Hitting a bit of a wall trying to resolve an issue. Poor reception across the entire FM range. No Am reception at all, not that I use it. Like one or two FM stations were strong enough to even register when I did a seek scan on the radio. I was going to replace the HU anyway, so I did so with an Alpine BT radio. I read up and made sure to connect the power antenna wire to the appropriate wire on the BMW harness to give the FM amp power. No difference in reception between to stock or aftermarket unit.

So I pull back the left rear panel to tinker with the FM amplifier. It is indeed getting voltage (about 3.5V) and I measured it at the back of the radio as well to verify this voltage was correct. Plugging the power line in and out caused no change. I pulled the antenna cables out of the FM amp and connected them straight together to bypass the amplifier and I got some decent signal. Decent, but not great. I could maybe live with it, but it doesn't make sense that this little circuit board would crap out and it bothers me that something doesn't work 100%.

So I pick up a cheap used FM signal amp online to give it a shot. Same situation. I can see it's getting voltage, but it's just not doing anything. Since I already have two, I decide to crack open my original one. There was indeed corrosion around all the ground solder mounts to the board as well as the antenna cable input and output. I gave them a good cleaning with alcohol, put it all back together, and STILL nothing.

So I'm totally stumped on this one. Unless the boards on these amps do fail and the used one I purchased was also toast, something else must be up. There are 4 plugs on the FM amp: antenna in and out, power, and a thin coax looking cable that I don't know anything about. Could the opposite end of this connection be my issue? Anyone had a similar problem with an FM signal amp before?

 

For reference, it's an HK (Harmon Kardon) amp.
Are you able to play CDs, or the aux-in successfully? Is it only the radio reception that's the issue?
Do you have navigation, or no? If you have nav, the antenna is integrated into the rear passenger window.

 

Sorry, no this is not the HK amp. This is the super base 2004 pre-facelift model with the boost CD player. No Nav, either. From what I can gather, the facelift models got a "diversity" antenna hence the roof antenna and window antenna, but mine came just before that and has only the basic FM signal amplifier with cables to the roof antenna and to the back of the head unit.. The diversity antenna amps had an additional input from the diversity switching unit or some such thing, and yes did run to both the roof and a window antenna integrated into the defroster lines. But yeah, mine has none of that.

I didn't even try a CD in the stock radio before changing it out, but the reception issues are the same on both the stock and aftermarket head unit. Honestly, I drove it around a bit today and the reception with the bypass seems to be doing alright. I was just hoping to get it that much better if I could sort out the issue. I did have to swap the stubby out for the stock antenna to get a better signal, but other threads I've seen indicate that a working system with the antenna amp working properly aren't as finicky with the stubby antenna mounted.

This could very well be a case of getting a used part that was also not working, but I'd hate to spend 100 bucks just to get a new OEM part that *might* fix the honestly very small/inconvenient issue.

 

For clarity, the part in question is PN 65201480015, part 1 in the diagram here:

https://www.realoem.com/bmw/enUS/sho...diagId=65_0708