Installing a Pickup and Preamp in a Yamaha FG700S Acoustic

Fig. 1. Yamaha FG700S

If this post isn't all that informative, that's at least partly because I didn't take that many photos. I was so intent on not destroying the guitar that it didn't even occur to me to take photos until after I successfully cut the first hole. But first things first.

I have an inexpensive Yamaha FG700S, which is a $200 guitar new, but I got mine used for $140. For a solid top guitar, it's hard to beat for the price, and I like this one a lot. However, shortly after buying it, I wished I had bought an acoustic electric. I put it on craigslist so I could upgrade to an acoustic electric, but got no bites. So I bought a $13 piezo pickup, and a $22 preamp and input on ebay. I didn't want to spend too much, because I felt there was a decent chance that I would destroy the guitar while trying to install the pickup, since I had never done this before. And I figured if I liked it, I could always upgrade to a better preamp and pickup.

The part I feared most, obviously, was cutting two holes in the guitar. Measuring and cutting a hole in a flat plane is easy, but the preamp goes on a curved section of the guitar. I measured the preamp housing several times, placed it along the edge of the curve in the guitar body to see where it would fit best. After finding a sweet spot, I used a razor blade to make little nicks in the finish marking the border of the section to be cut out. I then marked the border with masking tape (Fig. 2).

I drilled the corners with a 1/2" bit. I started to use a jigsaw to cut the hole, but the wood was too fragile. It made a sloppy cut. I tried a utility knife to score the border of the hole, and I kept scoring until it poked through. This didn't take as long as expected, and the cuts were pretty clean. I sanded the hole and inserted the preamp. It was a little tight here and there so I sanded it again and it fit. I didn't get photos of the preamp installation, so here is the during and after shots of the input and the after shot of the preamp:

Fig. 2. Cutting the hole for the input. Installed input and preamp.

I did the same exact thing with the input. Then I plugged the piezo pickup into the preamp and stuck it under the bridge.

Fig. 3. About to place the pickup under the bridge.

I should note that the preamp came with as under-the-saddle pickup (Fig. 4). I preferred the kind that mounts inside the guitar under the bridge (Fig. 3, above), so I snipped the input off the former and and soldered it to the latter and used that.

Fig. 4. Cheap under-the-saddle pickup

Everything's in, and although I made the input hole a hair too big (so there's not enough wood in one corner for the screw to grab), it went better than I expected.

It Pays to Have a Little Fun—Fixing a Marshall Master Lead Combo 5010

Have you ever checked to see if an amp or pedal was working by sticking a cable in and tapping the other end? Or noodled on the guitar for a moment to make sure that whatever you just fixed works? My latest repair highlighted the importance of taking the time to ensure that the problem is indeed fixed. And as a mere dabbler in repair, this is something I need to remind myself from time to time.

I recently was given an Marshall Master Lead Combo 5010 to fix (Fig. 1 below). When I tested it, it had output, but it was intermittent and there were occasional dropouts and static pops. Turning the Preamp control caused a lot of crackle and garbling of the signal. There was crackle with all the other controls as well, but not as bad.

Fig. 1. Marshall Master Lead Combo 5010 Solid State

I removed the chassis and thoroughly cleaned all the pots. I plugged in a guitar and it sounded fine, even when I turned all of the controls. No static. So I played for a few moments more and thought I heard a pop. I stopped and waited, but didn't hear it again. I played again, and after five or ten seconds, heard another pop.

I did the chopstick test, poking all of the components (of which there are not many—see Fig. 2). The corner of the board near the two big caps was pretty loose, and when I poked it, I heard loud crackling. I poked at the caps and one of them was markedly looser than the other. I looked at the solder joints with an inspection mirror while I wobbled it, and sure enough, I could see the wire and solder moving freely back and forth as I did so (Fig. 3)

Fig. 2. The simple layout of the board was refreshing when troubleshooting.

Figure 3. A very loose and sloppy solder joint

Apparently, when I played, the vibration was causing that bad joint to wiggle enough to cause the pops.

When I removed the board, it was immediately apparent that previous repairs and other sundry misfortunes had been visited upon this poor amp (Fig. 4). It all seemed to work except for the joint in question, so I ignored the other stuff, resoldered the bad joint, plugged in the guitar and played for a bit. None of the problems were evident, so that did the trick.

Fig 4. Some of the copper traces had previously been scraped and exposed for reasons unknown.

After cleaning the pots, if I had just done the cable-only test or noodled on the guitar for a moment, I would have assumed that the problem was fixed. The bad solder joint might not have had the opportunity to cause a problem, and I would have called it good and returned it as supposedly working. Playing it normally for a little while saved me that embarrassment.

When I fix an amp, I always play it just for fun for five to ten minutes. Often, it's likely the only opportunity I will have to try a particular amp. It's usually an amp I haven't played before and it's almost certainly one I will never own, so it's a nice chance to try a new amp in the comfort of my own home. But more importantly, this indulgence also serves the purpose of making sure I'm not too hasty in calling it fixed.