Finds: Kenwood Stereo

Check out those bling knobs.

Check out those bling knobs.

About a year ago, I found a 90s era Kenwood stereo system going spare. Destined for the trash, I couldn't let it go to waste. It looked in such good condition! Alas, when I did eventually manage to power it up, I was to be disappointed. 

Upon hooking everything up, there was very little working. Sound could be heard very quietly on the right channel, with nothing at all coming through on the left. Playing with the controls showed me that the volume control did nothing until it got to the bottom of its range, and balance was also completely inactive. This led me to suspect the volume control IC, as this stereo has soft controls. 

We initially wasted some serious time thinking we were working with the larger LC7535M. Awkward.

We initially wasted some serious time thinking we were working with the larger LC7535M. Awkward.

After opening up the unit, some investigation and Googling of chip names led me to find the stereo used the Sharp LC7535P, a volume control chip in a rather odd 22-pin DIP package. Already having determined it was non-functional, and pricing a replacement at >$20 on eBay, I decided to remove it completely, and hardwire the input signal to the output pin of the chip to test things (in this case, wiring the pin 1 trace to pin 6 and pin 22 to 17). Success... sort of. There was now a clean, loud signal on the right channel, but distortion on the left. 

Spot the SMD replacement transistor floating in free space. We globbed it in epoxy later to protect it from possible shorts. The red, blue and black leads head to the front of the stereo so we could splice a pot in place of the former volume IC.

Spot the SMD replacement transistor floating in free space. We globbed it in epoxy later to protect it from possible shorts. The red, blue and black leads head to the front of the stereo so we could splice a pot in place of the former volume IC.

Further hunting led to the discovery of a blown transistor on the vertical PCB next to the chip, which contains some of the amplification components. The transistor in question was a strange 100V+ bipolar type, which required the ordering of a specialty part from element14. The nearest replacement we could source was an SMT part, so it was soldered on with a few stiff pieces of solidcore wire and left to hang in free space. More leads were run to the front panel to replace the LC7535 volume control IC with a simple pot taken from the microphone input, which was deemed useless anyway. This allowed the new volume control to seamlessly fit in with the original fascia. There was just one problem. We'd lost all the original knobs and screws. Some fresh bling items were installed on the front panel instead (as shown in the opening pic, which I think look rather happening, don't you? 

Start to finish, diagnosing this problem and then implementing the fixes took around about 3 or 4 hours. The average service tech these days has to charge in excess of $50 an hour just to keep the doors open, let alone feed their family. So, when you add it up, this sort of job on a junked 90s stereo makes absolutely no financial sense to get done, but as a bit of a fun project on a slow weekend, it's certainly enjoyable, and you just might learn something.