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Rotel RA-931 pop right channel


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Getting an audible 'pop' from right channel (and large woofer movement) when powering on/off an old RA-931. Works/sounds great when powered on. 

Anyone got an idea where to look first of all? Cheers, Nic.

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17 hours ago, nosepeg said:

Getting an audible 'pop' from right channel (and large woofer movement) when powering on/off an old RA-931. Works/sounds great when powered on. 

Anyone got an idea where to look first of all? Cheers, Nic.

 

First up, check if it happens only when a particular source is selected, or regardless of which source is selected.

I had a similar issue in my RA-970BX.  The minimalist pre-amp design makes them quite sensitive to any trace of DC on any line level input.  If the issue is limited to when one particular source is selected, then the problem might not be in the amp.

 

I used to use the headphone output from an iPhone as a source and the tiny DC signal that those devices use for the inline switch feature was enough to cause the effect you describe. 

So, it is possible that a source has developed an issue on the right channel.


In my case, since the source was not actually faulty, I dealt with it by adding a suitable capacitor in series with each of the 'Aux 2' inputs and only used that input for the iPhone.  Later on, I added a Sangean WFT-3D to the system and didn't need to use the iPhone any more.

 

If the issue is not source-dependent, then next step would be to treat the input selector & tape monitor switches and all the pots with some contact cleaner/lubricant.  The 970BX needed a treatment on the input selector a few years ago to get rid of intermittent crackling.

 

The other issue these amps have (and many other brands from that period) is that infamous glue used on the PS caps that can corrode component leads that it comes into contact with.  In my 970BX it destroyed a zener diode in the power supply which then caused the associated regulator transistor and preamp IC to fail.  These faults would usually effect both channels, so probably not the issue in your case, but worth checking.

 

Beyond that, an oscilloscope and the service manual might be needed.

https://www.hifiengine.com/manual_library/rotel/ra-931.shtml

They are easy amps for techs to work on and well worth repairing in my opinion.

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Many thanks for the very useful info...

I've identified that it happens on all inputs, but one shortcut is having the speakers connected to the remote output and have it switched off when you power on and off.

Dug out my meter so will measure relevant DC offsets. Shall download the technical manual also. 

If there are any caps etc, I can ask a tech mate of mine to get his soldering iron out.

Cheers once again, Nic.

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The RA-931 does not have the "standard" protection circuit/speaker relay, only uses a 4A fuse. Consider fitting a speaker protection kit, found on the bay from our good friends in china. Such kits do power on mute and excessive Vdc detection.

 

Since no speaker relay is present then possible causes are

- leaky coupling capacitor, eg, C602 C502, C516

- resistor drifting off spec

- failed diode (D602,4)

- either +ve or -ve power rails not coming up gracefully due to leaky psu cap(s)

  (but this would affect both channels)

 

I'd be tempted to pull C602 to seperate the pre/power amp stages and then power up, if pop still heard then power amp issue. Hoever as a first step measure the dc voltage at the right speaker connector, important to also note the sign +/-...

 

 

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Many thanks for the info. Getting 67 millivolts right channel, 74 on the left.

Shall check speaker protection kits - I guess they are a bit fiddly to install?

Cheers, Nic.

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8 hours ago, nosepeg said:

Getting 67 millivolts right channel, 74 on the left.

That seems a little high. I did not see any adjuster for offset. You might be able to remedy the situation by replacing Q602, Q606 with a matched pair

 

 

Also did you check what the voltage was on the right channel when the pop occurs, ie, connect multimeter to right speaker connectors, select 20Vdc scale then power on, ok numbers will flash by, are we talking about 1-2 volts or 10+volts??

 

8 hours ago, nosepeg said:

Shall check speaker protection kits - I guess they are a bit fiddly to install?

Depends if your speakers are valuable.

While the protection kit does the power ON/OFF muting (of pops) the main motivation is to protect the spkrs in case of amp failure (eg, faulty transistor...) which could dump about 35V onto the speakers which may destroy the woofer (at least)

 

Yeah they can be fiddly, the issue is available space in the chassis. I've refurb'd a RA-920 and wanted to add a kit but there is "no room". The kit comprises of a small transformer and a circuit board, maybe wait on the protection kit, sort out your problem first.

 

Does the pop occur when you have to tone switch OFF/ON

 

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Reached my limited limit of knowledge measuring possible faulty components. I'll put it up for free for someone who can fix it.

By the way, measured the bias on both channels and it was spot on...

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On 14/06/2024 at 6:06 PM, nosepeg said:

one shortcut is having the speakers connected to the remote output and have it switched off when you power on and off.

or continue to use the amp using above workaround?

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