SX-950 volume pot replacement

HomerJSimpson · Sep 22, 2008

I have a 950 that has a broken volume pot. Iam having a hard time finding a replacement. It is a 2x50 ohm.

Has anyone found a suitable replacement?

markthefixer · Sep 22, 2008

HomerJSimpson said:
I have a 950 that has a broken volume pot. Iam having a hard time finding a replacement. It is a 2x50 ohm.

Has anyone found a suitable replacement?

It is an ACV-164, a dual 50k ohm loudness tapped audio taper pot. PC mount, with an undetermined number of detents.

It is only used in the sx-850 & sx-950.

It may be possible, by changing some other values of parts around it to get the loudness and -20 db muting correct, to use a 100k pot, which can be found in the sx-550 as an undetented control (acv-161: sx-450, 550, 580, 680, 780, 880), through the sx-3800 (acv-181) with 40 detents AND IS STILL AVAILABLE FROM Pioneer for about 20 bucks....

HomerJSimpson · Sep 23, 2008

I have a 100k pot from my 780 parts unit...

What would I have to change?

markthefixer · Sep 23, 2008

HomerJSimpson said:
I have a 100k pot from my 780 parts unit...

What would I have to change?

The 20 db attenuator needs no changes, the difference is within component tolerances, 10 percent.

The 950's R19 & R20 (6.8k) get replaced by the 780's R219 & R220 (15k)
The 950's C9 & C10 (0.082) get replaced by the 780's C205 & C206 (0.022)
The 950's C7 & C8 (470pf) get replaced by the 780's C203 & C204 (330pf)
The 950's R47 & R48 (22k) get replaced by 68k, blue-grey-orange resistors - I don't know if there are any in the 780...

This boils down to recreating the sx-550's loudness circuit and values in the 950......
(because of the 50k to 100k volume pot change)

The 780 has a VERY different balance control arrangement than the 550 or the 950's, thus the missing 68k resistors.

Don't worry about the 550 to 950 differences, the 950 has a buffer amp between the balance control and the volume control, thus eliminating any balance control effects from altering the loudness curve. Just find and get those 68k resistors in.

HomerJSimpson · Sep 23, 2008

markthefixer said:
The 780 has a VERY different balance control arrangement than the 550 or the 950's, thus the missing 68k resistors.

Don't worry about the 550 to 950 differences, the 950 has a buffer amp between the balance control and the volume control, thus eliminating any balance control effects from altering the loudness curve. Just find and get those 68k resistors in.

Where do I out thr 68k resistors? I have some laying around.

markthefixer · Sep 23, 2008

markthefixer said:
the 20 Db Attenuator Needs No Changes, The Difference Is Within Component Tolerances, 10 Percent.

The 950's R19 & R20 (6.8k) Get Replaced By The 780's R219 & R220 (15k)
The 950's C9 & C10 (0.082) Get Replaced By The 780's C205 & C206 (0.022)
The 950's C7 & C8 (470pf) Get Replaced By The 780's C203 & C204 (330pf)
the 950's R47 & R48 (22k) Get Replaced By 68k, Blue-grey-orange Resistors - I Don't Know If There Are Any In The 780...

This Boils Down To Recreating The Sx-550's Loudness Circuit And Values In The 950......
(because Of The 50k To 100k Volume Pot Change)

The 780 Has A Very Different Balance Control Arrangement Than The 550 Or The 950's, Thus The Missing 68k Resistors.

Don't Worry About The 550 To 950 Differences, The 950 Has A Buffer Amp Between The Balance Control And The Volume Control, Thus Eliminating Any Balance Control Effects From Altering The Loudness Curve. Just Find And Get Those 68k Resistors In.

...... ^^^^^ ..........

homerjsimpson said:
where Do I Out Thr 68k Resistors? I Have Some Laying Around.

Good... I was concerned you wouldn't be able to find any in the 780....

HomerJSimpson · Sep 23, 2008

markthefixer said:
...... ^^^^^ ..........

Good... I was concerned you wouldn't be able to find any in the 780....

D'oh ! I don't know how I missed that:bash:

I will work on it tonight and post the results.

Thank you Mark:thmbsp:

HomerJSimpson · Sep 23, 2008

The procedure is complete.

The patient is fine and in recovery...:thmbsp:

Thanx a million.

Now all I need is 2 knobs and she will be like new again, well sorta, needs a new cabinet too...

well there is one more thing, there is a POP when the tone control switch is flipped. This will cause it to go into protection for a second...

markthefixer · Sep 23, 2008

HomerJSimpson said:
The procedure is complete.

The patient is fine and in recovery...:thmbsp:

Thanx a million.

Now all I need is 2 knobs and she will be like new again, well sorta, needs a new cabinet too...

well there is one more thing, there is a POP when the tone control switch is flipped. This will cause it to go into protection for a second...

Dc coming out of the tone controls, although you can try deoxit, my bet is on a recap.... any blue sanyo caps or tantalum caps? Those would be your usual suspects... BTW you didn't say if it was one or both channels... that's important. When I get that info I will look at the schematic.

BTW, how many detents were on the original volume control, and on the 780's ? This is for future reference.

HomerJSimpson · Sep 24, 2008

markthefixer said:
Dc coming out of the tone controls, although you can try deoxit, my bet is on a recap.... any blue sanyo caps or tantalum caps? Those would be your usual suspects... BTW you didn't say if it was one or both channels... that's important. When I get that info I will look at the schematic.

Right channel

BTW, how many detents were on the original volume control, and on the 780's ? This is for future reference.

What do you mean by detents? It is is smooth when youn you turn it.

HomerJSimpson · Sep 25, 2008

the pop is only on the right channel.

HomerJSimpson · Sep 25, 2008

I replaced the 4 electrilytic caps, problem still exists.

HomerJSimpson · Sep 25, 2008

It also does this on the Low cut as well...

markthefixer · Sep 25, 2008

HomerJSimpson said:
I replaced the 4 electrilytic caps, problem still exists.

Which 4? Quite frankly I expected a recap of the entire board (12 caps).

Did you do C23 & C24 in the tone amp assembly AWG-039 (page 56 of 73 in the .pdf)?

Turn ON the tone amp, then try the low cut in and out to see if it pops. If the low cut pop only occurs on the tone amp being OFF, then the output cap C24 on the flat amp is leaking.

Also check the voltages on C13+ & C14+ on the flat amp board, they are the left and right output stage power and should be similar.

HomerJSimpson · Sep 25, 2008

markthefixer said:
Which 4? Quite frankly I expected a recap of the entire board (12 caps).

C19 C20 C21 C22

markthefixer said:
Did you do C23 & C24 in the tone amp assembly AWG-039 (page 56 of 73 in the .pdf)?

No

Turn ON the tone amp, then try the low cut in and out to see if it pops. If the low cut pop only occurs on the tone amp being OFF, then the output cap C24 on the flat amp is leaking.

When I do this it DOES NOT POP!!!

HomerJSimpson · Sep 25, 2008

Replaced C24 on flat amp, and problem is gone...

Thanx Mark....

Once again, you saved the day...:thmbsp:

HomerJSimpson · Sep 25, 2008

Well I am almost done with this nightmare.

I found and replaced some burnt up resistors on the phono board. While testing this unit I have nofound that the new resistors are starting to burn up....:tears:

I replaced R13 R15 and R17

markthefixer · Sep 26, 2008

HomerJSimpson said:
Well I am almost done with this nightmare.

I found and replaced some burnt up resistors on the phono board. While testing this unit I have nofound that the new resistors are starting to burn up....:tears:

I replaced R13 R15 and R17

search on awf011 equalization amp...

Rather than fooling around, figuring out which of the 3 on that channel are bad, just replace ALL 6 transistors a's for a' and c's for c's:

2sa726 to-92 BCE 50v 0.1a 0.2w 100mhz 500hfe watch out, the replacement is ecb
2sc1313 to-92 BCE 50v 0.1a 0.2w 150mhz 350hfe watch out, the replacement is ecb

512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.06 ea
512-KSC1845EBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.06 ea

These are tried and true, very quiet transistors.
You might want to get yourself a bunch, they're good for a LOT of Pioneer applications...
(I always keep a bunch around, and at 120v they're nothing to sneeze at, they're not zetexes with the high current capability, but they're great for low level preamp signals up through the differential input stages of the power amps.)

Recapping is also a VERY good idea too here, the transistor didn't go for nothing (usually).

from an older post:

markthefixer said:
Well, IF you were going to do it yourself, and had soldering / desoldering equipment, you could get away with spending far less than 10 bucks (edit $3.16 w/o ship) in parts, excluding shipping. There are SEVERAL threads on the Pioneer forum about rebuilding the phono amp in the 850, 950 and several remarkably similar phono's on other models.

<snip>
from mouser.com
Q1, Q2 : 512-KSA992FBU (low noise)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.06 ea
Q3, Q4, Q5, Q6: 512-KSC1845EBU (low noise)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.06 ea
C1, C2: 1uf 25v 647-UPW1H010MDD 1uf 50v $0.23 ea
C7, C8: 330uf 6v 647-UPW0J331MED 330uf 6.3v $0.39 ea
C11, C12: 3.3uf 25v 647-UPW1H3R3MDD 3.3uf 50v $0.23 ea
C17: 100uf 35v 647-UPW1H101MPD 100uf 50v $0.50 ea
C18: 220uf 25v 647-upw1h221mpd 220uf 50v $0.60 ea

all are currently in stock, changes in voltage reflect either availability or cost versus safety margin tradeoffs.

Note: the transistor lead arrangement on the originals are bce while the replacements are ecb, meaning the flat faces the other way
and the caps are polarized, look for the plus on the board and remember the MINUS lead is marked on the capacitor, be sure the board markings reflect the way the cap you are removing was installed.

<snip>

DAY4201 · Dec 25, 2018

markthefixer said:
It is an ACV-164, a dual 50k ohm loudness tapped audio taper pot. PC mount, with an undetermined number of detents.

It is only used in the sx-850 & sx-950.

It may be possible, by changing some other values of parts around it to get the loudness and -20 db muting correct, to use a 100k pot, which can be found in the sx-550 as an undetented control (acv-161: sx-450, 550, 580, 680, 780, 880), through the sx-3800 (acv-181) with 40 detents AND IS STILL AVAILABLE FROM Pioneer for about 20 bucks....

Hi Mark, you've helped with some other issues I had with my restoration of an SX-850. Here is another question. I have moved to the flat amp and it appears to me that the volume pot (ACV-161) is damaged. When I bought this a thrift store ($15) I notices that the control was slightly bent but don't see any damage to the face plate or case. I have deoxed it once early but it still is sort staticy (not that is a real word or not) but generally works. I have attached a photo or 2 of it as I can't really say if it is damaged or not. I have yet to find a good photo of one so I need to ask the question. Thanks for any help you can give. Best regards and Happy Holiday's.

EastPoint · Dec 25, 2018

Soak it in Deoxit and work it back and forth a few dozen times. It also may be getting DC leaking from caps going bad. Those bright orange caps and the blue blob tantalum caps should all be replaced, probably with Wima film caps.

SX-950 volume pot replacement

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