Skipping ahead 9 months...

When I last wrote in this blog last December I'd just completed Lesson 2 of FEDEVEL's "Advanced PCB Layout" course. By mid January I'd finished all the videos and then went back over several of the lessons I thought were most relevant. Robert Feranec is a good instructor with clear mastery of the topic.

As good as that class was, I still felt I needed a bit broader coverage. I've been doing digital design as a hobby for about 15 years, but I knew I was making some decisions based on guesses rather than good data. I decided to invest in FEDEVEL's "Advanced Digital Hardware Design" course, taught by Philip Salmony who also makes the "Phil's Lab" YouTube videos. The "Zett Brett" design he discusses in the video has many parallels with my latest project. I'd already completed much of my design by the time I started this course, but it gave me added confidence in many of my decisions.

FEDEVEL Advanced PCB Layout: Post Lesson 2 notes

I finished Lesson 2 of FEDEVEL Academy's Advanced PCB Layout course yesterday. As I expected, there ratio of review to new material (for me) is shifting toward the new stuff. There were a couple of confusing points, like the the instructor's strong recommendation to use a separate via for each power and ground connection on a BGA while watching him demonstrate routing two such adjacent balls to a shared via. Some of the recommendations  offered were things that I had read elsewhere, but it was nice to have it explained from another person's viewpoint.

And there were a couple of "Wait, what?" moments. 😯

Taking FEDEVEL's Advanced PCB Layout Course

As part of a hobby project I'm designing a PCB that will include a Xilinx Zynq-7000 System-on-Chip and a DDR3L SDRAM. Both of these come in ball-grid array (BGA) packages with balls on a 0.8mm grid. This is my first attempt at a PCB using BGAs, but more critically, it's my first attempt with high-speed DRAMs.

I'm fairly confident, but at the same time I'm a little bit intimidated.

A look at a vintage PC's disk storage subsystem

I've been doing a lot of work recently with vintage PC disk storage subsystems. I'm hoping to present a talk on this topic at the VCF East event in April 2026. Here's a preview of some of the material I'm planning to present.

 As always, comments and questions are encouraged. 

My current museum project

Here's what I'm currently working on restoring for the System Source Computer Museum:

Yes, this is an IBM System/360 Model 20 from the mid 1960s.

This machine's journey started with some friends drinking in a pub in the UK while browsing eBay. They came upon an ad for some computer equipment in a building due for renovation in Nuremberg, Germany, and decided to put in a bid. If this already sounds like a recipe for disaster, well, I'd agree. 

I really can't do justice to the effort they put into the recovery. It's best if you read it in their own words here: https://www.ibm360.co.uk/

To make a long story short (and you really should go read their blog), it's on long-term loan to our museum for restoration.

Computeriter Emulator wrap-up

Okay, so it's been 5 months since I updated this blog. I have some catching up to do!

Spoiler alert: I did get everything working in time for VCF West. Here's a pic of the CDC-160A system on display in sunny California:

 

What did it take to get it done? I pretty much lived and breathed for this project from mid April through early August. But let me catch you up on what you missed.

Heavy Metal from the 1960s

Any time you're working with older electronics, there's a risk that some of the components may have failed as a result of age. This is especially true of wet electrolytic capacitors. Given that the museum's CDC 161 Typewriter is at least 61 years old (there's an inspection sticker from 1964 in it), the sheer mention of a power supply problem led to a decision to pull the thing out for at least a visual check of its internals. We were also looking to find the source of a loud buzzing sound coming from within the cabinet.

The power supply in the CDC 161 hangs from the underside of the cabinet's top like a bat, held in by four hefty bolts. I took photos of the wiring before disconnecting everything and stowing the cable bundle out of the way. I tried to loosen all the bolts evenly to make it easier to remove, but when the first bolt on the left side came out the second on that side broke off. I found out later that it was badly rusted, and may have been shearing rather than unscrewing. It's fortunate that I'd put some packing material to help support the power supply, because otherwise it would have crushed my hand -- it weighed 40 to 50 lbs!

It seems the CDC 161 had suffered some water exposure, as some of the screws holding the power supply's cover were rusted. With sufficient persuasion, I was able to unscrew all but one of the screws. We removed the head of the last screw with a Dremel and a cutting disc.

With the cover removed I could see why the thing weighs so much. Take a look at the three huge transformers in this thing! That's some heavy iron.

Despite being told repeatedly that switching power supplies didn't exist in the 1960s, this COTS beast from the late 1950s is indeed a switcher.

Operated on the bench it produced clean, steady DC power on all three of its outputs with only a slightly audible 60 Hz hum, even when operated under load (note the 100W load resistor on the left side of the picture). Scoping the filter capacitors showed essentially no ripple, confirming that they were still doing their jobs.

But what about the overvoltage lock-outs I'd been experiencing?

To minimize the effect or voltage losses in the wiring between the power supply and the load, this power supply has separate voltage sense inputs (the top three connections on the terminal block shown in this pic). These allow the supply to sense the voltage at the load rather than inside the power supply. The problem with this scheme is that a poor or open connection in these sense leads will cause the supply to think it needs to increase the output voltage.

While inspecting these connections I discovered that the faces of the ring terminals were corroded under the terminal block screws. My theory is that this lead to the overvoltage condition, and it correctly shut down the supply when the output voltage rose to an excessive level.

At least that's my story, and I'm sticking to it!

After reassembling the power supply, I was faced with the challenge of getting it back into the cabinet. I decided the simplest way to do this was to flip the cabinet onto its top, supported by some cribbing. This allowed gravity to work in my favor while I fitted the bolts.

But if the loud buzzing wasn't coming from the power supply, where was it coming from?