Yamaha P2100 popping noise issue

[pradigmshift]

My Yamaha P2100 needed a revision, because the loud popping from the stereo/mono switch was becoming too annoying to listen to.

So I replaced all electrolitic capacitors, replaced the bias potmeters with nice Vishay multiturns and hardwired / bypassed the switch for always stereo.

I keep having some popping noise from the amp. In response I reflowed all solder joints on the main board (left the amp L and R alone they look ok). No effect. What could the popping noise be? Maybe the removable connector strips don't make adequate contact? Or do I need to look at replacing a dying transistor? Could a failing pair of differential transistors cause it?

PS the popping noise is sometimes loud, faint or even absent. It seems as if playing a tune makes the noise go away after a few songs, which is a good thing.

 

[avionic]

Probably a transistor.

 

[pradigmshift]

Thanks! I'm not sure how I can find out which to replace? Using the "shotgun method" would it be good to start with TR1 to TR4 - the differential pairs - and maybe the heatsinked transistors because they get crazy hot?

Schematic PDF is here. I have some fun things to show you when I get the noise out, some nice modifications.

 

[avionic]

Is this annoying popping coming from the speakers? If so,Left,right or both?

 

[pradigmshift]

Is this annoying popping coming from the speakers? If so,Left,right or both?

From the speakers, the popping sound comes from both sides at the same time, although the left side is a little louder than the right side.

 

[avionic]

From the speakers, the popping sound comes from both sides at the same time, although the left side is a little louder than the right side.

Does it pop without a source plugged in to the input?

 

[pradigmshift]

Does it pop without a source plugged in to the input?

Yes. This was also the case prior to the recap.

 

[avionic]

It has to be a common circuit/component to both channels then. Try jumpering out the power switch.

 

[pradigmshift]

It has to be a common circuit/component to both channels then. Try jumpering out the power switch.

Will do that tomorrow and report back. Thanks!

 

[pradigmshift]

Yamaha P2100 [issue fixed] modifications

Have found the problem! The amp suffered from LCD, a self-coined term for Loose Capacitor Disorder. It was one of the newly soldered caps (220 uF) having intermittent contact to B- connection, creating popping noises left and quieter noises on the right.

So here's the modifications I wanted to show you.

Modifications where inspired by this post and the Yamaha P2150-P2250 schematic. Amp is working great, although probably I should stress-test the amp and check for oscillations. But I'm still a rookie when it comes to repairs.
Things I did besides the above: added speaker protection (Velleman K4700), replaced all input connectors with 2x cinch, new speaker terminals that accept tulp, removed power button from circuit, new diode bridge (50A 1000V). Should add new heatsink paste, will do so after successfully running it for a while. Then I'll also secure the new caps with a decent amount of glue. Any suggestions which kind of glue to use?

Nice signature you have btw. :thmbsp:

 

[pradigmshift]

Forgot to mention, I also removed the volume control from the circuit (who needs volume control anyways) and upped the bias from about 33 mV to about 52 mV. The 33 mV was what I measured before, but I have not found any Yamaha reference of the stock bias voltage. Upping the bias really increases the sweetness of the sound, making the highs less harsh. The transistors get only a little warmer, you can put your hand on the heatsink. However, the transformer gets pretty hot. Maybe too hot? But you can still touch it.