legginswill
New Member
Preface: I tried searching for a thread that was similar to this because I know these FM tuners can be finicky and a lot of people have posted about them, but unfortunately I could not find anyone with the same description or anyone who replied what their fix was. If I missed it, I do apologize!
Hello AK, got a problem with a MAC 4100 FM tuner. To start, this is an early production unit that has been recapped all around by me earlier this year, mostly due to a 60hz hum in the background and some other audible issues within the tone controls. Another tech had been in it at some point with some sketchy repairs that I fixed as well. This FM issue started before I recapped the unit.
The FM tuner has a hard time outputting the right channel audio. When I power the unit on after working on it, the FM comes in on both channels, but after about 4-5 minutes, the right channel just disappears. Now this may be a coincidence, but twice now I’ve been able to tune the dial all the way down to about 88MHz, and it’ll come back for a minute or two, then randomly cut out again. This “fix” doesn’t work consistently though, normally only once from my studies.
I’ve pulled the board, cleaned the board contacts, replaced caps, and the same issue arises. I’ve also replaced/swapped around the LM301a ICs on the board from a suggestion on another person’s thread they posted in the past; same results. The signal strength indicator and the stereo light stay the same when the right channel drops. I will note, in mono both channels play. Sometimes I can faintly hear sound from the right channel after it does drop out, but eventually it goes away.
Any suggestions or where to look or what to test? I consider myself novice when it comes to vintage audio repair. I’ve been doing it for about 6 years now and have gained quite a bit of knowledge, but this one has stumped me. I added some pictures of the block diagram and schematic of the FM board and Pre-Amp Switching board. Any tips help! Thank you
Hello AK, got a problem with a MAC 4100 FM tuner. To start, this is an early production unit that has been recapped all around by me earlier this year, mostly due to a 60hz hum in the background and some other audible issues within the tone controls. Another tech had been in it at some point with some sketchy repairs that I fixed as well. This FM issue started before I recapped the unit.
The FM tuner has a hard time outputting the right channel audio. When I power the unit on after working on it, the FM comes in on both channels, but after about 4-5 minutes, the right channel just disappears. Now this may be a coincidence, but twice now I’ve been able to tune the dial all the way down to about 88MHz, and it’ll come back for a minute or two, then randomly cut out again. This “fix” doesn’t work consistently though, normally only once from my studies.
I’ve pulled the board, cleaned the board contacts, replaced caps, and the same issue arises. I’ve also replaced/swapped around the LM301a ICs on the board from a suggestion on another person’s thread they posted in the past; same results. The signal strength indicator and the stereo light stay the same when the right channel drops. I will note, in mono both channels play. Sometimes I can faintly hear sound from the right channel after it does drop out, but eventually it goes away.
Any suggestions or where to look or what to test? I consider myself novice when it comes to vintage audio repair. I’ve been doing it for about 6 years now and have gained quite a bit of knowledge, but this one has stumped me. I added some pictures of the block diagram and schematic of the FM board and Pre-Amp Switching board. Any tips help! Thank you
