Takamine EF 349 Acoustic Electric Guitar

Recently, I repaired a Takamine EF 349 acoustic/electric guitar. It was one of the older ones made with a Martin-like headstock and logo. It features what some call the “old brownie” TC-02M preamp. The TC-02M preamp features three sliders; gain, treble, and bass. There’s not much information about this preamp online so I hope this post can help out others in the future that try and repair these. The guitar had a couple issues. First, the preamp stopped working. Second, when it was working previously it was going through a lot of batteries.

I started investigating the preamp in the guitar and discovered one of the jacks in the internal preamp felt very loose when plugging and unplugging the jack. There are two jacks on the preamp. One is 2.5mm and the other is 3.5mm. The 3.5mm connects to the output jack on the end of the guitar, and the 2.5mm connects to the internal pickup. The jack for the internal pickup (2.5mm) was the one that was loose.

The next hurdle was trying to figure out how to get the preamp outside of the guitar so I could actually see it and work on it. After some research online and asking “Mr. Takamine” on Facebook, I found out the stickers for the bass and treble sliders have to be removed in order to expose two screws that hold the preamp in place. I also needed to unplug the cables for the pickup and output jack of course.

Preamp with stickers removed to expose screws
Underside of preamp. Picture was taken after 2.5mm jack replaced.

Once I had the preamp outside of the guitar I took it apart and discovered the 2.5mm jack for the pickup was completely broken. I knew that definitely had to be replaced but wanted to investigate more to see if there were other issues. I used some jumper cables with alligator clips to temporarily connect the pickup to the broken jack.

Broken jack that connects pickup

The battery connections were unsoldered as well so the preamp was completely free from the guitar. This allowed me to work on it much easier. I temporarily soldered a power jack to the board like you would see on a guitar pedal. This allowed me to test the preamp with a 9V power supply instead of going through a ton of batteries. When testing the preamp I noticed the bass and treble controls seemed to work fine but the gain control only worked when it was in the center. On both ends of the control the output was completely gone.

I desoldered and removed the gain slider from the board completely to test it. It tested perfectly fine so I knew the slider itself was not the issue. I searched online for a schematic of this preamp circuit but couldn’t find one at all. Since there weren’t too many components on the board I decided to go ahead and trace the circuit and draw my own schematic. You can see it below.

Takamine TC-02M Schematic

The circuit looks like a two-band Baxandall tone control with a gain control after it. Looking at the components around the gain control I suspected the 10uf tantalum capacitor (C7 in the schematic) might be the issue. Some say tantalum capacitors have a tendency to short out over time. I removed it to test it and sure enough it was shorted out. There were two other tantalum capacitors on the board as well (C2 and C6 in he schematic). I went ahead and replaced those as well just to prevent any possible issues in the future.

Tantalum Capacitors I replaced
Preamp board after I replaced the capacitors. The one on the bottom right is 10uF, the other two are 1uf.

After replacing the caps I tested the preamp again. Everything was working great. The gain control had full volume on 10 and no volume on 0 just like it’s supposed to work. I also replaced the broken jack with a high quality metal Switchcraft TR2A 2.5mm jack pictured below.

Board with new Switchcraft TR2A jack installed on the right

Then I started looking into why the guitar was going through so many batteries. I knew the output jack had been replaced in the past so I looked at that first. When I got the jack out of the guitar I could see it was a mono jack with only two connections. Usually with an acoustic electric you need a stereo jack with three connections. That third connection is used to connect the battery when a cable is plugged in and disconnect the battery when the cable is unplugged. That way the battery isn’t being drained when the guitar is not in use.

Old Mono jack with spliced wire.

To stop the guitar from eating batteries, I installed a replacement Gotoh stereo output jack very similar to what the guitar originally had. I had to enlarge the hole slightly to get it to fit. As you can see in the pictures, there were already three small mounting screw holes from the original jack.

Test fitting the new jack. There were already three mounting screw holes from the original jack. The new one lined up perfectly with the existing holes.

Next, I made a new cable to go from the output jack to the preamp. The old one had been shortened too much, was spliced together, and was unusable. I wanted cable that was shielded the entire length to prevent any noise. I chose Mogami W2582 microphone cable because it was readily available, features two conductors and is well shielded. Below is a picture of the replacement cable I made. One end is soldered to a high quality Amphenol KS3P 3.5mm plug and the other end is soldered to the new Gotoh output jack. Compared to the old cable it’s much thicker and better quality.

Once everything was back together I tested it. It works perfectly now! All the controls work, great sound quality, very good signal to noise ratio, and it doesn’t eat batteries anymore. The final step was to reattach the Treble and Bass stickers on the preamp. I just used some alcohol and a Q-tip to clean any reside and then attached the stickers with a dab of thin superglue.

New jack installed (left). Stickers reattached to preamp (right).

Again, I hope this post can help someone else in the future that’s trying to repair one of these old preamps. If you have any questions or if there’s any way I can help please email me at snestler86@gmail.com