EDIT 3/6/2021: I’ve got a new post up with a video of this repair. I’m embedding it near the bottom of this post as well for convenience
The world is a small place sometimes. Last year, I was cruising Craigslist and came across a Technics tape deck and some classic gaming accessories. Met the guy, made the exchange. Mentioned what I do — a few weeks later he texted me asking if I would like to take a look at seven or eight Takamine preamps that had low or no output. Turns out he’s the admin of a multi-thousand member Facebook group for the guitars! Some worked in ‘Pass’ mode or picked up on a digital tuner, but weren’t getting a signal to the amp.
I said that I’d take a look at them (I figured they just needed recapped), and he neglected to tell me that the units had been deemed “unrepairable” by most techs for some reason. So I designed a repair. Charlie supplied me with these schematics:
And I got to work. First, we’d need to depopulate the six surface mount caps present in between the sliders, the cap on the volume board, and the radial power cap on the mainboard.
The integrity of the pads isn’t of much concern. The plastic sliders make putting in new surface mount caps unfeasible, and the back of the board has adequate room in the plastic casing to insert radial caps in circuit. I started by yanking these and their pins off with needlenose pliers.
One quirk is that the volume board could have a tantalum capacitor or an aluminum electrolytic. I replace the tantalum when I recap them (because why not), though it’s worth noting that you’ll experience a degraded signal if you replace a tantalum with an aluminum — I suspect that a different component is used elsewhere though the board revision is the same. I have a photo of each type:
|Reference Designator||Capacitance/Voltage||Positive Lead||Negative Lead||Notes|
|C301||Aluminum: 4.7 uF/25V|
Tantalum: 4.7 uF/16V
|Marked on board||Marked on board||On some units, negative lead is bridged to R301; this is fine|
|C16||470 uF/16V||By wire pad 1||By wire pad 2||Polarity marked on board|
|C10||0.47 uF/50V||South pin of R1||North pin of R1|
|C11||0.47 uF/50V||South pin of R5||North pin of R6|
|C12||0.47 uF/50V||Point north of R22||North pin of R25||Orange Circle|
|C13||0.47 uF/50V||South pin of R30||Emitter of Q3||Q3’s Emitter marked with ‘E’|
|C14||4.7 uF/25V||Pin west of Q1’s Gate Ref||North pin of R7||Green Circle|
|C15||10 uF/16V||South pin of R14||North pin of R21|
With this parts list and connection points, one can restore an AAP to full functionality! If using quality caps, it’ll perform just like new. The cardinal directions from the table are based on the board’s orientation that I use, and I’ve marked some of the confusing spots:
Yet another issue can arise though — depending on whether or not a certain pad is lifted or corroded when ripping out the surface mount caps, you may require a lead wire to restore functionality to the battery LEDs. It doesn’t hurt anything to include it even if it’s not needed, so I put it in every one just in case. Put it between the south pin of R30 and the collector of Q2 (marked with a C, conveniently).
When you’re done it should look something like this:
I’ve embedded a video below, thanks to Peter for sending me one to repair and film!
That leads us to the question of how well this repair works, and if it’s worth the time. I did a spectrum analysis using RightMark Audio Analyzer on a completely dead unit, and performed it again after the repair with a test signal. Since the units only have one input, the white line is of interest on the following graphs, and unfortunately I couldn’t turn the other channel (green) off.
If you’d like to repair one of these yourself (or live overseas), I’ve created a repair document for these. One link is to Google Docs which displays better on a mobile device, and there’s also a .docx version that prints well. You’re free to use my repair and reproduce the document, as long as you don’t remove the attribution to me! Help keep these units out of the trash bin!
Microsoft Word (docx): https://drive.google.com/file/d/1zgc5hUz_NwyO6gO1QureVq51Zp5IEC7O/view?usp=sharing