Karaoke Machine Has Owner Singing the Blues

We purchased an inexpensive but fairly full-featured karaoke machine for the kids. One selling point for this particular model was that it accepted two separate inputs - for double the fun! But just one week later, one of the kids asked, "Can you look at the karaoke machine? One of the microphone inputs isn't working."

We swapped the microphones around and determined the problem was internal to the unit, so I decided to open it up and see what I could find. The back of the unit came off easily, as it was attached with standard Phillips screws, only one of which was hidden by a label. After detaching the power input panel from the back cover, I had clear access to the internals, shown in Figure 2.

My first impressions were good, with most connectors secured with an adhesive to help prevent them from vibrating loose. I started tracking the microphone signals through the plugs and cabling, eventually back to a board with potentiometers for sound adjustment. A check of the volume adjustment pots for the microphones seemed OK. The next step was to track the signals from the inputs, which led me to a dual op-amp. Here is where the monkeys had worked their mischief.

First, the op-amp is a JRC4558, which turned out to be quite obsolete. Second, a brief examination of the board revealed what appeared to be heavy flux residue and the beginnings of corrosion, shown in Figure 3. Cleaning this area and some others where the residue was less severe led to the system working again -- hopefully that did the trick. Incidentally, we can even identify the monkey in question: A sticker reads "Passed 24" in the photo a couple inches from the worst residue.

Tell us your experiences with monkey-designed products. Send stories to Jennifer Campbell for Made by Monkeys.