Replacing Vintage Pioneer broken Volume Control Dual Gang Pot *Need Help

[steve]

Hi DIY'ers,
I'm really happy I found this forum and hope there's someone out there to help me with this project. I joined this forum to ask this question but I know there will be more.

Equipment: Pioneer SA 6500 ii Amplifier
Issue: Replacing and installing Volume control potentiometer
Part: AVC - 150 - 0 B1 - 100KohmX2

I thought this would be a simple fix. Just replace a dual gang pot. But after looking more closely I realized that this pot had three lugs on top of each pot and one lug on the bottom of each pot totalling 8 lugs to be wired to the board. I have the schematic downloaded which I'd be happy to send to anyone if they're so ambitious(stevenpwilton@gmail.com). So the main problem is I don't know what to buy and I don't understand what the bottom lugs are for. Are they grounds? If they are grounds can I just buy a 6 lug dual gang pot and solder to the back of the pots? This amp sounded really good; how can I insure I'm buying a good quality pot to avoid a loss in quality? (also if anyone looks at the schematic, the numbers of the connections on the board that link to each bottom lug are 19 and 21.

When I was pulling off the volume know the turning peg slipped through the back and now continuously turns. It feels as if its stripped and maybe the wipers are broken. Is there any way of repairing this pot. Im a beginner so if this is very difficult its probably not for me.

Pictures:

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/20140104_024558.jpg.html

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/20140104_024515.jpg.html

Schematic(pdf): (uploaded titled hfe_pioneer_sa-6500_mk2_service.pdf)

http://www.aronnelson.com/DIYFiles/up/hfe_pioneer_sa-6500_mk2_service.pdf

Thanks in advance for anyone who helps.

Cheers,
Steve

[duck_arse]

wow, that's what I call a circuit diagram.

the extra pins on the vol pot are taps for the loudness function. it will take an expert to explain, as I've never understood them, but they are tapped at something like 20% of maximum, and you push the loudness switch to boost high and low freqs, but only at volumes below the tap.

try google the part, try alpha, you might find exact, but I think they are pretty much standard for the tap value. you can sub a normal dual gang if you can live without the loudness switch.

[digi2t]

This might do the job;



http://www.ebay.ca/itm/3-x-Genuine-ALPHA-A100K-Dual-Stereo-Center-Tap-Potentiometer-Pot-15m-/300921422458?pt=LH_DefaultDomain_0&hash=item461050827a

[Jmkrull]

I have a similar issue. It doesn't appear to be anything on the market and I asked a few of the NOS part dealers and they don't have anything. The pot is getting really bad and I really don't want to trash the receiver because of a pot. I would be fine putting in a replacement that doesn't have the balance control (as long as it can remain dead center balance) as long as I can get volume. Does anyone know a place that I could find a replacement or how I would go about putting in a replacement that locks the balance and give me volume? Also, if I change the value of the pot because there's another one of a different value, what would be the result (assuming higher value than lower), just a larger sweep?






Schematic:
https://drive.google.com/open?id=1Thx7L7Se-MRqO-0RBKXuXkO8o-paP2OR

[Mark Hammer]

Two things...

1) Traditionally, such "taps" on volume pots were not critical to the basic functioning of the amp and volume control.  They were used to provide equal-loudness compensation below designated volume settings, not dissimilar from the compensated volume pot on a Fender guitar.  It is a convenience to implicitly/automatically adjust top end as volume level is changed.  But it's usually not something you couldn't do using an untapped volume pot and tweaking the bass and treble controls a bit.

2) I recommend a product called Stabilant 22.  It is a contact enhancer - NOT a cleaner - that applies a thin non-drying electroconductive layer on whatever contact you want.  Not cheap, but a teensy bit goes a very long way.  I've been quite impressed with the controls, connectors, and appliances I've been able to bring back to life with the stuff.  Obviously if the pot in question is fractured or mechanically irreparable, that's one thing.  But if it is simply a case of crackly intermittent contact, this stuff can work miracles.  Pro repair benches recommend a good cleaning first (e.g., with DeOxit), followed by application of Stabilant.  A few sesame-seed-sized droplets are generally enough to bring a pot back to life.

[Jmkrull]

Two things...

1) Traditionally, such "taps" on volume pots were not critical to the basic functioning of the amp and volume control.  They were used to provide equal-loudness compensation below designated volume settings, not dissimilar from the compensated volume pot on a Fender guitar.  It is a convenience to implicitly/automatically adjust top end as volume level is changed.  But it's usually not something you couldn't do using an untapped volume pot and tweaking the bass and treble controls a bit.

2) I recommend a product called Stabilant 22.  It is a contact enhancer - NOT a cleaner - that applies a thin non-drying electroconductive layer on whatever contact you want.  Not cheap, but a teensy bit goes a very long way.  I've been quite impressed with the controls, connectors, and appliances I've been able to bring back to life with the stuff.  Obviously if the pot in question is fractured or mechanically irreparable, that's one thing.  But if it is simply a case of crackly intermittent contact, this stuff can work miracles.  Pro repair benches recommend a good cleaning first (e.g., with DeOxit), followed by application of Stabilant.  A few sesame-seed-sized droplets are generally enough to bring a pot back to life.

Thanks for the reply. Good to know about the taps, that widens the net for the search a bit. For the solution, I've tried a few cleaners and lubricants. It ends up not really helping as it seems to be more of a dead spot or worn through as the problem areas are always in the same positions before and after cleaning. I think the pot has just run its course, so to say.

[Mark Hammer]

Thanks for the reply. Good to know about the taps, that widens the net for the search a bit. For the solution, I've tried a few cleaners and lubricants. It ends up not really helping as it seems to be more of a dead spot or worn through as the problem areas are always in the same positions before and after cleaning. I think the pot has just run its course, so to say.

That's where I find Stabilant comes to the rescue, by providing a layer on top of the existing resistive strip.  The viscous nature means it's not going to dry out.  But the fact that it doesn't dry also means you can't build up layers.  I like to say that in the land of electrons, one micron may as well be the Grand Canyon.  So often one layer of the stuff will bridge a critical gap and function like a liquid solder joint between the wiper and resistive strip.  But, as I also like to joke, if you've been playing White Room and Tales of Brave Ulysses, nightly, 5 nights a week in a touring Cream tribute band since 1980, chances are poor that Stabilant will be enough to bring that wah pot back from the dead.

[Jmkrull]

Thanks for the reply. Good to know about the taps, that widens the net for the search a bit. For the solution, I've tried a few cleaners and lubricants. It ends up not really helping as it seems to be more of a dead spot or worn through as the problem areas are always in the same positions before and after cleaning. I think the pot has just run its course, so to say.

That's where I find Stabilant comes to the rescue, by providing a layer on top of the existing resistive strip.  The viscous nature means it's not going to dry out.  But the fact that it doesn't dry also means you can't build up layers.  I like to say that in the land of electrons, one micron may as well be the Grand Canyon.  So often one layer of the stuff will bridge a critical gap and function like a liquid solder joint between the wiper and resistive strip.  But, as I also like to joke, if you've been playing White Room and Tales of Brave Ulysses, nightly, 5 nights a week in a touring Cream tribute band since 1980, chances are poor that Stabilant will be enough to bring that wah pot back from the dead.

I guess it's worth a try for the short term. Ideally, I would like to find a replacement.

Do you have to disassemble the pot, or does it just shoot in there?

[Jmkrull]

Wow, that stuff is outrageously expensive...

[anotherjim]

A brand that still appears with a loudness tap are Alps - but those I've seen have 4 pins in a row with the loudness tap at one end of the row. Also the stockists I've found only carried one value. 50k is common. You might only find them with a motor and gearbox for remote control fitted on the back, although that can be removed.

I've had the same dilemma a few times and had some experimental success repairing the track with conductive graphite paint. The resistance of that depends on the thickness and I don't think its all that different from what the pot resistance tracks are made with anyway. However, I didn't trust it myself and fitted ordinary dual Alpha pots, even though I had some engineering to do to match it with the panel knob fitting.

[Mark Hammer]

Wow, that stuff is outrageously expensive...

It is. But 15ml will likely last you for another decade if not longer.  I was fortunate to be introduced to it very inexpensively.  A now-defunct electronics distributor near me was selling 0.5ml "tester" vials for 50 cents apiece.  I scooped up handfuls of them, giving out many to friends and industry folks I thought would find it handy.  I've only got about 2ml left, but it has lasted me years.  Had my first taste been a 15ml vial for $40, I think I would have left it on the shelf.  It was originally recommended to me by a buddy who operates a fairly busy studio that uses 2" tape for the master recording, with very long-throw fader controls at the mixing board.  Those suckers need to be absolutely dead quiet, and his use of Stabilant keeps them that way.

[Rob Strand]

Does anyone know a place that I could find a replacement or how I would go about putting in a replacement that locks the balance and give me volume?

The simple solution is put in two resistors for the balance control.  The value is half the balance pot value.
Some circuits you can just not connect the balance pot but in others the gain increases too much so it's best to use the resistors.

[Jmkrull]

Does anyone know a place that I could find a replacement or how I would go about putting in a replacement that locks the balance and give me volume?

The simple solution is put in two resistors for the balance control.  The value is half the balance pot value.
Some circuits you can just not connect the balance pot but in others the gain increases too much so it's best to use the resistors.

What do you mean by this? Could you please explain a little bit more on what you're talking about?

[Rob Strand]

What do you mean by this? Could you please explain a little bit more on what you're talking about?

Suppose you have a single-gang balance pot, say 50k, set to the centre position.  There will be 25kohm from the wiper to one side/terminal of the balance pot and 25kohm from the wiper to the other side/terminal of the balance pot.  So if you replace the pot with two 25k ohm resistors which join where the pot wiper goes, the circuit will look exactly like the balance pot is set to the centre position.

[Jmkrull]

What do you mean by this? Could you please explain a little bit more on what you're talking about?

Suppose you have a single-gang balance pot, say 50k, set to the centre position.  There will be 25kohm from the wiper to one side/terminal of the balance pot and 25kohm from the wiper to the other side/terminal of the balance pot.  So if you replace the pot with two 25k ohm resistors which join where the pot wiper goes, the circuit will look exactly like the balance pot is set to the centre position.

Sorry, I feel like an idiot, but say i replace the existing dual gang with a single gang for the volume. This would leave out a whole set of stuff for the balance control, so where would you affix these resistors? Or could you just put the value of half the balance pot between the wires for the balance portion (1 and 3 of the pot currently), leaving out the ground, and do that? I'm not able to conceptualize the two resistors. 

[anotherjim]

I think Rob was only suggesting replacing the stereo left/right balance control with 2 fixed resistors. The balance control has nothing to do with volume or loudness compensation and is typically a single gang potentiometer. Actually, if it comes to it, a faulty balance control can often just be removed if it causes a problem.

If you do remove the balance control, it can cause maximum volume to rise since the balance pot resistance usually provides some loading on the audio path, so replacing it with 2 fixed resistors to equal the balance pot set at the centre can keep the volume in the designed range.

[Jmkrull]

Thanks guys, but I'm lost. I guess this is too much for me to figure out/conceptualize. I'm not advanced when it comes to things like this. I'd have to see it.

To me, it doesn't make sense because there's too many wires going to the pot now. I wouldn't know where to put resistors, nor what I could omit or not.

[Rob Strand]

Thanks guys, but I'm lost.

OK. OK!  I understand why you don't get it.

In the post above you mentioned "locking the balance".  We are thinking that means locking the balance *control *to the central position.   When I look at the front of the unit it has a funky pot.   There is *no balance pot*.  The volume pot is a dual-concentric pot with separate left and right controls, except  (I think), it has some sort of mechanical linkage between the left and right control so the two pot gangs move together.

https://hifidealer.net/wp-content/uploads/2018/05/pioneer_sx-440.jpg

So ignore all comments about resistors and the balance pot.

So there simple options are:
1) Replace the pot with dual-concentric pot which has the mechanical locking.   That will require some digging around and it might turn out to be expensive.

2a) Replace the pot with a common dual-gang pot.  Cheap and will work fine.  However you will only be able to fit the outer knob and you lose the ability to adjust left and right levels individually.

3) Replace the pot with a dual-concentric pot.   A little more expensive than a dual-gang pot and more expensive.  The downside is the controls aren't locked.  It's pain to use since you have to keep setting both pot equally.

That aside, there's one other thing.   
2b) Your pot has 8 terminal instead of 6.   The extra lugs are used for the loudness control.   Such pots are common for stereo volume controls.   With some digging you should be able to get one of these however will will have to go with option 2 because tapped pots for options 1 and 3 aren't common at all.

To summarize:
Option 2a is very easy to do but you will loose the ability to adjust Left and Right volumes individually, which isn't a big deal.  You will also lose the loudness function.    So if you want to keep loudness you will need to go with option 2b.

I could suggest a third option "option 2c" where you use option 2a and modify circuit to implement the loudness function separately.  This is pain and probably requires more changes than you want to do, also, if you don't care about loudness it's a waist of time coming up with a scheme.

[PRR]

> When I look at the front of the unit it has a funky pot.   There is *no balance pot*.  The volume pot is a dual-concentric pot with separate left and right controls
? ? ? I see a Balance knob on the front panel. (It also does not look like a split Volume knob.)

Internally the Balance is indeed two section, but each is only "half a pot". One end is carbon (zigzag), the other is good conductor (solid). When centered, signal passes through 100%, off-center, one side is 100% and the other side is 99%-0%.

A repair is simple. Of the 6 terminals, two are grounded. Let that be. The other two wires on each gang, disconnect from pot and connect to each other.

You lose the Balance control action. It acts like the Balance is always dead-center, just like it did before, except now nothing happens when the knob is off center.

Nevermind.

[Jmkrull]

> When I look at the front of the unit it has a funky pot.   There is *no balance pot*.  The volume pot is a dual-concentric pot with separate left and right controls

? ? ? I see a Balance knob on the front panel. (It also does not look like a split Volume knob.)


Internally the Balance is indeed two section, but each is only "half a pot". One end is carbon (zigzag), the other is good conductor (solid). When centered, signal passes through 100%, off-center, one side is 100% and the other side is 99%-0%.




A repair is simple. Of the 6 terminals, two are grounded. Let that be. The other two wires on each gang, disconnect from pot and connect to each other.





You lose the Balance control action. It acts like the Balance is always dead-center, just like it did before, except now nothing happens when the knob is off center.

Unfortunately, not the same unit...