Loud pop sound at anytime in left channel Pioneer A-9

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#44
Ian Finch said:
Electrolytic capacitors contain fluid electrolyte, sealed only with a rubber plug. Thus they have a limited life, only guaranteed for a finite period at their rated maximum duty which is typically only 1,000- 3,000 hrs. In many applications though, the duty is light, occasional or the product may be unused for decades. The life of the cap then becomes simply related to age and possible damage to the "forming" of the anode by abuse. Commonly, after 20 years they begin to fail with some degree of predictability. After 40 years, good quality types may still be OK but from a service point of view, it's then high time they were all replaced.

Two meanings of leaky caps; physically leaky, as in electrolyte leaking from the lead exits, or around the rim, then electrically leaky, as in passing some DC current when there is an internal fault. If you want to know all about components and failure modes, Wikipedia is there. This reference is really comprehensive and helpful: Aluminum electrolytic capacitor - Wikipedia . BTW, you can't test caps with a simple multimeter!

Cheapo parts testers you find in the dozens on Ebay are not suitable for testing large capacitors. You can buy a real ESR meter and learn how to use it to save money on caps over several projects but not worth the expense for only the odd need. Once you know the duty and age of components that have limited life, the decisions are simpler anyway.

The A9 amplifier was produced around 37 years ago so you can see that there is a fair chance of capacitor problems unless someone before you has already addressed them. As implied in Adason's suggestion, the soldering on mass produced PCBs can be bad right from the start. It may not become a problem until many years of thermal cycling have taken place but intermittent faults like noises and dropouts are legion. A dab with the iron and maybe the tiniest spot of 60/40 flux to connections on the PCBs is easy. If you know the general location of the fault, it doesn't take long either.

I couldn't say what the greasy layer on the PCBs is, but it seems likely that someone before you may have tried to fix the problem with "the mechanic in a can" or just the wrong type of service product that left a deposit behind. A greasy film may also darken by absorbing airborne dust, soaking into the phenolic board etc. You could try a relatively safe solvent like isopropyl alcohol or methylated spirit with a small wipe test first so petroleum based grease/oil deposit will require a related solvent like white spirit but this could also soften the print which identifies the components. Test first.

Needless to say, using flammable solvents in the home, particularly an air conditioned one or an apartment, is a no-no nowadays. Consider your circumstances and experience before trying something out of your comfort zone. PCB cleaning can also be done by scrubbing with hot water and strong detergent but this requires disconnecting the boards, removing, and fully drying afterward. Sometimes this can't be done without unsoldering leads but make a sketch of where they connect first.

Pray that there's no problem with the input dual fet(s). There are expensive pulls and fakes around but any you see on Ebay are unlikely to be genuine parts though they may seem to function acceptably. The issues are not only parameters like Idss but the match between the devices. That's what makes genuine parts essential even if unobtanium. If you have sound, don't touch without a very good reason :Ouch:
Click to expand...

Yep; agree 100%. I would suspect Electrolytics drying out as the FIRST cause of this (they can also spew out or just slowly release the fluids; literally drying out). Most likely, it is not in the power supply section since 1 channel is doing this over the other. MTBF of some electolytics is 10 years; much higher quality ones can go 20 years or more. I had almost the EXACT same experience with a 20 year old Yamaha receiver. Replacing EVERY electrolytic may not be cost effective unless you truly want to restore this back to original condition!
 
#45
update - turns out it is not the speaker relay issue. I tested the system thoroughly , it took 1 week for the pop sound to come again . 1 WEEK ! i did all sort of testing. every day or every alternate day switch the system ON from cold state(for 2 hours ). then after 4 days switch it ON from cold state. there was just no pops and statics , until 8 days later.

I will find the problem , maybe leaky capacitors or faulty transistors . or loose solder or a bleeding leg of a transistor .
appreciate every ones help here . great forum . I will definitely keep updating results here

quick note. Since I have been ignoring the static pop sounds I turn the system volume up at 80% to avoid the pops as at this volume level it seems to not occur or get the chance to occur. of course with music playing . approx at 100 wpc output volume
 
#50
my english is very poor ,sorry 🙁
you have a good channel, so that's your reference.
you just have to compare the wrong channel with the good one until you find a difference.
you start by unplugging the preamp and the amp and you try one without the other to see if the problem comes from the preamp or the amp.
when you have found which one has a problem, you compare the failed channel to the other working channel starting at the input of the circuit (in) to the output (out) of the circuit.
 
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