Repairing A BPS-305 30V Bench Power Supply

December 31, 2024 by Maya Posch 6 Comments

When [Tahmid Mahbub] recently reached for his ‘Lavolta’ BPS-305 bench supply, he was dismayed to find that despite it being a 30V, 5A-rated unit, the supply refused to output more than 15V. To be fair, he wasn’t sure that he had ever tried to push it beyond 15V in the years that he had owned it, but it had better live up to its specs. Ergo out came the screwdriver to open the power supply to see what had broken, and hopefully to fix it.

After some more probing around, he discovered that the unit had many more issues, including a highly unstable output voltage and output current measurement was completely wrong. Fortunately this bench power supply turns out to be very much like any number of similar 30V, 5A units, with repair videos and schematics available.

While [Tahmid] doesn’t detail his troubleshooting process, he does mention the culprits: two broken potentiometers (VR104 and VR102). VR104 is a 5 kOhm pot in the output voltage feedback circuit and VR102 (500 Ohm) sets the maximum output current. With no 500 Ohm pot at hand, a 5 kOhm one was combined with a 470 Ohm resistor to still allow for trimming. Also adjusted were the voltage and current trimpots for the front display as they were quite a bit off. Following some testing on the reassembled unit, this power supply is now back in service, for the cost of two potentiometers and a bit of time.

The Mystery Of The Messed-Up Hammond X5

December 24, 2024 by Elliot Williams 4 Comments

[Filip] got his hands on a sweet old Hammond X5 organ, but it had one crucial problem: only half of the keys worked. Each and every C#, D, D#, E, F, and F# would not play, up and down the keyboard, although the other notes in between sounded just fine.

Those of you with an esoteric knowledge of older electric organs will be saying “it’s a busted top-octave generator chip”, and you’re right. One of the TOGs worked, and the other didn’t. [Filip] rolled his own top-octave generator with a Pico, in Python no less, and the old beauty roared to life once more.

But what is a top-octave generator, you may ask? For a brief period of time in the early 70s, there were organs that ran on square waves. Because a musical octave is a doubling or halving of frequency, you can create a pitch for every key on the organ if you simply create one octave’s worth of pitches, and divide them all down using something as simple as a binary counter IC. But nobody makes top-octave chips any more.

Back in 2018, [DC Darsen] wrote in asking us if we knew about any DIY top-octave designs, and we put out an Ask Hackaday to see if you all could make a top-octave generator out of a microcontroller. We got a super-optimized code hack in response, and that’s worth checking out in its own right, but we always had the nagging suspicion that a hardware solution was the best solution.

We love how [Filip]’s design leans heavily on the Pico’s programmable input/output hardware modules to get the job done with essentially zero CPU load, allowing him to write in Python and entirely bypassing the cycle-counting and assembly language trickery. The voltage shifters and the switchable jumpers to swap between different top-octave chip types are a nice touch as well. If you have an organ that needs a top-octave chip in 2024, this is the way we’d do it. (And it sounds fantastic.)

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Saving An Electron Microscope From The Trash

December 13, 2024 by Maya Posch 17 Comments

Who wouldn’t want to have a scanning electron microscope (SEM)? If you’re the person behind the ProjectsInFlight channel on YouTube, you certainly do. In a recent video it’s explained how he got his mittens on a late 1980s, early 1990s era JEOL JSM-5200 SEM that was going to be scrapped. This absolute unit of a system comes with everything that’s needed to do the imaging, processing and displaying on the small CRT. The only problem with it was that it was defective, deemed irreparable and hence the reason why it was headed to the scrap. Could it still be revived against all odds?

The JEOL JSM-5200 SEM after being revived and happily scanning away. (Credit: ProjectsInFlight, YouTube)

The good news was that the unit came with the manual and schematics, and it turns out there’s an online SEM community of enthusiasts who are more than happy to help each other out. One of these even had his own JSM-5200 which helped with comparing the two units when something wasn’t working. Being an SEM, the sample has to be placed in a high vacuum, which takes a diffusion vacuum pump, which itself requires a second vacuum pump, all of which requires voltages and electronics before even getting to the amplification circuitry.

Since the first problem was that this salvaged unit wasn’t turning on, it started with the power supply and a blown fuse. This led to a shorted transformer, bad DC-DC converters, a broken vacuum pump, expired rubber hoses and seals, and so on, much of which can be attributed simply to the age of the machine. Finding direct replacements was often simply impossible to very expensive, necessitating creative solutions along with significant TLC.

Although there are still some small issues with for example the CRT due to possibly bad capacitors, overall the SEM seems to be in working condition now, which is amazing for a unit that was going to be trashed.

Thanks to [Hans] for the tip.

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Close Shave For An Old Oscilloscope Saved With A Sticky Note

November 23, 2024 by Dan Maloney 12 Comments

When you tear into an old piece of test equipment, you’re probably going to come up against some surprises. That’s especially true of high-precision gear like oscilloscopes from the time before ASICs and ADCs, which had to accomplish so much with discrete components and a lot of engineering ingenuity.

Unfortunately, though, those clever hacks that made everything work sometimes come back to bite you, as [Void Electronics] learned while bringing this classic Tektronix 466 scope back to life. A previous video revealed that the “Works fine, powers up” eBay listing for this scope wasn’t entirely accurate, as it was DOA. That ended up being a bad op-amp in the power supply, which was easily fixed. Once powered up, though, another, more insidious problem cropped up with the vertical attenuator, which failed with any setting divisible by two.

With this curious symptom in mind, [Void] got to work on the scope. Old analog Tek scopes like this use a bank of attenuator modules switched in and out of the signal path by a complex mechanical system of cams. It seemed like one of the modules, specifically the 4x attenuator, was the culprit. [Void] did the obvious first test and compared the module against the known good 4x module in the other channel of the dual-channel scope, but surprisingly, the module worked fine. That meant the problem had to be on the PCB that the module lives on. Close examination with the help of some magnification revealed the culprit — tin whiskers had formed, stretching out from a pad to chassis ground. The tiny metal threads were shorting the signal to ground whenever the 4x module was switched into the signal path. The solution? A quick flick with a sticky note to remove the whiskers!

This was a great fix and a fantastic lesson in looking past the obvious and being observant. It puts us in the mood for breaking out our old Tek scope and seeing what wonders — and challenges — it holds.

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Simple PCB Repairs Keep Old Vehicle Out Of The Crusher

October 24, 2024 by Dan Maloney 21 Comments

For those of us devoted to keeping an older vehicle on the road, the struggle is real. We know that at some point, a part will go bad and we’ll learn that it’s no longer available from the dealer or in the aftermarket, at least at a reasonable cost. We might get lucky and find a replacement at the boneyard, but if not — well, it was nice knowing ya, faithful chariot.

It doesn’t have to be that way, though, at least if the wonky part is one of the many computer modules found in most cars made in the last few decades. Sometimes they can be repaired, as with this engine control module from a Ford F350 pickup. Admittedly, [jeffescortlx] got pretty lucky with this module, which with its trio of obviously defective electrolytics practically diagnosed itself. He also had the advantage of the module’s mid-90s technology, which still relied heavily on through-hole parts, making the repair easier.

Unfortunately, his luck stopped there, as the caps had released the schmoo and corroded quite a few traces on the PCB. Complicating the repair was the conformal coating on everything, a common problem on any electronics used in rough environments. It took a bit of probing and poking to locate all the open traces, which included a mystery trace far away from any of the leaky caps. Magnet wire was used to repair the damaged traces, the caps were replaced with new ones, and everything got a fresh coat of brush-on conformal coating.

Simple though they may be, we really enjoy these successful vehicle module repairs because they give us hope that when the day eventually comes, we’ll stand a chance of being able to perform some repair heroics. And it’s nice to know that something as simple as fixing a dead dashboard cluster can keep a car out of the crusher.

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Heathkit Signal Generator Gets An Update

October 22, 2024 by Al Williams 9 Comments

[DTSS_Smudge] correctly intuits that if you are interested in an old Heathkit signal generator, you probably already know how to solder. So, in a recent video, he focused on the components he decided to update for safety and other reasons. Meanwhile, we get treated to a nice teardown of this iconic piece of test gear.

If you didn’t grow up in the 1960s, it seems strange that the device has a polarized line cord with one end connected to the chassis. But that used to be quite common, just like kids didn’t wear helmets on bikes in those days.

A lot of TVs were “hot chassis” back then, too. We were always taught to touch the chassis with the back of your hand first. That way, if you get a shock, the associated muscle contraction will pull your hand away from the electricity. Touching it normally will make you grip the offending chassis hard, and you probably won’t be able to let go until someone kindly pulls the plug or a fuse blows.

These signal generators were very common back in the day. A lot of Heathkit gear was very serviceable and more affordable than the commercial alternatives. In 1970, these cost about $32 as a kit or $60 already built. While $32 doesn’t sound like much, it is equivalent to $260 today, so not an impulse buy.

Some of the parts are simply irreplaceable. The variable capacitor would be tough to source since it is a special type. The coils would also be tough to find replacements, although you might have luck rewinding them if it were necessary.

We are spoiled today with so many cheap quality instruments available. However, there was something satisfying about building your own gear and it certainly helped if you ever had to fix it.

There was so much Heathkit gear around that even though they’ve been gone for years, you still see quite a few units in use. Not all of their gear had tubes, but some of our favorite ones did.

Fail Of The Week: The Case Of The Curiously Colored Streetlights

October 9, 2024 by Dan Maloney 116 Comments

What color are the street lights in your town? While an unfortunate few still suffer under one of the awful colors offered by vapor discharge lamps, like the pink or orange of sodium or the greenish-white of mercury, most municipalities have moved to energy-saving LED streetlights, with a bright white light that’s generally superior in every way. Unless, of course, things go wrong and the lights start to mysteriously change colors.

If you’ve noticed this trend in your area, relax; [NanoPalomaki] has an in-depth and surprisingly interesting analysis of why LED streetlights are changing colors. After examining a few streetlights removed from service thanks to changing from white to purple, he discovered a simple explanation. White LEDs aren’t emitting white light directly; rather, the white light comes from phosphors coating the underlying LED, which emits a deep blue light. The defunct units all showed signs of phosphor degradation. In some cases, the phosphors seemed discolored, as if they experienced overheating or chemical changes. In other LEDs the phosphor layer was physically separated from the backing, exposing the underlying LEDs completely. The color of these damaged modules was significantly shifted toward the blue end of the spectrum, which was obviously why they were removed from service.

Now, a discolored LED here and there does not exactly constitute a streetlight emergency, but it’s happening to enough cities that people are starting to take notice. The obvious solution would be for municipalities to replace the dodgy units Even in the unlikely event that a city would get some compensation from the manufacturer, this seems like an expensive proposition. Luckily, [NanoPalomaki] tested a solution: he mixed a wideband phosphor into a UV-curable resin and painted it onto the lens of each defective LED in the fixture. Two coats seemed to do the trick.

We have to admit that we have a hard time visualizing a city employee painstakingly painting LEDs when swapping out a fixture would take an electrician a few minutes, but at least it’s an option. And, it’s something for hobbyists and homeowners faced with the problem of wonky white LEDs to keep in mind too.

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