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Help diagonizing Rotel RX 304
Help diagonizing Rotel RX 304
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Old 17th October 2020, 09:40 PM   #1
Labienus is offline Labienus
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Join Date: Oct 2020
Default Help diagonizing Rotel RX 304

Hi everybody,


that's my first post here, but let me already thank everybody - the last few days I was working on my old Rotel RX 304 and this forum was a great resource.


What I've done so far is replacing all old electrolytic capacitors with new ones. After the recap, the amp sounds way better than before, so I'm quite happy. There is still one problem with a background noise which wasn't fixed by the recap.


Here is a bit of initial diagnosis obtained by playing around with Online Tone Generator - generate pure tones of any frequency:

- There is already auditable noise coming at very low Hz, in fact there is a tone at approximately the input frequency (so if I put in 1Hz sinus, I get a tone every second).
- This tone disappears if the volume is lowered, but the exact volume depends on the frequency: if the input signal has a higher frequency, then the noise appears already at a lower volume. It doesn't matter if the volume is controlled from the PC or by the volume button of the Rotel.
- The noise is there over the whole frequency range, even if the input signal is 20kHz (where it sounds like a tone of about 10kHz).
- The noise (at least for lower input frequencies) is not affected by the treble control, but its volume goes down linearly with the bass control (but is still there even with bass -10).

- The problem is in both channels, but slightly different. The noise in the left channel has lower amplitude of the noise (as displayed in the Watt display from the Rotel) and twice the frequency of the right channel (for 1Hz input).
- There is also a white noise at higher volume (probably not related, but if somebody has an idea how to fix that...).


I sadly don't have good equipment to measure things (not even a voltmeter), but I'm planning to go to a repair cafe and check a few things. So would like to have advice what to check. Thanks!
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