Inside A Selective Voltmeter

May 9, 2025 by Al Williams No comments

[Martin Lorton] has a vintage Harmon 4200B selective voltmeter that needed repair. He picked it up on eBay, and he knew it wasn’t working, but it was in good condition, especially for the price. He’s posted four videos about what’s inside and how he’s fixing it. You can see the first installment below.

The 4200B is an RMS voltmeter and is selective because it has a tuned circuit to adjust to a particular frequency. The unit uses discrete components and has an analog meter along with an LCD counter.

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Restoring An Abandoned Game Boy Kiosk

April 18, 2025 by Maya Posch 8 Comments

Back in the olden days, there existed physical game stores, which in addition to physical games would also have kiosks where you could try out the current game consoles and handhelds. Generally these kiosks held the console, a display and any controllers if needed. After a while these kiosks would get scrapped, with only a very few ending up being rescued and restored. One of the lucky ones is a Game Boy kiosk, which [The Retro Future] managed to snag after it was found in a construction site. Sadly the thing was in a very rough condition, with the particle board especially being mostly destroyed.

Display model Game Boy, safely secured into the demo kiosk. (Credit: The Retro Future, YouTube)

These Game Boy kiosks also featured a special Game Boy, which – despite being super rare – also was hunted down. This led to the restoration, which included recovering as much of the original particle board as possible, with a professional furniture restore ([Don]) lending his expertise. This provides a master class in how to patch up damaged particle board, as maligned as this wood-dust-and-glue material is.

The boards were then reassembled more securely than the wood screws used by the person who had found the destroyed kiosk, in a way that allows for easy disassembly if needed. Fortunately most of the plastic pieces were still intact, and the Game Boy grey paint was easily matched. Next was reproducing a missing piece of art work, with fortunately existing versions available as reference. For a few missing metal bits that held the special Game Boy in place another kiosk was used to provide measurements.

After all this, the kiosk was powered back on, and it was like 1990 was back once again, just in time for playing Tetris on a dim, green-and-black screen while hunched half into the kiosk at the game store.

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A Tricky Commodore PET Repair And A Lesson About Assumptions

April 14, 2025 by Maya Posch 11 Comments

The PET opened, showing the motherboard. (Credit: Ken Shirriff)

An unavoidable part of old home computer systems and kin like the Commodore PET is that due to the age of their components they will develop issues that go far beyond what was covered in the official repair manual, not to mention require unconventional repairs. A case in point is the 2001 series Commodore PET that [Ken Shirriff] recently repaired.

The initial diagnosis was quite straightforward: it did turn on, but only displayed random symbols on the CRT, so obviously the ICs weren’t entirely happy, but at least the power supply and the basic display routines seemed to be more or less functional. Surely this meant that only a few bad ICs and maybe a few capacitors had to be replaced, and everything would be fully functional again.

Initially two bad MOS MPS6540 ROM chips had to be replaced with 2716 EPROMs using an adapter, but this did not fix the original symptom. After a logic analyzer session three bad RAM ICs were identified, which mostly fixed the display issue, aside from a quaint 2×2 checkerboard pattern and completely bizarre behavior upon running BASIC programs.

Using the logic analyzer capture the 6502 MPU was identified as writing to the wrong addresses. Ironically, this turned out to be due to a wrong byte in one of the replacement 2716 EPROMs as the used programmer wasn’t quite capable of hitting the right programming voltage. Using a better programmer fixed this, but on the next boot another RAM IC turned out to have failed, upping the total of failed silicon to four RAM & two ROM ICs, as pictured above, and teaching the important lesson to test replacement ROMs before you stick them into a system.

Repairing Classic Sound Cards

April 12, 2025 by Jenny List 15 Comments

Sound hardware has been built into PC motherboards for so long now it’s difficult to remember the days when a sound card was an expensive add-on peripheral. By the mid to late 1990s they were affordable and ubiquitous enough to be everywhere, but three decades later some of them are starting to fail. [Necroware] takes us through the repair of a couple of Creative Labs Sound Blaster 16s, which were the card to have back then.

The video below is a relaxed look at typical problems afflicting second-hand cards with uncertain pasts. There’s a broken PCB trace on the first one, which receives a neat repair. The second one has a lot more wrong with it though, and reveals some surprises. We would have found the dead 74 series chips, but we’re not so sure we’d have immediately suspected a resistor network as the culprit.

Watching these cards become sought-after in the 2020s is a little painful for those of us who were there at the time, because it’s certain we won’t be the only ones who cleared out a pile of old ISA cards back in the 2000s. If you find one today and don’t have an ISA slot, worry not, because you can still interface it via your LPC bus.

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Clever Engineering Leaves Appliance Useless

April 10, 2025 by Tom Nardi 23 Comments

Around these parts, we generally celebrate clever hacks that let you do more with less. So if somebody wrote in to tell us how they used multiplexing to drive the front panel of their latest gadget with fewer pins on the microcontroller than would normally be required, we’d be all over it. But what if that same hack ended up leading to a common failure in a piece of consumer hardware?

As [Jim] recently found out, that’s precisely what seems to be ailing the Meaco Arete dehumidifier. When his stopped working, some Internet searching uncovered the cause of the failure: if a segment in the cheap LED display dies and shorts out, the multiplexing scheme used to interface with the front panel essentially reads that as a stuck button and causes the microcontroller to lock up. He passed the info along to us as a cautionary tale of how over-optimization can come with a hidden cost down the line.

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Repairing A Legendary Elka Synthex Analog Synthesizer

March 17, 2025 by Maya Posch 16 Comments

Handy diagnostic LEDs on the side of the tone generator boards. (Credit: Mend it Mark, YouTube)

Somehow, an Elka Synthex analog synthesizer made it onto [Mend it Mark]’s repair bench recently. It had a couple of dud buttons, and some keys produced the wrong tone. Remember, this is an analog synthesizer from the 1980s, so we’re talking basic 74LS chips and kin. Fortunately, Elka helped him with the complete repair manual, including schematics.

As usual, [Mark] starts by diagnosing the faults, using the schematics to mark the parts of the circuitry to focus on. Then, the synth’s bonnet is popped open to reveal its absolutely gobsmackingly delightful inner workings, with neatly modular PCBs attached to a central backplane. The entire unit is controlled by a 6502 MPU, with basic counter ICs handling tone generation, controlled by top panel settings.

The Elka Synthex is a polyphonic analog synthesizer produced from 1981 to 1985 and used by famous artists, including Jean-Michel Jarre. Due to its modular nature, [Mark] was quickly able to hunt down the few defective 74LS chips and replace them before testing the instrument by playing some synth tunes from Jean-Michel Jarre’s Oxygène album, as is proper with a 1980s synthesizer.

Looking for something simpler? Or, perhaps, you want something not quite that simple.

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Tiny Laptop Gets A New Case And An Unlocking

March 11, 2025 by Dan Maloney 21 Comments

Unless you’ve got an especially small lap, calling the Toshiba Libretto a laptop is a bit of a stretch. The diminutive computers from the mid-1990s had a lot of the usual laptop features, but in an especially compact and portable case that made them a great choice for anyone with an on-the-go lifestyle.

Fast-forward thirty years or so, and the remaining Librettos haven’t fared too well. Many of them have cases that crumble at the slightest touch, which is what led [polymatt] to undertake this meticulous case replacement. The effort started with a complete teardown; luckily, the lower aluminum-alloy shell was in fine shape, but the upper case parts were found to be almost too deteriorated to handle. Still, with a little patience and the judicious application of tape, [polymatt] was able to scan the case pieces on a flatbed scanner and import them into his CAD package. Great tip on the blue-tack for leveling the parts for accurate scanning, by the way.

After multiple rounds of printing and tweaking, [polymatt] had a case good enough to reassemble the Libretto. Unfortunately, the previous owner left an unwanted gift: a BIOS password. Disconnecting the CMOS battery didn’t reset it, but a little research told him that shorting a few pins on the parallel port on the machine’s dock should do the trick. It was a bit involved, requiring the design and subsequent bodging of a PCB to fit into the docking port connector, but in the end he was able to wake up a machine to all its Windows 95 glory. Better get patching.

In a time when laptops were more like lap-crushers, the Libretto was an amazing little machine, and thirty years on, they’re well worth saving from the scrap heap. Hats off to [polymatt] for the effort to save this beauty, and if he needs tips on reading data from any PCMCIA cards that may have come with it, we’ve got him covered.

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