Sansui 5050 AM Stereo Conversion Upon arrival, I took the top cover off of the unit and tested it to see how high it tuned. It went up to 1670 already, so the only adjustments needed was a tweak on C5 to get it to 1705, and a touch-up on C2 for upper band sensitivity at 1400. Dial tracking was only slightly affected from about 1400 on up. The lower cover was then removed. The next step was the long and arduous process- creating a new stereo indicator that looked like it came from the factory that way. I removed the knobs and the control shaft nuts to remove the front panel and pulled the tuning pointer off and out of the way, leaving it on the string as there was enough slack to do so. Using a pencil I traced where the sides of the scale were, then I was able to remove the dial scale from the metal back plate by running a single edge razor CAREFULLY between the plastic and the metal along the top and sides so as not to scratch the plastic. The old adhesive was rubbed off afterward. Then the back plate itself was removed by first taking out the screws that held the dial light assembly which were accessed through holes in the top of the back plate, then the meters had to be unsoldered and removed, the dial cord unstrung, then the flywheel was removed. Next was removal of all the screws which held the plate to the main chassis as well as the switches on the plate, plus a second set of control shaft nuts. The back plate is somewhat of a stair-step arrangement. Taking a 3x5 index card, I rested it on the lower ledge of the back plate like the dial scale does, and through the back I traced out the hole for the FM stereo indicator on it, then I took a razor and cut out the traced area. The dial lamp assembly had numerous spots for extra indicators, and the longest ones were at opposite ends, the one on the left being used for the FMS indicator. Putting the dial lamp assembly up to the top ledge of the back plate as a guide, I marked on the front of the plate where the edges of the new indicator would be, then sliding the card over so the hole lined up with the marks, I traced the card’s hole onto the back plate using a fine-tipped felt marker. Note- this indicator’s hole is a tad smaller than the FMS indicator’s, so be aware of this. At this point I could have drilled some holes at the corners and tried cutting the longer edges with a Dremel tool, but the hole was small enough I didn’t think it would produce good results. So my next task was finding a machine shop to do it for me. Warning- this could be expensive as a lot of shops quoted me $50, most of that for set-up time. I eventually found a small, hole in the wall shop who did it for $25. After that, I was able to remount the back plate and get at least a few things put back on the unit. Then it was a question of what to do about the dial scale. On the back of it I took an awl and scratched 5050 in small numbers to see how easily the paint came off; noticed it came off fairly easily. I did this on the far left part of the scale where it wouldn’t be visible once reassembled. Studying the scale carefully, I noticed the FM STEREO letters were somewhat indented. Evidently the first layer of paint was black, with the lettering and scale markings either masked or engraved later. Then a layer of blue paint was put down over the dial area itself, red over the FMS indicator, and on top of those was a layer of white paint, most likely for better light diffusion. I noticed the shop milled the hole for the new indicator a bit bigger than what I had traced, so through the plate’s hole, with the index card in front of the scale, I lightly traced the card’s hole onto the back of the scale using a pencil. The scale was then taken to a shop that does trophies, and had them engrave AM STEREO inside the area traced. Even this cost $21 to do. Taking into account that the indicator would be a tad smaller, I had them compress the letters somewhat. Then I had to determine what would work for coloring the new letters. I settled on sun catcher paint, available at arts and crafts stores such as Ben Franklin. I put on a couple layers of green, then a layer of white, allowing each to dry before adding the next layer. The dial scale was reglued to the back plate using mucilage. Adding the stereo decoder This was by far easier, though not without its problems. The decoder board was first hot-melt glued to a horizontal cross-brace near the HA1197 AM receiver chip. Decoder power was initially tapped off at pin 11, ground at pin 10, and the best IF signal was found at pin 9. However I discovered the decoder worked better tapping the power from the front of the circuit board where it comes direct from the power supply, as the resistor values were such that it wouldn’t allow for running both the HA1197 and the decoder. When the stereo kicked in it would cause the voltage to sag and the oscillator to drift, causing it to drop back to mono. A small 7805L regulator was added to keep the decoder Vcc down. Bought a 6v grain of wheat lamp for the new indicator in the interim. The stock AM audio is on a yellow wire which runs from a plug on top of the chassis to a set of double terminals on the front wafer of the function switch (the only ones on it.) I removed the switch for easier access, and noticed in this one position it had a built-in jumper to get the mono audio to both sides. The left audio was soldered to the rear of these two terminals, and on the front side of the front wafer I pried the stationary contact away from the sliding one. Right channel audio was then connected to the terminal 180 deg around the switch from the L ch. The last task was creating a circuit to power the new indicator lamp. The LED was removed from the decoder board. A second circuit was built with a small scrap of perf board using an LED/photo transistor optoisolator, a junk silicon NPN transistor and a few resistors. The LED side of the opto was connected to the decoder board where the standard LED would be, and taking power from the high side of R89 for the rest of the lamp circuit, I put a 1K resistor in series with the collector of the photo transistor, and connected its emitter to the base of the junkbox transistor. That transistor’s emitter went to ground, its collector going to the lamp, through a 130 ohm resistor (selected so the lamp voltage would be about 5v) to R89. The lamp was glued into place on the dial light assembly with a bit of silicon RTV. I drilled a small hole in the same cross-brace and used a screw to mount the driver board. This completes the conversion process. Scott Todd 9/5/03