system with the re-cleaned SDU installed and… hey! After a few seconds, just the RUN and SET UP lights were on. Looks like I got lucky here. Still nothing on the terminal, though. Hm.

Wankel-Rotary Switch

I consulted further with Daniel Seagraves and he suggested checking the rotary selector switch on the rear of the cabinet; this selects one of several actions when the system is powered on or reset. Normally it should be at “0” to force the monitor console onto the serial port, but depending on the revision of the SDU’s ROM monitor, it might want to be at “1” instead. I turned the switch to “1”, turned the system on and:

Success!

Alright! Now we’re cookin’ with gas. The SDU is talking to me at last, the power supplies are working acceptably, and the faint musty odor of the air being wafted at me out of the cabinet by the chassis fans smells like victory.

My plan now is to hunt down a suitable 9-track tape drive so that I can use it to load diagnostics into the system and test the various components in the system. While that’s going on, I’m going to take a look at the Lambda’s High-Resolution Terminals (aka “monitors”) and see what needs to be done to make them work again. Stay tuned for the next exciting installment!