15 October 2014

Fixing a Sears Silvertone 1448 Guitar Case Tube Amplifier

Fig. 1a. Sears/Silvertone 1448 Guitar Case/Tube Amplifier
Fig. 1b. The innards—a 3-watt tube amplifier with a 6-inch speaker.

In the summer of 1984, I bought a Sears/Silvertone 1448 guitar and amp-in-a-case at a flea market for $3. Several years later, before I starting buying and selling on eBay, I sold it for $125. It currently goes for $500 and over on eBay these days, depending on the condition. Whenever I regret selling it for only $125, I remind myself that at least I didn’t sell it for $3. I only played through it a few times, and the amp had a sweet bit of overdrive when cranked.

The one you see in these photos did not. I was charged with repairing this one, which had very low output and an input jack that had been pushed through the control panel (Fig. 2). The first thing I did was to secure the input jack, so I could play through it. I had a washer on hand, drilled the center hole big enough for the jack and tried that. It’s not pretty (Fig. 6), but it’s cheap and solid.

Fig. 2. The input had been pushed through the control panel. Also note the missing bolt to the right of the Volume knob.
With the input jack in order, I tested it myself, and the output was very low, with a lot of crackle. The first thing I noticed was bad connections on the speaker wires (Fig. 3), which had been pretty poorly spliced and not even soldered. I found two other bad joints under the panel, which was a mess (Fig. 4). Five bolts were missing, so the transformer and capacitor were just hanging loosely, as was the bracket holding the tubes.
Fig. 3. The speaker wires were just pinched together, like the red one in this photo. (Another missing bolt in the upper right.)

Fig. 4. This is seriously what the guts looked like when I removed the control panel. The missing bolts attach the big capacitor, the transformer and the bracket holding the tubes.

I re-soldered the bad joints and bolted the cap to the back of the panel. The bolt head looked like hell, and I couldn’t justify using it on such an old amp, so I removed that and used rivets instead (Fig. 5). These rivets alone were too small for these holes, so I had to buy some rivet washers. The rivets I used for this were 1/8” (3mm) diameter with a 1/4” grip. The washers were 1/8” (hole diameter). This worked very well. When I was done, everything was securely attached—no wobble—and looked much better on the front of the panel (and on the back of the panel for that matter—see Fig. 6).

Fig. 5. The rivets and rivet washers used to secure the chassis, capacitor and transformer

Fig. 6. These rivets were the perfect size. Unobtrusive, but big enough to secure everything tightly.

Before looking too much further for any other problems, I plugged in and tried it again. It worked and sounded pretty good. There was a little crackle when adjusting the volume, so I sprayed the pot and called it good. Here's the finished panel:

Fig. 6. Good to go. The rivets fit in with the vintage vibe. The input jack less so, but it is sturdy and easily replaced.

04 October 2014

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.