Hello!
I’ve got an Alesis QS 8.1 that for some reason is fine on the left channel, but quiet on the right. I thought it was probably the J111 FET transistors so I replaced them and…same problem. There was also a capacitor (can’t remember which one as I’m not currently near the keyboard) that looked like it had burst and leaked so I replaced that. I’ve actually replaced several of the capacitors (again, not at home so I can’t give you which ones at the moment) and the 78L15 and 79L15 voltage regulators. Still no change. I don’t mind replacing all the caps since they’re old, but I hate just throwing stuff at the wall to see what fixes it (and yes, if you’ve seen my DX7 thread, you know I tend to do that), so I’m here asking for help.
The issue is in BOTH the LR outputs AND the headphones, so it’s before those split off. I may eventually sell this keyboard, so while it doesn’t bother me personally and can be compensated by panning it very far to the right, I’d sure love to figure this out and get it working properly. The midi functions work, the card works and can be written to via midi. The sounds are fine as are the sliders, buttons etc. I just can’t fix this weird low volume on the right channel.
Thanks!
Fortunately, there’s not much circuitry involved. The sound is generated in U4 and goes out through two identical amplifier circuits.
Check U6, pins 1 & 7 using your scope. This should be an 8 pin TL084.
From there, the audio goes through the stereo volume pot and into pins 3 & 5 of U7. This should be an NJM4580E.
Audio branches out at pins 1 & 7 of U7, and you’ve already ruled out anything beyond that point.
IMO, half of each op-amp is working, so I don’t suspect U6 or U7. My suspects include the stereo volume pot, which appears to be external to the board, (so wipers, cable, connectors,) C12, C23, C26, and C28
As for throwing stuff at the wall… I’ve always thought that you should be able to narrow it down to just one failed component. But with the common wisdom regarding aged caps, it might not be practical to spend hours tracking down that one part. I’ve been working on an arcade board in bad shape: With the hours I’ve spent on it, I would be billing a small fortune - I’d be done with it if I’d just removed all ICs and rebuilt it.
FWIW, I found some basic schematics on this page:
Thank you! I do have the schematics and have been hunting around on them for the different caps and other components. Your suggestions have helped immensely. I’ll keep you posted.
I’m home and looking over the board. I’m not sure I have the skill to replace any of the tiny, surface mounted compoents, so for now I’m going to look at the stereo pot.
Following up on this, measuring pins on U7, I get signal bouncing on my scope when I hit a key and probe pins 5, 6 and 7 (left, loud channel) and pretty much nothing at all when I probe 1, 2 and 3 (right, quiet channel).
U6 appears to be a 14 pin chip, and if I start at the bottom left reading the label on the chip correctly (Let’s call it pin 1), only pin 1 bounces when I hit a key. Pin 7 does get signal if I bang the keyboard really hard.
Going back to U4, that thing is darn near impossible to count the pins lol. I think it’s 12 on each side. I think I got 16-17 (right) and 18-19 (left) and all 4 read the same (2-3 volts). You may be right about it being C12 or C23 at this point.
As far as the stereo volume pot, I’m not even sure how I’d test the left and right on that. When the board is on, all the pins up at the slider board measure about the same (9 volts I think, I checked it earlier and didn’t write it down).
BIIIG question!
Have you observed the same issue at the AUX outputs!?
AUX is using a separate DAC from the Main - if the issue is apparent on both AUX and Main the problem is Digital !!
I have NOT tried that yet. Mostly because I wasn’t sure if they were fed the same way as the mains. Meaning are they treated like Left and Aux 1 get the same feed, Right and Aux 2 get the same feed. I’ll still test them just in case. Thanks!
They are exactly same except via two different DACs (U8 is AUX, U4 is Main). If both Main and AUX have the same issue then the problem is on the digital side.
changing a program to aux with sound 1 panned left and sound 2 panned right gives me perfect balance in both sides. Switching back to main L and R gives me left heavy again. That means it’s in the main path somewhere and not digital.
… or the DAC itself … of course!!
You’re now down to this little section (pic attached)
You can shorten the two pins on the volume pot board (yellow section pins 5-6 and pins 2-3) - that’ll eliminate the volume pot
when you say shorten, do you mean short together (pin 5 to pin 6, pin 2 to pin 3)?
YES - if you have a look at the schematic - LEFT channel, DAC Out L, goes to U6A buffer and then in at pin 6 to the pot - the wiper comes out at pin 5 and goes to the last buffer amp (U7B) then out to the LEFT jack. RIGHT goes pin 3 in 2 out to U7A and RIGHT Jack.
