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AU-9900 Tone control static

KeithD

KeithD

AK Subscriber
Subscriber
After recapping and replacing all transistors on the tone control boards (F-2575 midrange and F-2576 treble and bass) of my AU-9900, I am getting some low level static, mostly on the right channel. I'm wondering if there is a way to isolate whether the static is coming from the F-2575 or F-2576 board? The problem is the signal goes from the defeat/filter/filter & tone switch to F-2575 then to F-2579 (then to the filter board) when tone controls are selected. Is there a way to check the signal from one or the other boards? If I can't do that, I may need to try to pick up spare (working) boards to swap and see what makes a difference.
 
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Is the static there all the time, or only when you are turning one of the tone controls?
 
All the time. When the selector is in “defeat” or “filter” the amp is dead silent. It’s only in the tone controls, and it’s pretty steady. Sound like classics transistor static, but I’ve replaced the ICs and transistors on both boards.
 
If the static is new, and wasn't there before the work, I would start by putting those IC's (dual pnp transistors) back, as well as the other transistors. I would try the IC's first, and then the transistors.
 
Tom B said:
If the static is new, and wasn't there before the work, I would start by putting those IC's (dual pnp transistors) back, as well as the other transistors. I would try the IC's first, and then the transistors.
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The static started after I replaced the ICs and the caps. On the chance it was coming from TR01/02 I replaced those. The legs were black, so they were easy candidates to do. However, that didn't change the static. I'm pretty sure it's the ICs that are the issue. They are extremely sensitive to movement of any kind. If I get near them with a wood chopstick, I get hum in that channel. If I touch them with the stick, I get static. I was just hoping to narrow down which board the background static is coming from. I'm starting to wonder if the cause is just the slightest vibration on one of the boards that is causing the IC to move enough to create a low background static. My guess is it's the midrange board. That board is held in place only with the nut on the VR pot that holds the board to the frame. The treble and bass board is the same, but each pot has a nut to hold the board.
 
just make sure you installed the KSA992's as B C E C B, or clean the black off the originals and reinstall.
 
Tom B said:
just make sure you installed the KSA992's as B C E C B, or clean the black off the originals and reinstall.
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Yes, the 2 x 992s are installed correct. I already tried reinstalling the original 2SA798s a couple of weeks ago with no effect.

Did some testing and touching with a wood stick today. It seems to come down to one IC on the midrange board. The only thing I can think of is one of the traces got damaged in the replacement? When I touch that IC, I get enhanced static (beyond the background) on that channel. I don't get static when touching any of the other IC positions on either board. I also reflowed the solder at all IC junctions, but no change.
 
BilboBaggins said:
Have you tried reflowing the solder at all the junction points of the signal going through the interconnections?
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No, I only started getting static after re-capping and replacing the ICs on the tone control boards. As I mentioned above, I get enhanced static on the R channel when touching IC02 on the midrange board, so that one seems to be the main issue.
 
The reason I ask is if you have any flaky contacts, they could start acting like an antenna, which is what your symptoms sound like. The interconnects are areas that would get stressed as you are working.
 
BilboBaggins said:
The reason I ask is if you have any flaky contacts, they could start acting like an antenna, which is what your symptoms sound like. The interconnects are areas that would get stressed as you are working.
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Ok, I might not be understanding what you mean by “interconnects” - do you mean the solder pads where the through holes are for the components? A few solder pads did lift off the board on the ICs during the replacement process. I think that is where the trouble is, and agree with you if that’s what you meant. I have reflowed all of those solder joints, but the one IC still seems to be the source of static. Any suggestions would be appreciated. Thanks!
 
Just remembered, in addition to the static, if I put my finger or even the wood stick close to that IC, I start to get a hum which increases the closer I get to the IC.
 
KeithD said:
Ok, I might not be understanding what you mean by “interconnects” - do you mean the solder pads where the through holes are for the components? A few solder pads did lift off the board on the ICs during the replacement process. I think that is where the trouble is, and agree with you if that’s what you meant. I have reflowed all of those solder joints, but the one IC still seems to be the source of static. Any suggestions would be appreciated. Thanks!
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Yes, that's exactly what I mean :)

That IC is acting as an antenna, and your challenge is to find out why. I ran into this once many many years ago, and never forgot about it. I worked on that board on and off for close to a week. I was getting so frustrated! The fix in that situation was a noisy ground that re-flowing the solder solved. The reason I think it is in the interconnects is because you have already reflowed the solder at all the ICs, the problem appeared after you started working on it, so you've introduced something, and these interconnects are notorious for being problematic.
 
Another thing just occurred to me. Check all the pins for shorts. This can happen in the board itself if there are two traces close to each other and the solder flows through. I've seen this in multi-layer boards before. Just put one lead of the MM on a pin and run the other lead across the other pins to see if you have a short.
 
BilboBaggins said:
Another thing just occurred to me. Check all the pins for shorts. This can happen in the board itself if there are two traces close to each other and the solder flows through. I've seen this in multi-layer boards before. Just put one lead of the MM on a pin and run the other lead across the other pins to see if you have a short.
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Thanks for those insights. On these ICs I checked for shorts after each installation or flowing of solder. The pads are so close together, I was worried about shorting one as you mentioned. I will check those again. Will also check the ground connections as well as the solder points for the capacitors I replaced.

I agree, something I did in the rehab of this board is causing the problem - it's just to find out what! :confused: Glad you've seen this issue before. I was hoping someone had to help narrow down the issue. :thumbsup:
 
Sounds like a bad contact that is acting like an antenna as someone says. I had a similar problem with a Yamaha amp, in that case the problem was with a transistor. I changed it and the problem was gone. One of the new devices probably is failing, it happens even with new devices.
 
I probably won't get to this until the weekend. Need to take the faceplate off to get the board out. Of all the boars in this unit, the tone controls are the most tricky to get out. Not much room in there. Will report back what I find.
 
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