My A-X9 is making a strange sound in the left channel-Video

GD70

GD70

AK Subscriber
Subscriber
Seems I'm suddenly having lousy luck in the amp dept!
This just started happening 3 days ago. Turns on fine, comes out of protection normally. The volume has no effect regarding how loud the sound is. None of the inputs, Tuner, Phono, Aux, Tape monitor, Speaker 1/2, balance etc... make any difference. It goes away when the speakers are selected off.
It starts happening maybe 4-5 minutes after I turn it on.
Here's the video where you can hear it.

Any ideas and have you heard this kind of sound before?

Glenn
 
Hmmmmm.....

Hey fellas, no body has any thoughts on what could be causeing this?

Thanks again, Glenn
 
Might be a bad solder joint. Especially if it clears up by cycling the switch. It "might" also be a transistor failing- unless it comes from both channels, in which case focus on the power supply. Also check your grounds to chassis. It would not hurt to clean up the controls and switches with LPS-1 (it is safer than the much touted product).

You might also check the relays for arcing, which the contacts you can try clean up with the much touted product, while it would only be temporary, for purpose of troubleshooting, it would be fine.
 
Couldn't play your video due to the annoying amount of pop-ups that appear in photobucket when I clicked on your link. Could you describe the sound for us? If it's a popping or extended cracking, it could be something as simple as a dry joint, or slightly less simple as transistor with black leg syndrome. I've seen this happen on Pioneer units, the legs of the transistor corrode and turn black, and eventually this corrosion makes it's way inside the transistor and cause crackling and other weird noises.

Lee.
 
Trio said:
Might be a bad solder joint. Especially if it clears up by cycling the switch. It "might" also be a transistor failing- unless it comes from both channels, in which case focus on the power supply. Also check your grounds to chassis. It would not hurt to clean up the controls and switches with LPS-1 (it is safer than the much touted product).

You might also check the relays for arcing, which the contacts you can try clean up with the much touted product, while it would only be temporary, for purpose of troubleshooting, it would be fine.
Click to expand...

Thanks Trio!
Which switch to cycle are you referring to?
Sound is just the left channel.
All the controls and switches were cleaned prior to my purchase.
Would I actually see arcing?
I'll pull the cover, see if I can see anything.

This is so frustrating. It's been working perfectly.

Glenn
 
leesonic said:
Couldn't play your video due to the annoying amount of pop-ups that appear in photobucket when I clicked on your link. Could you describe the sound for us? If it's a popping or extended cracking, it could be something as simple as a dry joint, or slightly less simple as transistor with black leg syndrome. I've seen this happen on Pioneer units, the legs of the transistor corrode and turn black, and eventually this corrosion makes it's way inside the transistor and cause crackling and other weird noises.

Lee.
Click to expand...

It's a weird combination of some popping, crackling, some louder some lower volume, no rhythm to it, and a sound that sounds like a needle sliding across an album which the volumes and length varies. Can be all at once, or any one of the sounds at any time.
I'll put the video on YouTube later tonight. That may work better.

Glenn
 
Any other thoughts on this?
I hope to open it up this weekend, but I must say, this amp is packed. I'm apprehensive to start pulling it apart. I will remove the top and bottom covers to see if anything jumps out at me.

Thanks again, Glenn
 
I think I found the problem. I checked all my connections on the back panel and saw two RCA plugs with male-female connections touching. I must have moved the amp a bit causing the cables to touch. It seems fine now. I did a 15 min test this am before work and it sounds fine. I hope this didn't cause any internal damage.
 
Ahhhh

The problem persists! Crap!
It's good for 15-20 min and the sound comes back. Thought I had it solved.

After (hopefully) the M-7050 is repaired, this will need to go to the shop.
I'm not going to screw around with it.
The tech was hesitant to take the 7050 because of previous repairs, and me being in there, I don't want that with the A-X9.
 
Update-still the same-Pics

My friends brother is a mechanical electric engineer & tech. He has worked with some big name bands including Santana, maintaining their gear.
We looked, listened to the amp, the static, and audible sounds it makes intermittenly.(sp?)
From what he can tell, it's most likely a bad cap, as he's heard this kind of sound in the past. Most likely the power caps. These are 4700uf/71w caps. My question is are these two pin or three pin?:scratch2: Looks like 3 solder joints on the PC board. Also is higher voltage ie; 75-80V ok? How high can I safely go?

Here's two pics. Also, where to get these? As usual, I have trouble with Mouser's cap selection page, I always get "0 results" and not much luck on Digi Key's page either.

Thanks guys.

 
I wouldn't be rushing off and replacing them on a speculative basis (what's the plan if that doesn't work?). If you can get access to a scope I would be verifying that there is something happening on the supply rails when the noise occurs to indicate the supply caps are causing a problem. From my experience with noises like this it tends to be a leaky transistor between the input differential and pre-drive in the power amp. That said it could also be many other things. Does the behaviour seem to be temperature dependant? I.e. Worse when cold, worse when warm or no difference?

Does the volume of the noise change with the setting of the volume control? Without access to a scope the first thing I would be doing is hooking another amp either to the pre-outs (if it has them. I don't think it does) or the tape out connection and seeing if the noise is in the preamp or power amp section and start narrowing it down from there.

This amplifier is from the peak of the "Low TIM distortion" wars. Hence the transistors all have fairly high fT and the open loop bandwidth is going to be pretty high. Because the output is not properly following the input when these noises occur there's a risk it may also be oscillating at high frequencies (above the audible range). I would be careful because it could lead to tweeter damage (you won't hear a thing, it'll just cook them). Scoping the output when the noise is occurring would be a good idea. Just to check there's no ultrasonic content on the speaker terminals.
 
Prime Mover said:
I wouldn't be rushing off and replacing them on a speculative basis (what's the plan if that doesn't work?). If you can get access to a scope I would be verifying that there is something happening on the supply rails when the noise occurs to indicate the supply caps are causing a problem. From my experience with noises like this it tends to be a leaky transistor between the input differential and pre-drive in the power amp. That said it could also be many other things. Does the behaviour seem to be temperature dependant? I.e. Worse when cold, worse when warm or no difference?

Does the volume of the noise change with the setting of the volume control? Without access to a scope the first thing I would be doing is hooking another amp either to the pre-outs (if it has them. I don't think it does) or the tape out connection and seeing if the noise is in the preamp or power amp section and start narrowing it down from there.

This amplifier is from the peak of the "Low TIM distortion" wars. Hence the transistors all have fairly high fT and the open loop bandwidth is going to be pretty high. Because the output is not properly following the input when these noises occur there's a risk it may also be oscillating at high frequencies (above the audible range). I would be careful because it could lead to tweeter damage (you won't hear a thing, it'll just cook them). Scoping the output when the noise is occurring would be a good idea. Just to check there's no ultrasonic content on the speaker terminals.
Click to expand...

Hi Prime, and thanks for responding.

Here's a few answers. Maybe this will help.

The noise starts within a few seconds after the protection relay clicks. Temperature has no effect.
Nothing in the pre amp section has an effect on the noise, volume setting, selector switches which led us to believe it something after this stage. Only turning off speaker A or B stops it.

We put my multimeter at the speaker terminals, the right side was steady, the left side was jumping all over the place while the static was happening.

I can try another amp to see if anything changes.

I don't have access to scope. Maybe an AKer near me does?

Thanks for your help!
Glenn
 
No problem. From what you've tested it sounds like it's definitely in the power amp section. I've had a quick glance at a schematic and if it was the supply caps it should affect both L and R channels equally as they are common to both. Based on that I suspect it's something else.

If you don't have much access to test equipment but are okay with a soldering iron you could try swapping components (particularly transistors then maybe caps) from one side to the other, one at a time, to see which is the culprit. There are a lot of transistors in that design though so that could take a while. Some might be tempted to do a shotgun replacement of components in that section but it might be difficult to find acceptable equivalents for some of the parts from that era.

The service manual does describe a procedure for shortening the negative feedback loop. That is just what you need to do to divide and conquer the problem in this case. If you do get access to a scope, that procedure would allow you to narrow the problem down further. It's a bit too long of a swim to drop mine over unfortunately. The manual also covers a good deal of what you should do in the diagnostic procedures section (way more than most service manuals) and lists what voltages you should check. If you are confident then you could run through those checks and list anything here that shows up as out of spec.

Do be careful if you do start probing around. It's a shock hazard with the voltages that are exposed and it's also all too easy to slip with the probe and make matters far worse. If you feel it would be best tackled by a pro then don't hesitate to find one near you. Don't send yourself or the amp to heaven.
 
Prime Mover said:
No problem. From what you've tested it sounds like it's definitely in the power amp section. I've had a quick glance at a schematic and if it was the supply caps it should affect both L and R channels equally as they are common to both. Based on that I suspect it's something else.

If you don't have much access to test equipment but are okay with a soldering iron you could try swapping components (particularly transistors then maybe caps) from one side to the other, one at a time, to see which is the culprit. There are a lot of transistors in that design though so that could take a while. Some might be tempted to do a shotgun replacement of components in that section but it might be difficult to find acceptable equivalents for some of the parts from that era.

The service manual does describe a procedure for shortening the negative feedback loop. That is just what you need to do to divide and conquer the problem in this case. If you do get access to a scope, that procedure would allow you to narrow the problem down further. It's a bit too long of a swim to drop mine over unfortunately. The manual also covers a good deal of what you should do in the diagnostic procedures section (way more than most service manuals) and lists what voltages you should check. If you are confident then you could run through those checks and list anything here that shows up as out of spec.

Do be careful if you do start probing around. It's a shock hazard with the voltages that are exposed and it's also all too easy to slip with the probe and make matters far worse. If you feel it would be best tackled by a pro then don't hesitate to find one near you. Don't send yourself or the amp to heaven.
Click to expand...

Thanks PM,

I can look around and see if I spot something obvious, but my electronic knowledge is limited to tackle something like this. I had a good tech that fixed my M-7050 amplifier, but he recently moved. :-(

I'll have to track down another tech within driving distance so I don't have to ship this thing if possible. I really appreciate your input on this.

Cheers, Glenn
 
Prime Mover said:
I wouldn't be rushing off and replacing them on a speculative basis (what's the plan if that doesn't work?). If you can get access to a scope I would be verifying that there is something happening on the supply rails when the noise occurs to indicate the supply caps are causing a problem. From my experience with noises like this it tends to be a leaky transistor between the input differential and pre-drive in the power amp. That said it could also be many other things. Does the behaviour seem to be temperature dependant? I.e. Worse when cold, worse when warm or no difference?

Does the volume of the noise change with the setting of the volume control? Without access to a scope the first thing I would be doing is hooking another amp either to the pre-outs (if it has them. I don't think it does) or the tape out connection and seeing if the noise is in the preamp or power amp section and start narrowing it down from there.

This amplifier is from the peak of the "Low TIM distortion" wars. Hence the transistors all have fairly high fT and the open loop bandwidth is going to be pretty high. Because the output is not properly following the input when these noises occur there's a risk it may also be oscillating at high frequencies (above the audible range). I would be careful because it could lead to tweeter damage (you won't hear a thing, it'll just cook them). Scoping the output when the noise is occurring would be a good idea. Just to check there's no ultrasonic content on the speaker terminals.
Click to expand...

Hey PM,

Since you downloaded the SM, could you point me to the page that has the PC board diagram that has the input differential transistors you mentioned and the pre drive? Schematics are still somewhat Greek to me, but I'm a visual guy and knowing what board to look for and the transistors in question would be of tremendous help. I could then probably go to the schematic and find the area on it as well.

Cheers, Glenn
 
The schematic is pretty awful. The input differential pairs are X301 and X302. It looks like X301 is for the left and X302 for the right. They are shown on the left side of the schematic on page 26.

They are dual FETs which don't have a good reputation. It might be worth a shot swapping them from one channel to the other to see if the problem follows. Be careful as they could easily be damaged by excessive heat when soldering. If it's not the problem it will be hard to dig much further as the Super A bias arrangement make the rest of the amplifier a fairly complicated beast after that and it really could be anything between the input and output.

If its not X301 I would look at X505, X507, X509 and X511 but that's based more on a hunch which is all I can do without the amp in front of me.
 
Some pics of the potential problem

Prime Mover said:
The schematic is pretty awful. The input differential pairs are X301 and X302. It looks like X301 is for the left and X302 for the right. They are shown on the left side of the schematic on page 26.

They are dual FETs which don't have a good reputation. It might be worth a shot swapping them from one channel to the other to see if the problem follows. Be careful as they could easily be damaged by excessive heat when soldering. If it's not the problem it will be hard to dig much further as the Super A bias arrangement make the rest of the amplifier a fairly complicated beast after that and it really could be anything between the input and output.

If its not X301 I would look at X505, X507, X509 and X511 but that's based more on a hunch which is all I can do without the amp in front of me.
Click to expand...

Thanks PM!
Very helpful! I'll take a look.
I also discovered this!
Looks like these two caps leaked, or just en excessive amount of glue? Looks like there's a bit of corrosion on the transistor right next to the heat sink. Think this may be the reason? TXX-183B Driver Amp & Tone Control PC board.

Thanks, Glenn



 
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