Pioneer-A66X recap and restore

[goodpsusearch]

I bought this vintage amp 2 years ago for 60€

It worked but the source selector buttons needed maintenance/restore.

When I selected a source, one of the 2 channels was more silent than the other, some times while I was listening to music, one channel would start to disappear and I also noticed that if I turned up the volume both left and right channels would return, but in low volume there would be some noise and would loose a channel like a bad contact issue.

I figured that after 35 years of life the contacts of the buttons and the potentiometers would get dusty/oxidized and need maintenance.

So, I finally found the time during second lock down due to Covid to remove both the upper cover and the bottom cover of this thing and take a look. I also removed the front cover revealing some lights and switches.

Looks to me to be a high quality unit and everything is designed to be easily accessed and serviced.

I really like that the capacitors are safely distanced from the hot parts, compare this to modern devices where the capacitor is nearly touching the heatsink of the voltage regulator / transistor.

The phono preamp is located on an add on board and looks impressive. The green caps are Nichicon Muse audiophile capacitors of that age. I checked all the caps, some of them out of circuit and replaced 4 caps with Rubycon YXF.

I am also thinking of replacing the 2 12000uF primary caps with 2x 15000uF ones to help the amp have even better dynamic response to load.

I also cleaned the contacts of the buttons and the pots, I would like your opinion if I did a mistake. I first used CRC QD-Contact Cleaner to clean the dust/dirt/deoxidize contacts and then apllied CRC 2-26 to preserve good contact.

CRC 2-26 leaves residue where it is sprayed. Do you think this might have been wrong?

Btw I attach 10 pics before recapping the amp

[spiros.p]

hi change all caps small and big ones.crc 2-26 is ok but dont throw to much i do all this to my 30 years old technics amp and radio all so to a maranz eq and they sound like new for the next 30 years.

[goodpsusearch]

The reason I did a partial recap and didn't change all the capacitors on this amp is mainly that I was out of stock on several values that this amp used.

Sure, my stock always has been based on power supplies and especially ATX psus and only just recently I started playing with hi fi stuff (vintage stereo amps, multichannel AV amps, active sub-woofers, dj mixer consoles, turntables, etc).

Another reason is that the 36 year old capacitors looked fine and I proceeded to even test them one by one in circuit and some of those out of circuit and they all were fine, normal ESR for their specs, close to nominal capacitance and low leakage current.

I now attach pics after the partial recap.

Unfortunately, I think I applied too much 2-26. I couldn't access the internals of the buttons and the buttons had serious contact issues, so I got curried away and spilled too much.

I tried then to wipe the excessive oil that was left over.

The amp works and the sound is just wonderful. I noticed that if it runs for several hours at normal levels of sound, I can smell the oil from the areas that the amp gets hot during operation. I hope it is not cooking oil now

The 2x 15000uF caps that I ordered are here and I am planning to replace the 12000uF ones with those to improve the amp power handling capability

[goodpsusearch]

Originally Posted by goodpsusearch

The reason I did a partial recap and didn't change all the capacitors on this amp is mainly that I was out of stock on several values that this amp used.

Sure, my stock always has been based on power supplies and especially ATX psus and only just recently I started playing with hi fi stuff (vintage stereo amps, multichannel AV amps, active sub-woofers, dj mixer consoles, turntables, etc).

I revisited the internals of this amplifier and went on and replaced more caps with 105C good brand caps entry level esr. Pics after recap attached

Originally Posted by goodpsusearch

Unfortunately, I think I applied too much 2-26. I couldn't access the internals of the buttons and the buttons had serious contact issues, so I got curried away and spilled too much.

I tried then to wipe the excessive oil that was left over.

I removed face plate and then another cover hiding the input selector switches, unscrewed several pots and switches and then very carefully cleaned with cotton buds, cotton and paper the excess 2.26 I had spilled on it. I also used again CRC Contact cleaner to remove it from spots that were otherwise unreachable to me.

Originally Posted by goodpsusearch

The 2x 15000uF caps that I ordered are here and I am planning to replace the 12000uF ones with those to improve the amp power handling capability

This didn't go very well. Before ordering the caps, I measured the height and the widths of the original ones but failed to measure lead spacing.

Turns out the new caps have vastly different lead spacing and the only way to fit them there would be to drill holes to the PCB and I didn't want to go that far with that kind of vintage gear...

The new caps have also 2 legs per cap while the original caps have 4 legs, 2 legs are connected to electrolyte and 2 not connected to anywhere.

So, I decided to leave unchanged the original 2x12000uF caps. But first I measured them both, out of circuit and they were totally fine in terms of capacitance, ESR and leakage current.


The amplifier works but when I compare it with a Sony AVR that I got I think it has issues with sound. Among others, I think it overemphasizes the "s" letter, for example when there are lyrics like this:
https://www.youtube.com/watch?v=KQBoeBgb0uk

"The sound is in my ears
I can't believe the things you say
They echo what I fear"

It overemphasizes it like:

"The SSound is in my earSS
I can't believe the thingSS you SSay
They echo what I fear"

Sorry, I cannot explain it better, I tried searching online this symptom using many posssible terms explaining and got nothing.

Has anyone any idea what might be the issue? I am quite sure that the amplifier did this from the time that I got it, before actually changing anything to it.

[momaka]

Originally Posted by goodpsusearch

The amplifier works but when I compare it with a Sony AVR that I got I think it has issues with sound. Among others, I think it overemphasizes the "s" letter, for example when there are lyrics like this:
https://www.youtube.com/watch?v=KQBoeBgb0uk

"The sound is in my ears
I can't believe the things you say
They echo what I fear"

It overemphasizes it like:

"The SSound is in my earSS
I can't believe the thingSS you SSay
They echo what I fear"

Sorry, I cannot explain it better, I tried searching online this symptom using many posssible terms explaining and got nothing.

From what I've seen online from pro headphone reviews (like rtings.com), over-emphasized "S-es" means the device is said to produce "sibilant" sound, which generally means frequencies around the 10 KHz range (8-12 KHz, typically) are strong or over-emphasized.

Originally Posted by goodpsusearch

Has anyone any idea what might be the issue? I am quite sure that the amplifier did this from the time that I got it, before actually changing anything to it.

Assuming nothing is wrong with the tone controls (you can try switching the amp to Direct mode, if it has it)... then it's probably just the way it sounds.
Actually, according to the info on HiFiEngine here, that's a pretty good amp - only 0.007% THD and frequency response from 5 Hz to 100 KHz. No wonder it will sound "sharp" with treble. If you're used to listening from more modern amps (that usually have much worse THD and frequency response), you might be just used to a more "muffled" audio.
Then again, it's also possible that may not be the case and maybe there is something wrong with the amp... but for "Ses" to stick out / get over-emphasized, that means either all other frequencies are getting lowered relative to them, or somehow the tone controls are boosting the mid and upper sections of the treble, producing a more "sibilant" audio. In any case, if there is a problem anywhere, it would be in the pre-amp section, likely.
Have you tried using different inputs and different sources too?
Also, have you tried the amp with headphones, just to see how they sound or if the over-emphasized Ses are still there?

------------------------
By the way, that's a really good amplifier! Totally worth the 60 Euro any day. You simply won't find modern stuff built like this anymore - ever. With modern stuff, any chance they get to cheap out anywhere in any section - they usually will. On the other hand, old amps like this are gold.

[goodpsusearch]

Originally Posted by momaka

From what I've seen online from pro headphone reviews (like rtings.com), over-emphasized "S-es" means the device is said to produce "sibilant" sound, which generally means frequencies around the 10 KHz range (8-12 KHz, typically) are strong or over-emphasized.

I really cannot find the words to say how grateful I am for your help once more!

Now I have a term for my listening experience that I can google and actually , I already started online search since reading your post. "Sibilance"

Found several people claiming that when they were switching amps they noticed a difference in how sibilant the sound became. They changed the amp and nothing else in their system.

Even though I read many forum pages about this, unfortunately I have yet to find something technical that could explain this, or a suggestion what to check on the amplifier and replace and then test sound to see if it got any better.

Originally Posted by momaka

Assuming nothing is wrong with the tone controls (you can try switching the amp to Direct mode, if it has it)... then it's probably just the way it sounds.
Actually, according to the info on HiFiEngine here, that's a pretty good amp - only 0.007% THD and frequency response from 5 Hz to 100 KHz. No wonder it will sound "sharp" with treble. If you're used to listening from more modern amps (that usually have much worse THD and frequency response), you might be just used to a more "muffled" audio.
Then again, it's also possible that may not be the case and maybe there is something wrong with the amp... but for "Ses" to stick out / get over-emphasized, that means either all other frequencies are getting lowered relative to them, or somehow the tone controls are boosting the mid and upper sections of the treble, producing a more "sibilant" audio. In any case, if there is a problem anywhere, it would be in the pre-amp section, likely.
Have you tried using different inputs and different sources too?
Also, have you tried the amp with headphones, just to see how they sound or if the over-emphasized Ses are still there?

I always set controls to total flat and if there is a direct mode / analog sound direct / tone defeat mode etc I always enable it. Sorry, I should have mentioned it.

I tried several inputs and many sources. 3 different laptops, sound effects disabled on sound card drivers and windows audio settings and effects.
Edit: I also tried 3 different phones, and I checked online measurements about their 3.5mm mini jack audio quality and it was excellent.

Tried the headphones too, but it is not a very useful test, because back then the headphone output was just the amplifier output with some high value resistors in series

Service manual attached bellow

Originally Posted by momaka

By the way, that's a really good amplifier! Totally worth the 60 Euro any day. You simply won't find modern stuff built like this anymore - ever. With modern stuff, any chance they get to cheap out anywhere in any section - they usually will. On the other hand, old amps like this are gold.

Indeed, that's why I can't let go, I need to find if anything it's wrong and fix it. I am not sure I believe in audiophile myths such as pairing. I am doubtful that the sibilant sound might be due to the amp not pairing well with the speakers.

Unfortunately, my knowledge in this field is limited.

Are there any components on the amp board that could affect sound like this? Could this be due to bad ceramic caps for example?

Since, the amp was made in 1985 there is no logic IC inside, which I like a lot btw
All the buttons and switches and trimpots are mechanical, meaning that they handle the actual signals, these fat ribbons you see all over the place actually carry the inputs to the input selector switches. I wonder if a worn / corroded switch could distort the sound like this.

I also wonder if after all these years the transistors could be aging and affecting sound quality.

Could the remaining unchanged electrolytic caps cause this? I left unchanged some caps in power supply section but they tested ok in circuit and also left unchanged the 2 green audio grade audiophile capacitors located on the main board as you can see in the picture:

https://www.badcaps.net/forum/attach...0&d=1642280105

Of course I am making stupid speculations here, as I have no clue where I should start looking.

[momaka]

Originally Posted by goodpsusearch

Found several people claiming that when they were switching amps they noticed a difference in how sibilant the sound became. They changed the amp and nothing else in their system.

Even though I read many forum pages about this, unfortunately I have yet to find something technical that could explain this, or a suggestion what to check on the amplifier and replace and then test sound to see if it got any better.

Well, when the high frequencies appear much "brighter" than the rest of the frequencies in the audio range, that could be caused by a bad contact or a bad cap (usually electrolytic) in series with the signal path. So maybe check all of the small caps that handle audio signal (i.e. not big caps that handle power filtering.)

Another approach might be to input white noise into the amplifier and record the speaker output (with correct resistive divider) on a computer. Then look at the frequency plot (Audacity can do that - both white noise generation and Spectrum analysis of a recorded signal) to see if there are any spikes in the high-frequency range or dips in the bass and mid-frequencies, making the high frequencies stand out. That should give you an idea if what you're hearing really is happening or if there is something else involved.

Originally Posted by goodpsusearch

Tried the headphones too, but it is not a very useful test, because back then the headphone output was just the amplifier output with some high value resistors in series

That's better then, at least when it comes to seeing if the "sibilant" sound really comes from the amplifier or not. On that note, you may just feed the headphone output of the amplifier to another laptop PC, and record the sound with it in Audacity. Then check the Spectrum Analyzer for any anomalies.

Originally Posted by goodpsusearch

I am not sure I believe in audiophile myths such as pairing. I am doubtful that the sibilant sound might be due to the amp not pairing well with the speakers.

Unfortunately, my knowledge in this field is limited.

Pairing is indeed kind of a myth.

I say "kind of", because sometimes distortion from a "lesser" amplifier may actually produce a more "pleasant" sound to some listeners. It really all depends on the audio content being played and preferences of the listener himself/herself. For example, I tend to be a bit of a bass head and usually never run speakers without the tone controls turned On (typically I like to add a little... or a little more than a little ... to the bass.) Tone controls are technically a form of distortion to the original signal. But again, depending on the listener and the music content, distortion may be a desirable thing.

That being said, a "good" amplifier will simply amplify the signal/audio being sent to it without adding distortion, regardless of whatever speakers are connected to it (well, speakers that are within the size and output power levels of the amplifier anyways.) If there is any distortion heard, that would be from the speakers themselves and not the amplifier.

So going back to that example where you said you've read that some people swapped their amp and heard a more sibilant sound... maybe this is because the first amplifier they used wasn't "as good" and actually sounded "softer" naturally (i.e. attenuating / distorting the high frequencies) and the replacement amplifier produced an output that was "truer" to the input signal.

Originally Posted by goodpsusearch

Are there any components on the amp board that could affect sound like this? Could this be due to bad ceramic caps for example?

Sure.
Though bad ceramic caps tend to short-circuit, so I don't think those would be the issue. Again, the more likely culprit for a "bright" / sibilant sound would be possibly bad electrolytic or metal film coupling caps in series with the signal.

Originally Posted by goodpsusearch

Since, the amp was made in 1985 there is no logic IC inside, which I like a lot btw
All the buttons and switches and trimpots are mechanical, meaning that they handle the actual signals, these fat ribbons you see all over the place actually carry the inputs to the input selector switches.

Those are the best!
All the more modern stuff with digital "soundstage mixing" and whatnot sound much worse IMO - at least for simple stereo audio, like CDs and whatnot.

Originally Posted by goodpsusearch

I wonder if a worn / corroded switch could distort the sound like this.

Possibly.

Originally Posted by goodpsusearch

I also wonder if after all these years the transistors could be aging and affecting sound quality.

That's more unlikely, though not impossible from what I've read... usually transistors start getting electrically leaky with age, so I think the distortion will likely be a lot worse than just a slightly sibilant sound. But if someone has first hand experience with this, please correct me if I'm wrong.

Originally Posted by goodpsusearch

Could the remaining unchanged electrolytic caps cause this? I left unchanged some caps in power supply section but they tested ok in circuit and also left unchanged the 2 green audio grade audiophile capacitors located on the main board as you can see in the picture

Hmmm... Are those the only unchanged caps? If so, then I don't think those would be the issue.

[goodpsusearch]

Thank you so much for the time and effort you put into your answer.

Currently, I am working on another amplifier, a Sony STR-DB940 with MANY bad caps and dry solder joints, I will revisit this thread when I find some time.