wales@ucla-cs.UUCP (11/09/85)
Well, after two months and three trips to the local Kenwood service center, my TR-7950 finally works again -- almost. For those who may not remember, the problem was that the receiver sec- tion of the rig would "motorboat" whenever a signal was received. (I don't know whether there were any problems with the transmitter portion of the rig, because since I was unable to hear anything intelligible I was unable to get a signal report from anyone else!) Anyway, they said they had found (and fixed) some cold solder joints in the PLL (phase-locked loop) board. I had previously supposed that cold solder joints were primarily a "dead on arrival" or "infant mortality" problem (I've had this rig for over a year and a half). Then again, the vibrations attendant to operating "mobile" might (I suppose) aggravate a latent weakness in the rig and eventually cause a failure. Now for the bad news. Whenever I receive a signal, there is now a faint high-pitched whistle in the received audio (very similar to the sound of a "singing" flyback transformer in a TV set, except that in this case the whistling sound is definitely coming through the speaker and varies in loudness according to the setting of the volume control). The rig was NOT doing this before the problems arose; just to be sure, I checked another Kenwood TR-7950, and this high-pitched whistle does not appear to be a generic failing of this entire make of radio. This "whistle" is not very loud, by the way -- just loud enough to be heard and to be very annoying. People, when asked how my signal sounds, almost invariably tell me that I sound just fine. They specifically deny hearing any kind of high- pitched noise on my signal. (Of course, their rigs might have low-pass filters in the audio section -- in which case they wouldn't hear a whistle even if it were there. I eventually discovered that I could get rid of the whistle by tweaking a trimmer capacitor in the voltage-controlled oscillator on the PLL board (TC2, near Q9; the VCO is mounted in a little metal shield box on the PLL board). Unfortunately, if I tweak the trimmer enough to get rid of the whistle, the rig will not transmit! (There is a small "window" of adjustment in which the receiver section continue to work fine, even though transmission on part or all of the band becomes impossible.) I have inspected the boards and the inter-board cabling thoroughly, and there do not appear to be any stray paths for a signal to be getting into the audio from the VCO. Under no circumstances am I going to take the rig back to Kenwood to have this fixed! There's no way I'm going to give up my rig again for weeks on end. Any ideas on how to find and fix this problem? -- Rich Wales // UCLA Computer Science Department // +1 213-825-5683 3531 Boelter Hall // Los Angeles, California 90024 // USA ARPA: wales@LOCUS.UCLA.EDU -or- wales@UCLA-LOCUS.ARPA UUCP: ...!(ucbvax,ihnp4)!ucla-cs!wales