bew4568@zeus.tamu.edu (WILSON, BRUCE ERIC) (04/20/91)
Well, I got one of those nifty Ramsey FM-10 stereo transmitter kits earlier this week. Assembled in in an hour or so. Fairly simple work. There were some anomalies in the kit, however. My power switch didn't fit the holes provided for mounting, and the leads were too short to be bent to fit the mounting holes. It's not much of a biggie to me since I plan on having it turned on whenever I turn my stereo on--I'll use the switched 120VAC plugs on the back to power the transmitter. That brings up another point. The circut is not very immune to hum comming from an ac to dc converter. Even with a 1000 uFd electrolytic across the incomming power terminals the hum over the radio was as loud as the music. I plan on using a fixed voltage (7808) regulator to get rid of most of the hum. Wish me luck. Battery operation is great, though. The unit draws 50 mA at 9V. There are several modifications listed in the kit I got. The values of two capacitors and two resistors were changed. The electrolytic cap on the B+ leading directly to the chip was increased to 470 uFd. The capacitor that helps C8 (the variable cap used to adjust the subcarrier) was increased from 47 to 82 pFd. When I assembled the kit the sound was never quite right 'till I put the 47 pFd cap (which the Ramsey folks thoughtfully included with the kit) back in. Then I could adjust it properly. Sounds pretty good, now. The line-level inputs aren't line level. The kit comes with a voltage divider at the inputs with R1 = 4.7k and R2 = 1k (where the input is across R1 + R2 and the output is across R2). Right now I have R1 = 20k and I still clip occasionally when I use my CD player. (note: I still notice some sibilance at high frequencies but I think it's normal--driving 20 kHz into a 19 kHz multiplexer is bound to result in some non-linearity!) I'm not getting astounding range. My radio is in my house (aluminum siding) and I lose the stereo light when I'm about 40 feet from the radio. I don't know what it'll do in the open. All in all a nifty little toy if you have the need for one. And boy do I have the need. Now my roomie won't have to listen to the TV as he tries to sleep while I watch Letterman. The PC board is about 5" x 4", in case you care about cases. Bruce Wilson