Current Limiting Diode Use and Tutorial

Current limiting diode 1

Not that this happens often, but what do you do when faced with a repair where you don’t know the power source but you do know you have to drive LED backlighting? When faced with this dilemma [Eric Wasatonic’s] solution was to design for ambiguity. In this interesting hack repair [Eric] needed to restore backlighting for an old car stereo LCD display. First he guaranteed he was working with a DC power source by inserting a small full-wave bridge rectifier. Then knowing he needed 4 mA to power each LED for backlighting he used some 1978 vintage current limiting diodes designed to pass 2mA each regardless of voltage source, within limits of course.

Sure this is a simple hack repair but worthy of being included in anyone’s bag of tricks. Like most hacks there is always knowledge to be gained. [Eric] shares a second video where he uses a curve tracer and some datasheets to understand how these old parts actually tick. These old 1N5305 current limiting diode regulators are simply constructed from a JFET with an internal feedback resistor to its gate which maintains a fixed current output. To demonstrate the simplicity of such a component, [Eric] constructs a current limiting circuit using a JFET and feedback potentiometer then confirms the functionality on a curve tracer. His fabricated simulation circuit worked perfectly.

There was a little money to be made with this repair which is always an added bonus, and the recipient never reported back with any problems so the fix is assumed successful. You can watch the two videos linked after the break, plus it would be interesting to hear your thoughts on what could have been done differently given the same circumstances.

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Repairing Bose Active Noise Cancelling Headphones

[Mansour] was disappointed to find out that his Bose QC15 headphones had a dead right channel. These headphones have active noise cancelling, which uses a microphone to capture ambient noise and digital signal processing to insert an out of phase signal. Since they’re quite expensive, [Mansour] was determined to resurrect them.

First, he determined that the right speaker had died, so he found a replacement on eBay. These were designed for a different set of headphones, but matched the impedance of the original Bose part. After replacing the driver, it seemed that the repair was a failure. The sound cancelling wasn’t working, and a the playback was high-pitched. As a last attempt, he potted the speaker with glue, to match the original construction. Much to his surprise, this worked.

The problem was that the new driver didn’t have sufficient sound isolation from the microphone, which is meant to pick up passive noise. This feedback likely caused issues with the noise cancelling DSP. A little glue meant a $20 fix for a $400 pair of headphones.

Repairing Dead USB Flash Drives

Over the last few years, [Tobias] has repaired a number of USB Flash drives. This strikes us as a little odd, given small capacity Flash drives are effectively free in the form of conference handouts and swag, but we’re guessing [Tobias] has had a few too many friends lose their thesis to a broken Flash drive.

In all his repairs, [Tobias] found one thing in common The crystal responsible for communicating with the USB controller is always broken. In a way, this makes a lot of sense; everything else on a Flash drive is silicon encased in an epoxy package, where the crystal is a somewhat fragile piece of quartz. Breaking even a small part of this crystal will drastically change the frequency it resonates at making the USB controller throw a fit.

[Tobias]’ solution for all his Flash drive repairs is to desolder and change out the crystal, bringing the drive back to life. Some of the USB Flash drives even have multiple pads for different crystal packages, making it easy to kludge together a solution should you need to repair a Flash drive five minutes ago.

Simple Dremel TRIAC Hack Repair

Dremel Repair

It’s a wonderful thing to see a clever hack repair instead of disposing of a product. The best repair approach is finding exact replacement components, but sometimes exact components can’t be sourced or cross-referenced. Other times the product isn’t worth the shipping cost for replacement parts or you just don’t have time to wait for parts. That’s when you need to really know how something works electronically so you can source suitable replacement components from your junk bin to complete the repair. This is exactly what [Daniel Jose Viana] did when his 110 volt Dremel tool popped its TRIAC after he plugged it into a 220 volt outlet.

[Daniel] knew how the TRIAC functioned in the circuit and also knew that a standard TRIAC of sufficient specifications could be used as a replacement even if it didn’t have the correct form factor to fit the PCB layout. For [Daniel’s] tool repair he had to think outside the box enough to realize he could use some jumper wires and snuggle a larger TIC206E TRIAC that wasn’t meant for the device but still applicable into the housing where there was enough free space. A little shrink-wrap and all was good again. Sure the fix was simple, but let’s not trivialize the knowledge he needed for this repair.

And if you’re wondering if it worked, he notes that he’s been using this tool for three years since the repair. We thank [Daniel] for sharing this tip and allowing us to add this to our tool belt of Dremel repair tricks.

Diagnose and Repair a Yaesu FT-7800 Ham Radio

Yaesu FT-7800 Ham Radio

[Alan Wolke] aka [w2aew] was challenged to repair a friends Yaesu FT-7800 ham radio. This radio operates on two ham bands, 2 m VHF and 70 cm UHF. The complaint was that the 2 m side was not working but the 70 cm was transmitting fine. Alan started by verifying the complaint using a Bird watt meter with a 50 watt slug and terminating the signal into a 50 W dummy load. [Allen’s] bird meter is the type that has an RF sampler that can be connected to an oscilloscope for added signal viewing and validation.

After verifying that the radio was not working as described, Alan starts by glancing over the circuit board to look for any obvious damage. He then walks us through a block diagram as well as a circuit diagram of the FT-7800 radio before stepping us through the troubleshooting and diagnostics of radio repair. Even when he realizes he might have found the problem he still steps us through the remainder of his diagnostics. The skills and knowledge that Alan shares is extremely valuable to anybody looking to repair radios.

Spoiler alert. At the end of the first video he determines that the pin diodes near the final VHF output were bad. In the second video he reveals that he could no longer source these bad components. Through some clever evaluation of a more current Yaesu radio, [Allen] was able to find suitable replacement components. Lesson two ends with some surface mount solder rework tips as well as testing that the repair was successful.

And just in case you don’t know what a pin diode is, or is used for, Alan shares a third video covering just what this component is and does in a radio. You can follow the jump to watch all three videos.

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The trials of repairing a MacBook


As a favor to a friend, [Phil] traded a unibody MacBook logic board for one with a broken headphone jack, a busted keyboard controller, and a nonfunctional fan. Not one to let bad hardware go to waste, he set off to repair this now-broken laptop by scavenging parts wherever he could. The whole thing ended up working, and became a very impressive display of soldering skill in the process.

The first step for the keyboard transplant was to cut a properly sized hole in the newer unibody MacBook for an older, pre-unibody MacBook Pro 17″ keyboard. This was done by cutting out the keyboard pan of the pre-unibody case and very carefully epoxying it into the unibody chassis. The MBP had a separate keyboard and trackpad controller, so of course [Paul] needed to find some space inside the chassis for these new electronics. This space was found next to the internal hard drive, and a liberal application of hot glue held everything together.

In the future, [Phil] plans on adding more LEDs, a 3.5 mm jack, and a USB to TTL converter – a necessity for any true ‘hacker’ laptop. It’s still a wonderful piece of work, and an incredible amount of effort and skill to get it where it is today.

[Quinn] resurrects an amplifier that experienced death-by-capacitor

[Quinn Dunki] is adding wireless audio to all of the rooms in her home. She’s going with Airplay, snatching up used or refurbished Airport Express units because of their ability to work with both her existing WiFi and the Airplay protocol. The last piece in the puzzle is to get an Amp and she chose the small unit seen above. The problem is that it was dead on arrival and she couldn’t get the company to respond to her issue. So she cracked it open and fixed it right up.

The offenders are the three electrolytic capacitors at the top of the picture. She took some close-up images of each and you can’t miss the fact that they’re blown out. These are often among the higher price-per-unit parts and manufactures try to pinch the penny as much as possible. Add to it the heat in a small enclosure like this one and you’ve got a failure. [Quinn] dug through her junk bin but the size of the replacement had to be a perfect match so she ended up putting in a parts order. The new caps fit and work perfectly as you can hear in the clip after the break.

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