rocknroller
Active Member
Problem statement - The FM tuner was not getting any (or hardly any) FM Stereo signals identified with a good 300 ohm antenna connected. Mostly stations coming in mono with no FM stereo light (lamp is good) Also the Tuning indicator is not anywhere near center, quite a bit left (about 7 or 8 o'clock). These two issues may be related or not, I'm not sure.
Background - I don't have the proper tools to be calibrating the FM circuit, no oscilloscope or freq generator so please postpone all the comments to that regard and focus or what I can do anyway with making the calibration worse than it is. I do have the service manual. In a environment where I test lots of receivers so it's not a coverage/antenna related issue. It's specific to this unit.
Action taken - Checked the electrolytic caps on the board, many out of tolerance so replaced all but a couple that were good tolerance and esr . In general, the FM Stereo improved. Now get many stations decoded with FM stereo, but still a fair number identified by the FM light, but highly garbled, like two stations on top of each other. Also, no improvement to the tuning indicator as a result of the cap changes. It matters not what station I'm tuned to, FM mono or stereo, I cannot achieve center of this gauge (or even close)
The Service manual says VR101 can be adjusted for the signal meter. As I rotate this counterclockwise the tuning indicator moves slowly right towards center, but never reaches the center before I run out of range on the pot. Further, as I rotate it, the signal audio gets weaker and weaker. Returning it to it's original position restore the signal audio fully, but also returns the tuning indicator back to it's 8 o'clock position, so this control does not resolve anything, at least by itself.\
Thoughts as to the cause of the FM heavy overlap on some station points and the off center tuning indication? Are they related? Any components I should specifically be checking?
Background - I don't have the proper tools to be calibrating the FM circuit, no oscilloscope or freq generator so please postpone all the comments to that regard and focus or what I can do anyway with making the calibration worse than it is. I do have the service manual. In a environment where I test lots of receivers so it's not a coverage/antenna related issue. It's specific to this unit.
Action taken - Checked the electrolytic caps on the board, many out of tolerance so replaced all but a couple that were good tolerance and esr . In general, the FM Stereo improved. Now get many stations decoded with FM stereo, but still a fair number identified by the FM light, but highly garbled, like two stations on top of each other. Also, no improvement to the tuning indicator as a result of the cap changes. It matters not what station I'm tuned to, FM mono or stereo, I cannot achieve center of this gauge (or even close)
The Service manual says VR101 can be adjusted for the signal meter. As I rotate this counterclockwise the tuning indicator moves slowly right towards center, but never reaches the center before I run out of range on the pot. Further, as I rotate it, the signal audio gets weaker and weaker. Returning it to it's original position restore the signal audio fully, but also returns the tuning indicator back to it's 8 o'clock position, so this control does not resolve anything, at least by itself.\
Thoughts as to the cause of the FM heavy overlap on some station points and the off center tuning indication? Are they related? Any components I should specifically be checking?