Help: Problems with Balance and Tone controls on old Yamaha amp

[armandhammer]

The original problem with the Balance control has been solved, but now I'm having issues with the Tone control and Subsonic Filter.

Jump to Post #9 for details.

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I have an old Yamaha AX-500 amp, and the "250KMN" pot for the balance control is not functioning properly (and replacement is unavailable in the exact form factor), so I'd like to bypass it. But I don't know exactly how it works or what to bridge.

I attached a picture and labeled the pins.

Here are the measurements I get across various pins at CCW, CENTER, and CW positions. I'm not sure how reliable some of these are, since the pot is staticky and doesn't work right.


PIN  CCW   CENTER   CW
1+2  3R     235k   235k
3+4  4R       1k   225k


I don't measure a connection across any other pins (not even 2+6 or 3+5, which appear to be ends of the same tracks). Nor have I been able to figure out what pins 5 + 6 do.

Any idea how to bypass this thing?

[Ian.M]

You can see there's a ridge of copper track under the outer carbon track and the inner 'donut' one, so pins 1 and 4 are the two wiper contacts.  The contact layout on the rotor tells us they each contact the adjacent carbon resistance track.  Therefore the track ends for wiper 1 are 2 and 6, and for wiper 4, 3 and 5.

It looks like there may be copper under the carbon half way round each resistance track, which would  make it work as two pots, one for each channel, that fade one chanel as you move away from center leaving the other at full output and visa versa in the other direction.

Note that there appear to be a crack in the track PCB disconnecting pins 5 and 6.  If you scraped away just enough solder mask to let you solder bridge it and reinforced the track CB from the back with epoxy, you might bring it back to life.

Otherwise identify which of pins 2 and 6 is the signal in and jumper it to wiper pin 1, and similarly which of 3 and 5 to jumper to 4.   It *MAY* also need a pair of 240K resistors from 2 to 6 and 3 to 5 to load the previous stage correctly, but odds are it works OK without them.

[coromonadalix]

service manual

check page 6 of the manual  or page 8 of the pdf,  you see  the print out  on the pcb

Normally  you have to remove physically the balance control and do  2x bypass on the pcb with a small wire on each side

[Ian.M]

As I thought.  Now all you've got to do is identify which pins of VR102 (Balance) come from the VR101 (Volume Loudness) wipers.  Trace tracks, or use a DMM with a AC mV range or a scope to probe for signal.

[armandhammer]

Note that there appear to be a crack in the track PCB disconnecting pins 5 and 6.  If you scraped away just enough solder mask to let you solder bridge it and reinforced the track CB from the back with epoxy, you might bring it back to life.

Otherwise identify which of pins 2 and 6 is the signal in and jumper it to wiper pin 1, and similarly which of 3 and 5 to jumper to 4.   It *MAY* also need a pair of 240K resistors from 2 to 6 and 3 to 5 to load the previous stage correctly, but odds are it works OK without them.

Thanks, yes you are right I checked with my DMM and pins 5 and 6 are not connected. However, the pot in general is messed up in other ways because the wipers were loose when I opened it up. I tried to superglue them, but I don't know if it's effective.

I appreciate the additional info about the ~240k resistors, I wasn't sure from my measurements if it needs resistors as jumpers or just bare wire. I will try it first with the resistors.

PINS 2 + 5 come from the Volume control
PINS 3 + 6 go to the Loudness control

So do I jumper 1+2 and 4+5? And what happens with pins 3 and 6?

[Ian.M]

Sorry should have been  "loudness", not "Volume" in my reply #3 above.

Seeing the circuit, (thanks Coromonadalix) it should be fine without the 240K resistors I suggested, so you'll leave the two pins of VR102 (Balance that go to signal ground not connected, but your pin numbers in reply #4 don't make sense - they don't match the picture of the pot track PCB you posted.   Each track has a pin at each end. Each wiper contact has a single pin (at one end only for the outer one).   

Coromonadalix's diagram shows signal in from the VR101 (Loudness) wipers, going to VR102 track ends, with the opposite ends to ground, and  signal out from VR102 wipers going to SW105 (CD direct) switch, . . .

Double check your traced connections and confirm you haven't flipped the pin numbering of the main PCB footprint for VR102 (or if its got a marked pin 1 on the main PCB, confirm it matches pin 1 of your photo)!

[armandhammer]

Sorry yes I made an error. Thanks for being patient. Does this make more sense?

PINS 2 + 5 seem to be GND
PINS 1 + 4 go to Volume
PINS 3 + 6 come from Loudness control

So do I jumper 1+3 and 4+6? And do nothing with 2 and 5?

[Ian.M]

Only if you want to swap left and right!

As I said at the beginning, 1, 2 & 6 are one section and 3, 4 & 5 the other, so jumper 1-6, 3-4, and leave 2 & 5, the grounds not connected.

[armandhammer]

Ah, I now see how it all fits together. Thank you so much. This has been a great learning experience. It's 1am here in Japan so I have to go snooze, but I'll reassemble everything tomorrow and report back how it goes.

[armandhammer]

Okay, so the Balance has been successfully bypassed. But now I realize there's two other problems.

1) The Bass and Treble adjustments only affect the Right channel. It sounds like the Left is turned all the way to -5 no matter what I do. Since both controls seem to be busted for the Left, I don't think it's a bad pot (unless both are bad in the exact same way!). I guess the question here is, what could be wrong such that the signal gets through the circuit, but the tone control has no effect on the Left? And what do I check? I looked at the schematic but the Tone section really has a lot going on. I already replaced the electrolytics with Nichicon of the same value, and the mylar caps with Wima of the same value, as well as the resistors with 1%.

2) When I turn on the Subsonic Filter, I get hum and a staticky sound (almost like the sound of dirty vinyl). I've replaced all the electrolytic on this PCB -- C153 and C154 with regular Nichicon 10/16, and C119 and C120 with 0.1/50 Wima film caps. I do notice that the hum gets louder if I bring my finger near the top of the Nichicon caps, but I don't know if this is normal behavior or not (it's also strange, because the problem only happens when the Subsonic Filter is ON, but these caps are only in the circuit when it's OFF, right?)

Thanks for helping me solve this and learn what's going on

By the way here's the SM: https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Yamaha-AX-500-Service-Manual.pdf

[coromonadalix]

did it receive an impact or a hit of some kind ?,  you seem to have tons of problems with all the front pots / switchs  etc ...

[armandhammer]

did it receive an impact or a hit of some kind ?,  you seem to have tons of problems with all the front pots / switchs  etc ...

Not that I know of, and there's no apparent damage inside or outside (the crack on the balance pot could have been from me, when I pried it open). I bought it used for $20, so I'm fixing it up as a way to learn how these things work.

[armandhammer]

Hmm well this is interesting, I took the Bass/Treb PCB off the front panel to check things, and now it seems to work fine.

I'll post again if I need further help. Otherwise I'm just going to leave it in Direct mode, plug in my iPod, and enjoy some tunes in my workshop. That's really all I need it to do.