Rotel RX400A hum

[PaulR]

I have resurrected the above receiver which I bought new in the early 70s. It has a background hum which is loud enough to be somewhat intrusive and is more pronounced through one channel. It disappears into a background hiss that the set has always had if the volume control is turned right up and is somewhat reduced if it is turned to minimum.

I have replaced the electrolytics in the power supply but am not sure where to go next. I have a copy of the service manual available here https://elektrotanya.com/rotel_rx-40.../download.html

Can anyone suggest what to try next? I have a decent oscilloscope if required.

Thanks
Paul

[Skywave]

Is the hum related to the selected position of the rotary function switch, or is it present for any selected input source?
Example: is the hum present on 'Phono' but not present on 'Aux.' etc.?

(I am assuming that for that test - and also the conditions of use when the hum is present - that there are no external devices connected to the amplifier).

Al.

[Nickthedentist]

Did it always do this, or is it something that developed later?

[PaulR]

The hum is odd. It is there on all settings of the selector switch, but is worse at around 1/3 of the volume setting. It decreases to an extent as the volume control goes past or before that point but not suddenly as it would if there was a problem with the control itself. It also seems to get worse as it warms up. It isn't bad, my wife says she can hardly hear it, but is intrusive to me and has only started since I brought it out of storage. It is worse in the left channel and is not affected by the balance or tone controls.

I initially suspected the power supply electrolytics but I changed them with no improvement. My next thought is the electrolytics in the power amp board but I don't understand enough about solid state electronics to do more than a blanket change of them. This may eventually be the best but I would like to try to understand what is wrong.

[Nickthedentist]

Check any riveted/screwed earth interconnections are sound. Slight corrosion after storage can cause all manner of odd faults.

N.

[PaulR]

Apologies for the delay in replying. There aren't any apparent interconnections that may have become "bad" so I replaced the electrolytic output coupling capacitors. This seems to have cured the problem.

I am now wondering whether I really should replace them all in the power amp.

Thanks

[Edward Huggins]

Others will know this Rotel model better than me but as a general rule, just do not just go ahead and replace "all" capacitors. If it sounds just as it used to before then leave well alone. If you had one worn tyre on your car would you go replace them all!?
BUT if it soulds thinner, weaker or has more noticable distortion than you have been used to hearing then by all means investigate futher.

[PaulR]

Thanks Edward

I have noticed some leakage of electrolyte under the large value ones and have replaced those. I will start to use it again and see how it goes.