Repair Rotel RL850s

Joined 2011
Are you sure it's the speakers, and not the amplifier?
If not, try a different amplifier. The controls and switches could be dirty.
Check the speaker wires at both ends, and make sure they are not shorting.

If the speakers are at fault, are there level controls or switches on the speakers? Try cleaning them.
Have you checked the woofer surrounds?
This should be in multi-way really.

It's not clear to me, but the Rotel 850 bookshelf speaker is a ca. 1980 design.

6 or 8" Elac paper bass and Peerless 1" soft dome tweeter.

Various mark models, but usually a good standard if it's like other Rotel products.

As rayma says, a bit of contact cleaning might help. And amps get dirty contacts too, so unplug and replug all the cables. You can buy aerosol switch cleaner for all the knobs and buttons on amps. Corrosion never sleeps and makes for a fuzzy sound.

Distortion is a vague term. If its the bass end, well that tells you something. You can put your hand over the tweeter to listen to the bass on its own.

Tweeters often have an oil in them called ferrofluid, which dries up and clogs the tweeter solid over many years, resulting in loss of top end.

What I would do is unscrew the bass, and even the tweeter and have a look inside. Mark the topside of the bass with a black marker, and check the internal crimp connections if it has them, but keep a note on how they are wired positive (red) and negative (Black) so you can put it back together right.

Crimps pull off and push on again. And thus clean themselves in the process. Sometimes you tighten them with pliers.

Old woofers often sag a bit and start to rub voicecoil. You can feel this if you gently push them in. The fix is to rotate them 90 degrees. Which is why I said mark the top.

Check the wiring on what seems a simple 4 element crossover. Might be broken solder joints.

These are decent loudspeakers. So worth a bit of work with a screwdriver. Tweeters can be replaced. Ferrofluid can be very cheaply replaced. Broken woofer might be more problematic, but I could probably think of something if I know the cutout size.

Hope it helps!

Best regards from Steve in Portsmouth UK.
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Or it could be as simple as the foam surround of the bass units have "crumbled". Far from uncommon given the age.
Remove the covers and look.
If it looks anything like this, there´s your reason.

images (1).jpg
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If you have a sine wave oscillator, try sweeping across the frequency range, and you may find that certain frequencies excite a mechanical resonance somewhere so the tone changes from pure to a harsh buzz or whine. That can help isolate the problem.

Crossover caps might be bad, although I've only come across that on Altec pro crossovers that were abused and stored in damp conditions.