04-05-2014, 08:17 PM
I forgot to try grounding 4D. I measured 0.5V at TP402 (4D) in FM stereo and FM auto; in the millivolt range in FM mono.
I tested all six transistors and they all test good. I went ahead and replaced the two 2N2924s and the 2N2925 since I have some of those. No change.
I pulled the board and was going to replace the electrolytics, but apparently I used the 27 uF caps already, or misplaced them. In any event, I shall have to order more. Once that's done, I'll try it again, and try grounding 4D in stereo mode.
One thing I noticed this evening - on the schematic, there is a 200 uF cap (C30) which is switched in when the unit is in the FM stereo position. This also connects to point "AA" (4F on the MPX board, which also connects to the Stereo Beacon bulb). Also connected in this circuit are a few resistors (R65, R67 and R68 ). This does not make sense to me. It is applying the voltage from "AA" through those three resistors to 1E, 1H and 1L on the preamplifier board. Is this why I lose audio when the unit is in the FM Stereo position but not in FM Auto or FM Mono?
C30 is buried within the selector switch, and I haven't replaced it. Not sure how to go about replacing it, either, since it looks like it is nearly impossible to get to.
I tested all six transistors and they all test good. I went ahead and replaced the two 2N2924s and the 2N2925 since I have some of those. No change.
I pulled the board and was going to replace the electrolytics, but apparently I used the 27 uF caps already, or misplaced them. In any event, I shall have to order more. Once that's done, I'll try it again, and try grounding 4D in stereo mode.
One thing I noticed this evening - on the schematic, there is a 200 uF cap (C30) which is switched in when the unit is in the FM stereo position. This also connects to point "AA" (4F on the MPX board, which also connects to the Stereo Beacon bulb). Also connected in this circuit are a few resistors (R65, R67 and R68 ). This does not make sense to me. It is applying the voltage from "AA" through those three resistors to 1E, 1H and 1L on the preamplifier board. Is this why I lose audio when the unit is in the FM Stereo position but not in FM Auto or FM Mono?
C30 is buried within the selector switch, and I haven't replaced it. Not sure how to go about replacing it, either, since it looks like it is nearly impossible to get to.
--
Ron Ramirez
Ferdinand IN
