Second life Defender Mercury 50A. Need advice?

Question

Do it yourself

Good day!
Actually there is this miracle: Defender Mercury 50A .
They are already about 9 years old. About 3 years ago, sometimes the sound began to disappear in a large speaker on one of the speakers. But disappearing is not constant, and returned as strange as it disappeared. There were wheezing. I sinned on the speaker at first, took it apart, like the whole thing. Now the subject is in this state: 1 column plays normally completely, something is barely audible in the second from the low-frequency speaker, but the diaphragm moves, the tweeter seems to be normal.
When I took it apart, I didn’t see swollen conders ... maybe I overlooked it.
Actually questions:

1. According to the symptoms, what is it? Still electronics? How to check?
   
2. Maybe just order an amplifier and a couple of frequency dividers for different speakers and thereby revive this whole miracle? Or do you need the other way around, one divider and 2 two-channel amplifiers?
   I watched this: one , two , three .
   
3. Other options? Apart from buying new...
   

PS: I would like to revive, without altering the case. Maximum - holes for new speakers, if they do not fit. Electronics ready to change all. I would like to know what =)
PPS: I'm friends with a soldering iron, but I know only general points about sound amplification.
Thank you!

Answer 1

something is barely audible from the woofer, but the diaphragm moves, the tweeter seems to be normal.

Perhaps all the same, the speaker is covered, since the HF is playing. It's easy to check - you need to take a working speaker from the 1st column and connect it instead of the "not playing". And vice versa, connect a “non-playing” speaker to the working “column” instead of the one playing.
You should also check all the wires: from the signal source, from one speaker to another.
But maybe something was covered in the amplifier itself, and then, for example, only high frequencies can go to the output, which the 2nd (HF) speaker plays from the column.
You can also twist all the knobs / buttons. For modern technology, this should not happen, but in the old technology, contacts were oxidized from time to time.

Answer 2

I had similar problems with the amplifier due to dried electrolyte at the input. I replaced it with a film, glitches and distortions at certain frequencies disappeared. In your case, you need to start from the first OSI level , start the check from a simple one, as 4k4 recommends