Rotel RX-402 loud 'pop' upon turn on and off

pingu_turbo

On a constant search...
Hi Guys,

I have been using my recently acquired Rotex RX-402 receiver alot recently having now made some adaptors to connect the rca's to the 5 pin din connector inputs. Frankly it doesn't emit a bad sound. Quite pleased with it. :thmbsp:

However when I turn it on and switch it off, it makes a loud 'pop' through the speakers, no matter what the volume is set at.

My marantz and sansui units make a low pitched thump through the speakers, nothing like the 'pop'. Kind of sounds like a temporary short across the speakers or something.

Is this a result of some caps dying and generally giving out or does anyone else have this experience with this unit?

Any ideas? I can probably make a video of it if that will help anyone?
 
Thanks for your replies. :yes:

petehall347, your suggestion worked, although I'm sure it isn't meant to 'pop'.

bobaloniny, it could be cap. It is going to get a recap at some point anyway.

bk, once again you have come up trumps. :thmbsp: Thank you. :thmbsp: I checked the bias on the amp and low and behold both channels were set too high. The RH channel was 11.7v & LH channel was 13.7v! Both should be 7.5v. Problem fixed, no more 'popping'. :music:

On a side note, the service manual is very good for this model, you don't have to half guess like some I have seen!:scratch2:Top marks there.

So here we go, before and after.

DSCF9380.jpg


DSCF9381.jpg


Problem sorted. :smoke:
 
surely you meant mv ?
i am a little surprised setting the bias helped it .
 
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surely you meant mv ?
i am a little surprised setting the bias helped it .

Yes sorry, mv. I got a little excited. :yes: It hasn't done it yet. Turned on and off half a dozen times so far. It would 'pop' without fail on switch off.

I will know for certain in the morning when the caps have fully discharged. Ill repost then.:thmbsp:

Pete, is that how you normally turn on your receivers/amplifiers? I may move to doing that to avoid the gentle thump I sometimes get. :scratch2:
 
BIAS won't do the popping or at least if it's low it shouldn't. It's the DC offset that would cause the popping. Maybe the lowering of the bias had an effect on the offset. Measure the speaker out's and post back.
 
as far as i am aware there isn't a protect circuit in that amp . protect circuits normally take a few seconds to activate and will allow circuits to stabilise some so to lesson the dc at the outputs thus stopping the pop .
 
Sorry BK, I misread your post.

Pete, As far as I can see, it doesn't have a protection circuit. No relays, nothing.

Umm, it popped again after posting last time just before I went to bed but didn't first thing this morning. :scratch2:

I measured the offset as suggested, across both speaker teminals, and I have -18.4mv in one channel and -33.3mv in the other. Looking at the service manual I cannot see how to adjust this. Isn't it supposed to be as close to zero as possible?

The only two pots on any of the boards are the ones I adjusted last night (apart from the tuner. There is not even any adjustments on the power supply...

I would post the manual, but its too big to attach. :(
 
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i have the manual at home . i am wondering if the power switch is arcing .you could try and leave power switch on and use a switched socked to turn it on and off
 
That isn't a bad idea. I will try that. If it turns out to be the power switch at least I can try some deoxit first. :)
 
Well, I tried switching on using the wall socket. It makes no difference at all.

Basically (I was paying more attention this time) all within the space of about 2 seconds, the run of events are:

I throw the switch, lights on, no sound immediately.

A short 1/4 second electrical buzz,

Then the 'pop' and then the sound of the source fades up quickly.

That is the best I can do in the way of a description I am afraid. :(
 
Measure AC and DC voltage across the Main FILTER CAPS. No more than a couple volts AC. Sounds like the Mains are pretty well used up(DRY) and leaking excessive voltage. The pop at turn on is the surge thru the cap that isn't getting blocked and the fade out at turn off is the bleed off past the caps. The longer you wait the worse it's gonna get. ALSO when they leak electrically they heat up. Switch off the speakers, then turn it on and let it warm up for 5-10 minutes. The caps should be room temp or slightly lukewarm. If they feel warm to hot, turn it off and replace them before using it again.

Larry
 
Thanks Larry,

When I am home I'll be sure to measure the a/c to double check. Thanks for the tip. :thmbsp: As I said earlier, this one was going to get a recap anyway. It has just moved up the queue a couple of places. :) I'll start assembling a list soon and get an order in for them.

That is a shame, visually they look fine and internally it is the cleanest receiver I own. And how am I going to fix my workshop receiver without a radio? :scratch2: I am sure I can manage. :yes:
 
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