audible pop in pioneer sx-950

[karel.b]

After recapping my SX-950 (all 86 electrolytic caps, tantalum included), there is an audible pop when turning the tone amplifier switch ON/OFF. When in ON position, the low 30Hz filter does the same thing. The pop is louder right after turning the receiver ON. Also turning the receiver OFF (main switch) gives the same pop. Other than that the receiver sounds fantastic. Nothing gets hot and there is no distortion of any kind. I asked and paid an "EXPERT" on Pioneer receivers, but did not get anything specific, jut a lot of "try this and try that". I was wondering, if there would be someone, who experienced same or similar problem with the SX-950. I am only an amateur that loves vintage Pioneer receivers. Please help me.

 

[BinaryMike]

Popping tone/filter switches usually indicates leaky coupling caps. Your new caps might be the wrong type, or perhaps they just need time to re-form if they're old stock.

A popping power switch often means failure of the the arc suppression network. That problem should be corrected soon, before the power switch contacts get cratered.

 

[karel.b]

Power supply are Nichicon FG and the rest are Elna Sillmic II RFS except the big soup cans, which are Kendeil. When You say "arc suppression network", do You mean "protection circuit" ? I have to emphasize I am an amateur, but I am willing and eager to learn.

 

[BinaryMike]

The arc suppression network is normally connected directly across the terminals of the power switch and consists of a series R/C combination -- often in a single package.

I wouldn't know without checking datasheets if the Elna caps have sufficiently low leakage current for this application. Perhaps someone in the Pioneer section of AK will have this information handy. Make darn sure that you didn't install any caps with reversed polarity.

 

[karel.b]

I replaced the oiginal cord (2 prong) with new one (2+ground) and I connected the ground wire to the tranformer shell. I left the ceramic cap and the resistor as they were. Would the ground wire be a problem? As for the Elnas, I was told they are one of the best caps. Not so??? The schematics of some boards calls for low leakage caps. But the "PARTSCONNEXION" guy where I bought them told me that the Elnas will work fine. Wrong??? And I was extra carefull with the polarity!!

 

[BinaryMike]

I'm not a big fan of 3-wire power cords where they weren't factory-supplied. Why add a potential ground loop when the equipment already has good electrical safety features?

Never believe a salesman.

 

[Detailman]

I'm not a big fan of 3-wire power cords where they weren't factory-supplied. Why add a potential ground loop when the equipment already has good electrical safety features?

Never believe a salesman.

Agreed. just asking for another problem.

The pop on the tone switch just could be needing a good cleaning.
The point about the r/c network on the power switch is a good one.

 

[karel.b]

I cleaned all pots and switches first with Electrosolve zero residue cleaner, and after that I used Deoxit D5. As of the ground wire, I will disconnect it to see if it will solve the ON/OFF switch problem. If not I will test both of the big ceramic caps and the resistors. And I will clean the tone switch again with Deoxit. Guys I am very grateful for Your help.

 

[cademan]

You say you recapped the entire unit? Are there ceramic disc capacitors wired right at the tone & low filter switches?

 

[karel.b]

I recapped only the electrolytics. I did not touch the ceramics or the mylars.

 

[karel.b]

After disconnecting the ground and cleaning the tone amplifier switch, there is no improvement. Is there anything else I can do? The values of the big ceramic caps and the resistor in the arc suppression network are correct.

 

[karel.b]

Hi guys,
Finally I found some time to work on my SX-950. I replaced all ELNAS caps (where CSSA, CEANL, CSZA were called for in the spec) with NICHICON UKLs (low leakage) caps on all boards. I cleaned all switches again with DEOXIT G5 PRO GOLD, checked all electrolytics for polarity. I also checked all voltages given on the schematics (there are up to 1.5V variances) and made adjustment to VR1, 2, 3 and 4 on the power amp as per service manual. Got some improvement. The tone control switch and the filters (Hi and Lo) are silent, but only after about one minute of warm-up. No change to ON/OFF switch. I'm about to give up. Any suggestions?;(other then get it fixed by a pro shop)

Thanks

PS. Turning the tone switch ON/OFF right after turning the receiver ON causes the relay to disconnect.

 

[Artie]

I probably can't add much, but I did want to verfy one thing . . . the unit does not pop on "power up", but only if you operate the tone bypass switch shortly after power up. Is that correct? What I'm getting at is I don't think this has anything to do with the power switch RC network.

You say it goes away after about a minute? It sounds to me like you may have one power supply rail thats coming up a bit slower than the other. Also, it may be only in the audio section that precedes the tone amp. So, the tone amp is putting out a small amount of DC just until both rails stabilize.

Here's where an o'scope would be helpful, but a meter might do it. I'd watch the voltage on the input to the tone amp as you power the unit up. (You'll probably want a schematic for this.) Anyway, thats where I'd look. Then, if that turns out to be the case, re-examine the cap values and polarity of any you changed in those circuits. (That feed the tone amp, that is.)

The fact that it trips the DC protect circuit reinforces this idea.

Hope this helps.
Artie

Edit: Btw, do you own a tape deck with meters? The reason I ask is the output of the tone amp board goes direct to the pre-out jacks. You could pull the pre-main jumpers, run a cable to your deck, and monitor the "pop" with the tape deck meters. Might save some tweeters while troubleshooting.

 

[Artie]

Part duex: This happens in both channels, right? Monitor pin 1 of the tone amp board, (AWG-039), for a DC voltage at power up. Then turn the unit back off and give the caps time to completely discharge. Then monitor pin 7, (same as the pre-out jack), for the same thing. See if you have a DC voltage coming in or going out that slowly recedes to zero after power up. (Pin 1 is the tone amp input. Pin 7 the output.)

And one last thought, I notice that everything prior to the main amp board is run by a single polarity supply. That means that all that circuitry is capacitively coupled to the main amp board through caps C1/C2. If you changed those caps, I'd double check their voltage, uf's, and polarity. The negative terminal should point to the input pin.

 

[EchoWars]

If DC offset is properly set to zero, and the speaker protection relay is disconnecting immediately when the power switch is flipped to the 'off' position, there should be NO audible pop.

So, either DC offset is whack, or the relay is not opening when the power is removed (or not opening quickly enough). There's a bit of circuitry on the protection board that is designed to detect the loss of AC power and open up the relay. Perhaps it is not working, or the relay is fused closed, damaged, or otherwise modified in some undesirable fashion.

The tone control issue may be a totally separate issue, but double-check the grounding to all the preamp boards to make sure they are not floating, or only grounding through some high-value resistor.

 

[karel.b]

I'll work on it possibly tomorrow and I'll let you know of the results. Couple of things I'd like to mention. Most of the caps are higher voltage then originals;(I was told it's OK). If C1&C2 are on the power amp (AWH-050), then the negative terminals point away from the input terminals (18&3), but are connected to them. But that is probably what you meant. The values are 2.2uF@50V UKLs. There is also some mix-up with resistors R21 & R22 on the same board. Schematics calls for 3.6kOhms, but originals were 3.0kOhms. I replaced them, thinking they were wrong. (Da?)
Thank you guys again.
One more thing; the pop appears no matter what input I use, both channels.

 

[karel.b]

If DC offset is properly set to zero, and the speaker protection relay is disconnecting immediately when the power switch is flipped to the 'off' position, there should be NO audible pop.

So, either DC offset is whack, or the relay is not opening when the power is removed (or not opening quickly enough). There's a bit of circuitry on the protection board that is designed to detect the loss of AC power and open up the relay. Perhaps it is not working, or the relay is fused closed, damaged, or otherwise modified in some undesirable fashion.

The tone control issue may be a totally separate issue, but double-check the grounding to all the preamp boards to make sure they are not floating, or only grounding through some high-value resistor.

DC offset is zero and the relay seems to be working fine;(it disconnects the moment I turn the main switch off). I checked ALL grounds at each board and there is zero resistance.

 

[EchoWars]

If the relay is disconnecting immediately as the power is removed, and offset is zero, there is nothing that can go 'pop!'.

 

[qsaudio]

My 2 cents is there is an imbalance in the power that runs the tone amp board.The difference is heard and amplified as you flip switches around.

I always go back to start, since you changed caps, the problem is most likely the caps you changed on the tone amp board are doing something funny causing the imbalance of power giving you an offset felt throughout the rest of the signal path, and heard as a "pop".

 

[Artie]

If C1&C2 are on the power amp (AWH-050), then the negative terminals point away from the input terminals (18&3), but are connected to them. But that is probably what you meant.

We're probably just talking semantics here, but I'd say the negative terminal points towards the input. But yes, the negative terminal should be connected to pins 3 and 18.