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Old 2nd Dec 2019, 11:34 am   #1
qualityten
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Default Rotel RX402 soft put-put

I have a Rotel RX402 which I restored over the summer, replacing all the capacitors in the power supply, preamp and phono stages, but leaving those in the tuner section. It sounds very good and I'd like to give it away as a Christmas gift.

However, a very soft put-put sound remains, which is audible with headphones. It becomes inaudible when the volume pot is turned down, becomes audible at normal listening levels, but does not increase further.

As it is not present when using the power stage only, and I've replaced the capacitors in the preamp, is it likely to be one or more of the transistors in the preamp?
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Old 2nd Dec 2019, 2:17 pm   #2
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Default Re: Rotel RX402 soft put-put

Sounds like low-frequency instability. Check the values of high ohmic value resistors and check / replace electrolytic capacitors, especially those associated with PSU decoupling. I'll be very surprised if the cause turns out to be due to defective transistors.

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Old 2nd Dec 2019, 2:33 pm   #3
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Default Re: Rotel RX402 soft put-put

Thanks for ruling out transistors Al.

I have already replaced the PSU capacitors, but will check high value resistors.
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Old 2nd Dec 2019, 2:40 pm   #4
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Default Re: Rotel RX402 soft put-put

I agree that this is unlikely to be a transistor fault, and it does sound like some sort of stability issue. It will help a lot if you can identify the section of circuit which is responsible. There appears to be some interaction with the volume control rather than the control simply acting as a variable attenuator, so look very closely at things around there. Double check you have fitted any replacement electrolytics the right way round.

If the symptoms are identical on both channels, that normally points to some sort of PSU issue.

Do you have a scope? If so, look for anything strange on the power rails.
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Old 2nd Dec 2019, 2:46 pm   #5
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Default Re: Rotel RX402 soft put-put

That does sound a strange issue. I would begin by disconnecting everything external and having just the receiver and headphones connected.

Is it located physically near anything that could be emitting a pulse of energy... and I'm thinking even of things like proximity to airport radar dishes and the like.
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Old 2nd Dec 2019, 4:47 pm   #6
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Default Re: Rotel RX402 soft put-put

Mooly has a good point. In this I am referring to the extensive MV, HV & EHV activities undertaken within the Trafford Park area of which I believe remnant activity still exists. Here I am referring to the manufacture of Metrosils (Varistors) and Midol and so forth that will undergo HV & EHV testing at power frequency and basic impulse level.
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Old 2nd Dec 2019, 6:33 pm   #7
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Default Re: Rotel RX402 soft put-put

Were you able to test the receiver before the renovation work? If so was it motorboating then like it is now?

Alan
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Old 2nd Dec 2019, 7:35 pm   #8
qualityten
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Default Re: Rotel RX402 soft put-put

Thank you all for your thoughts. Mooly's advice to disconnect everything but the phones rang a bell, as it reminded me that I'd encountered a similar problem of instability after repairing a NAD receiver a year or so back. It turns out that this noise is being generated by something in the mains. I have my wifi router in this room and wifi is distributed to other rooms in the house with a TP-Link through the mains.

I first thought is might be the noisy power supply in my elderly Arcam Alpha CD player, but when I disconnected the RCA plugs the instability noise continued. But when I removed the DIN interconnect from the input socket of the Rotel, it went away. Are the interconnects functioning as aerials?

I've just found out that this explains why my beautifully rebuilt stereo Mullard 5-10 has the same 'problem'. I've just replaced the input cable with shorting plugs and voila, that 'instability' has gone too. I've just removed the TP-Link and the issue is gone. Now I just have to choose between on demand TV and the Rotel!

Last edited by qualityten; 2nd Dec 2019 at 7:45 pm.
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Old 3rd Dec 2019, 2:19 am   #9
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Default Re: Rotel RX402 soft put-put

Oh those damn things are notorious for causing a whole world of interference!
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Old 3rd Dec 2019, 9:01 am   #10
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Default Re: Rotel RX402 soft put-put

Pleased to hear you found the cause of this.

Finding why it affects them is a whole different ball game, there could be a combination of both radiated energy involved (leads acting as aerials as you mentioned) and also having the energy superimposed on the household ground leads which will have significant impedance away from mains frequencies and so allow voltage to appear. If the audio set up has multiple ground connections (amp and source components individually earthed) then you have a second possible route for this stuff getting into the audio chain.

The energy would also likely couple quite well through the transformer primary/secondary barrier and again enter the equipment.

It would be interesting to put a scope on the SECONDARY side of the power supplies and see if anything was visible even though the amp in isolation is quiet.
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Old 3rd Dec 2019, 11:32 am   #11
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Default Re: Rotel RX402 soft put-put

The amount of interference generated by powerlink systems does vary a lot, but it's not normal for it to be as bad as this. TP-Link kit is usually OK, so you may have a faulty unit somewhere.

You could try distributing the broadband using wifi range extenders rather than powerline adaptors. Some range extenders have one or more RJ45 connectors if you need a wired connection at the other end.
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Old 3rd Dec 2019, 1:32 pm   #12
qualityten
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Default Re: Rotel RX402 soft put-put

Thanks for the powerline vs range extender advice Paul. I'll review the position of the TP-Link boxes. One is also a range extender, but can't be located where the phone line enters the house as its range wouldn't go far enough.
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