New Power Supply

 I bought a beautiful Farnell L30-5 PSU - sold as faulty, not working. It's a thing of some beauty, with big meters on it. The former owner was correct - out of the box, it produced no output. The fuses were all intact, but when I turned it on, the overcurrent LED lit up, regardless of the setting of the knobs. Now though I was on a conference call at work at the time, I turned the camera off, carried on listening to the conversation, and got my screwdrivers out. 

So, time to go inside. I found a schematic online, and I found my way to the overcurrent LED driver. It was holding the output transistors in high impedance. It was a fairly simple thing to find that it was being driven by an op amp, that compared two voltages - one generated across a low value resistor in series with the output, and one from the current limit pot. Interesting to see that one of the voltages on the op amp was at or near the negative rail. I measured the resistance between the two pins on the op amp - pins 4 and 5, and it measured a few ohms. I concluded that the op amp had failed, I checked on t'Internet to find a close equivalent, and discovered that a good old TL072 would do the trick, and we have oodles of those around for the synth stuff. I even found an 8 pin socket ready to go in. 

Took the board out - and I wish I had taken pictures, but there was a small area, about a centimetre across on the coppers side of the board, which looked like a fuzzy crystalline structure had grown out of the solder connections. Bright, shiny solder except for this one grey, fuzzy mess - like a metallic mould. 

I've read about whiskers affecting older equipment before - it happens with different metals - sometimes zinc, sometimes lead and with a voltage across. Never seen it - a bit apocryphal I have always thought. I wiped the crystals with a cloth, then with a small bit of sanding block used for cleaning PCBs. I then took a soldering iron to it, just to tidy it up. Measured the resistance again, and the near short was gone. Not even a need to replace the op amp? 

Excellent repair. By the time the conference call was over, I had cleaned the insides with isopropyl, hoovered out the dust bunnies and put the screws back in the case. 

An accidental acquisition with an unexpected addition

 So I bought a Selectric III typewriter from eBay. 

I've been fascinated by IBM Selectric typewriters ever since I began working for IBM in 1977. Actually, my interest in IBM typewriters probably started when I was given a Model D during my University degree, which I took apart, lovingly repaired and typed my final year project on (OK, so Chrissie did most of the typing). 

When I began at IBM, all of the secretaries had Selectrics, and later Mag Card machines with the huge card reader/writers beside their desks. I adored the sound they made. Later I graduated on to using an IBM 2741 with an APL golfball, connected from IBM Hursley to the machine in Santa Teresa to run delay simulations on logic circuits. 

Now, many years later I am nearing (OK, shot well past) retirement age, I decided to indulge myself and bought a Selectric III from that auction site, with a view to making it one of my future retirement projects - once I have repaired several motorcycles, the music synthesisers and other bits of professional music gear.  I didn't pay a lot for it; I put it on my watch list, didn't think much more about it, and the auction finished without a bid. The seller reached out to me with a reduced price, so I was hooked. 

When the beast arrived this week - in much, much better condition than I expected, I noticed that just by the power cord, there was a ribbon cable with a D-shell connector on it emerging. So, against my better judgment I decided just to open it up to have a quick look. The ribbon cable with 20 conductors terminates at a third party add-on - a proper commercial product, made in Denmark - with the number GNT3602 on it. The add-on is fixed underneath the keyboard frame and seems to attach to all of the code levers, the space and carriage return and so on. 

Milli ohm meter now working .. !

 Gosh darn it, it seems to function quite well. 

Milli ohm meter getting warmer ...

So the aforementioned PCB arrived back, have assembled. 

I had to play with resistor values and so on,  but now I have a circuit that will pump 500mA through the device under test. There is a x100 amplifier which boosts up the voltage developed across the DUT to a suitable voltage for an Arduino 10 bit ADC to convert. 

So, a 100milli ohm resistor will develop a full scale reading. I'm going to have to add a range switch to lower the current if I want to measure bigger resistors than that. But then we have multimeters that can go down to a few hundred milli ohms in general. 

In general, quite pleased so far. I am going to assemble this into a bench power supply which happens to have one of my old boards that can read switches, write data to a flourescent display (which is only 30 years old). I am also going to float the power supply ground so we don't liberate magic smoke from things when I am using it. 

Yet Another Transistor Matcher - my version

 I found an article or a blog post on how to match transistors with a bit of slick interface. Matching transistors is pretty essential for exponential converters and ladder networks for filters on synths. Now you can do it manually with precision measurements, or you can use this circuit to show a quite groovy method. 

YATMA

I adapted it slightly - I have a whole drawer of old LF13331 analogue switches which I have used with an Arduino to sequence through, and a x1000 amp on the back side. It takes up to 8 NPN or PNP transistors, and plots their Vbe on a scope. Now since I am an old git and I like to be able to see the components I am soldering,  I am using pretty much all PTH components. How quaint, I hear you say. 

So, I have a small KiCad PCB made by Seeed Studios: 

The small Arduino Pico on the top just runs a small loop that switches two of the 16 switches on, connecting a current source (a chosen resistor) to one of the eight transistors in the ZIF socket on the other side. I had to put the ZIF socket on the back, so I can mount the board backside to the front panel with the socket sticking through. 

The jumpers can be moved to allow either NPN, or PNP (or simpler devices) to be selected. Then, the big BNC socket on the board is used to connect to a scope - I found the small old world scope managed to hold a sync much better than the digital one. 

The net effect is in this bit of a scope picture. I have AC coupling on so the tops of each step slope slightly in this picture - but you can easily see how you can select pairs of transistors with similar characteristics. 2 and 4 in this case are really close, 6 and 8 quite close and so on. It's surprising how quickly the levels change with a finger on the tranny - so it takes a bit of time to settle once the transistors are put in the sockets. 

And it works!  Quite pleased to be honest. 

More Kicad stuff ...

 Playing more with KiCAD these days. I designed a milli-ohm meter (it's not original - it's another of Rod Elliots designs, but I have done a schematic and laid it out. Quite pleased with the results. KiCAD is slowly getting easier to find my way around - clearly a lot of hot key work needed to make it slick. 

Archer 2779601 Power Supply

 So I have an ancient (1986) bench power supply from Tandy in the US, which has a spot on my bench. I have other power supplies for specific reasons, but this beast is 20v, 2.5A with a current limit and seems to survive lots of abuse. I have used it for decades. 

It does suffer from some ageing components though, and I dug around in the paperwork from my stay in the US and found the manuals for it. Someone on T'Internet might find them useful ...

Please find PDFs of the manual (with schematics and wiring diagram) and the assembly instructions, bill of materials and so on. 

Archer 2779601 Power Supply

Archer 2779601 PSU Assembly Instructions