60 Hz Hum/Buzz Only When Inputs Connected - Yamaha A760

[Agvo]

Heeelp!

Don't ask me why—it's a very long story—but I need a working Yamaha A760 amplifier. So far I've bought 3, and spent a fortune on refurb jobs by local techs that work on vintage gear. One of these techs has now passed away, and I want to avoid bringing things back to the other two shops because they've kept machines for over a year before!

I currently have one that works perfectly except for one thing: there's a hum/buzz on the speakers whenever any line level RCA input/output is plugged in to the machine.

I'm in North America, and the fundamental frequency of the hum seems to be 60Hz, with higher frequency noise harmonics, so this seems to be an AC grounding issue between the amp and other components. The noise is relatively quiet and stays the same volume regardless of where the volume control's at, but it seems to cause slight distortion in the audio signal.

When everything is unplugged from the line level input board, the hum disappears. There's no hum if just the speakers, turntable RCAs (and turntable ground wire) are connected. There is also no hum if the device plugged into the line level RCA board is not running on AC, like if it's an iPod running on battery. The RCA cables I have are decently shielded, and there is no hum problem on my other A760 amplifiers when I plug the same devices/cables in. (Those amps just have other problems I can't fix.)

While working on a prior A760 of mine with no hum issue, one tech told me there was a grounding problem with the chassis and he soldered a wire between the ground rail of the RCA inputs, the input switches and the chassis to correct it. So I used a multimeter to test continuity and resistance between the RCA ground and the chassis on this current unit. I assumed there may be oxidation in the chassis or a failing solder joint somewhere on a circuit board but there's good continuity for all RCA plugs to the chassis.

So I put this out to the community...what else could be going on here? Is there a specific component that may have begun to fail, and how can I test for that myself with a multimeter?

A few components have already been replaced, but I don't think the power supply in it has been touched. I know these machines have a weird "X" power supply, so maybe something's gone bad in that.

I tried testing the grounding of the two pairs of filter capacitors but finding the negative pin became confusing as they seem to be wired in series on the board and I'm not sure about discharging them etc.

(It may sound like I know what I'm doing but I've just done a lot of googling. I'm new to using a multimeter and not great at soldering so very simple instructions without abbreviated techy words would be best!)

 

[ThatSoundsGood]

Are the components that you're plugging into the amplifier also plugged into the same AC outlet? Is everything plugged into the same AC outlet? Are they all plugged into the same power strip?

 

[Agvo]

Hi, yes, they're all plugged into the same AC outlet. More detailed info:

The amp's AC cable is non-polarized 2-prong, and flipping the plug in the socket causes no improvement to the hum/buzz.

The line level sources I'm connecting to the RCA inputs/outputs all have 3-prong AC cables.
The amp and these sources are connected to the same power bar (and therefore same AC wall socket & circuit). The wall socket isn't switched at the wall.
Key thing: if I swap in any other (same model) amplifier in the setup, connecting these same sources, there's no hum/buzz.

Connecting my 2-prong turntable with grounding wire (and/or speakers) to the amp causes no hum/buzz. It's just AC-powered devices into the line-level RCA board.
With the turntable RCAs, turntable ground wire and speakers disconnected, the hum/buzz is still audible on headphones (again, only when one of the AC-powered sources is connected to any of the line level inputs/outputs).

Thanks

 

[ThatSoundsGood]

Hi, yes, they're all plugged into the same AC outlet. More detailed info:

The amp's AC cable is non-polarized 2-prong, and flipping the plug in the socket causes no improvement to the hum/buzz.

The line level sources I'm connecting to the RCA inputs/outputs all have 3-prong AC cables.
The amp and these sources are connected to the same power bar (and therefore same AC wall socket & circuit). The wall socket isn't switched at the wall.
Key thing: if I swap in any other (same model) amplifier in the setup, connecting these same sources, there's no hum/buzz.

Connecting my 2-prong turntable with grounding wire (and/or speakers) to the amp causes no hum/buzz. It's just AC-powered devices into the line-level RCA board.
With the turntable RCAs, turntable ground wire and speakers disconnected, the hum/buzz is still audible on headphones (again, only when one of the AC-powered sources is connected to any of the line level inputs/outputs).

Thanks

Have you checked the internal connections of the RCA inputs on the receiver? Since the external components don't buzz with the other receivers I suspect the they aren't the problem. And since the Phono input doesn't buzz then I would assume that the problem lies somewhere between the RCA line input jacks and the amplifier section of the receiver.

 

[restorer-john]

Your A-760 is a two wore double insulated unit, correct?

I would say the chassis is possibly 'floating' at some arbitrary nuisance voltage, could be caused by the cap to chassis in the "X" power supply leaking and when you connect another item to the amp, these issues can become a whole lot worse.

You'd need to look at every chassis/pcb screw where an 'earth/gnd' is made to the PCB track and the casework. Sometimes Yamaha (and other companies) would design a PCB to chassis point and then discover it made things worse, so they'd put a fibre/insulating washer between the PCB track and the chassis to 'isolate' it. All it takes is a washer to go missing and you have another path to chassis ground that can cause a hum when say a loop is made from RCA outers etc.

 

[Agvo]

Thanks guys—I will check these things. I don't believe the A-760 uses double-insulated grounding, I think it's a floating chassis design...but if I understand right, it shouldn't 'float' at the wrong voltage...?

 

[BDWoody]

Is there a Coax cable TV connection coming into the house? I have found that system to consistently cause grounding issues, as often they are grounded separately outside from the main ground for the power. If you can figure out the specific cable that is introducing the buzz you will be making progress.

 

[AnalogSteph]

The A-760 is an IEC Class II (double-insulated) device, well nominally anyway, but it has rather substantial leakage currents from the mains filtering associated with the "dimmer" circuit for the regulated power supply.

If the unit does not hum on open inputs (which they normally have a habit of doing if not connected to anything but speakers), I would suspect that someone may have previously converted it to "dumb" IEC Class I. Should be easy enough to check - does it have a grounded power cord and does the ground connect to chassis?

The 220/240V models are almost trivially easy to convert to "IEC Class II with ground", using the mains PE only for bleeding off the excessive leakage currents while still leaving one Y capacitor - there is a handy wire bridge on the board that one can solder to. The same ease unfortunately does not apply to the 120V model, though I don't remember the specifics and would have to look at the service docs again.

---
EDIT: So here's the problem:
The 220/240V as well as 115 V (General) models have this kind of circuit:

with the junction of C504-505-506 easily accessible via a wire link:

(the vertical one next to D504)

The U/C models, however, sport this circuit:

with this associated board:

C504 and C505 go straight to a ground fill screwed to chassis.

However, I think this is the same exact board, just populated differently. You should be able to convert the mains filtering to General spec by doing the following (red: delete, green: add):

So procure 3x 0.0068µF = 6.8 nF Y1/Y2 (check pin spacing required and opt for a slightly different value if need be), remove old C505, replace C504 by one of your new caps, remove the two designated wired bridges and replace them by two more of your new caps, and recycle the wire bridge from the "C504 new" position to add a new vertical one next to D504 as shown, which can then be used to attach the PE from a 3-wire mains cable.
---

Something that I would want to check: Do you have any way of easily reversing the mains live and neutral? It shouldn't make a major difference to the hum. If it does, I would be very worried about a potential isolation problem inside the mains transformer. (If so, good luck having that one rewound.)

Given that you have multiple units, I would suggest comparing them, visually at first. You never know what people before you have been up to in there.

Oh, if there are any old RIFA X and Y class capacitors (metallized paper with clear plastic casings) left on the power supply primary side, those should be scheduled for replacement immediately. Those guys have a habit of letting the magic smoke out in a rather spectacular fashion after cracking and letting humidity in. C504/505 in the U/C version should be plain ol' high-voltage ceramics though.

 

[Agvo]

Is there a Coax cable TV connection coming into the house? I have found that system to consistently cause grounding issues, as often they are grounded separately outside from the main ground for the power. If you can figure out the specific cable that is introducing the buzz you will be making progress.

No coax connection, no. The amp is being used in a recording studio, not a home setup with TV.

 

[Agvo]

The A-760 is an IEC Class II (double-insulated) device, well nominally anyway, but it has rather substantial leakage currents from the mains filtering associated with the "dimmer" circuit for the regulated power supply.

If the unit does not hum on open inputs (which they normally have a habit of doing if not connected to anything but speakers), I would suspect that someone may have previously converted it to "dumb" IEC Class I. Should be easy enough to check - does it have a grounded power cord and does the ground connect to chassis?

The 220/240V models are almost trivially easy to convert to "IEC Class II with ground", using the mains PE only for bleeding off the excessive leakage currents while still leaving one Y capacitor - there is a handy wire bridge on the board that one can solder to. The same ease unfortunately does not apply to the 120V model, though I don't remember the specifics and would have to look at the service docs again.

---
EDIT: So here's the problem:
The 220/240V as well as 115 V (General) models have this kind of circuit:
View attachment 431997
with the junction of C504-505-506 easily accessible via a wire link:
View attachment 431998
(the vertical one next to D504)

The U/C models, however, sport this circuit:
View attachment 432001
with this associated board:View attachment 432002
C504 and C505 go straight to a ground fill screwed to chassis.

However, I think this is the same exact board, just populated differently. You should be able to convert the mains filtering to General spec by doing the following (red: delete, green: add):
View attachment 432009
So procure 3x 0.0068µF = 6.8 nF Y1/Y2 (check pin spacing), remove old C505, replace C504 by one of your new caps, remove the two designated wired bridges and replace them by two more of your new caps, and recycle the wire bridge from the "C504 new" position to add a new vertical one next to D504 as shown, which can then be used to attach the PE from a 3-wire mains cable.
---

Something that I would want to check: Do you have any way of easily reversing the mains live and neutral? It shouldn't make a major difference to the hum. If it does, I would be very worried about a potential isolation problem inside the mains transformer.

Given that you have multiple units, I would suggest comparing them, visually at first. You never know what people before you have been up to in there.

Oh, if there are any old RIFA X and Y class capacitors (metallized paper with clear plastic casings) left on the power supply primary side, those should be scheduled for replacement immediately. Those guys have a habit of letting the magic smoke out in a rather spectacular fashion after cracking and letting humidity in. C504/505 in the U/C version should be plain ol' high-voltage ceramics though.

Hey Steph, thanks—no difference in the hum when flipping the mains live and neutral (and the unit has not been modified with a 3-prong cord).

I was concerned about the exploding power supply capacitor in all of these and asked techs who worked on them to replace them, but they all simply didn't do it—we're in Canada, these are the 120V machines and they said it hadn't been an issue with the lower voltage version. In fact one tech, on one machine, glued the cover on the power supply down to dampen a mechanical resonance and it seems impossible to remove now! But yes, I really hope those capacitors hang in.

Very much appreciate the suggestion for the mod but I'm afraid it goes over my head—I'm not experienced enough to do this, or to properly understand the difference between the various grounding methods. May bring the idea to another tech, if I can find one, though! Thanks!

 

[AnalogSteph]

Hey Steph, thanks—no difference in the hum when flipping the mains live and neutral (and the unit has not been modified with a 3-prong cord).

Then the sensitivity of this particular unit to ground loops remains a bit of a head-scratcher. Hmm... (If we assume that the sources might be floating a fraction of mains due to a grounding problem in the electrical system, then it could be related to missing shielding.)

Very much appreciate the suggestion for the mod but I'm afraid it goes over my head—I'm not experienced enough to do this, or to properly understand the difference between the various grounding methods. May bring the idea to another tech, if I can find one, though! Thanks!

Sounds like the better idea then. (In the meantime, my post is not going anywhere.) Good luck.

 

[Agvo]

Thank you Steph!

 

[Speedskater]

Does the A-760 have an exposed metal chassis or is totally enclosed in plastic?

 

[antcollinet]

What are the sources? Can you describe the full setup? All inputs, all outputs. How they are connected, how they are grounded.

Can you try each connection one at a time to find out which connections introduce the hum?

 

[terryforsythe]

Is there a Coax cable TV connection coming into the house? I have found that system to consistently cause grounding issues, as often they are grounded separately outside from the main ground for the power. If you can figure out the specific cable that is introducing the buzz you will be making progress.

The OP started a thread for the same problem on the diyAudio website. My post there correlates with your's. We both have encountered the same problem.

FYI, I solved mine with a terminal strip that included surge suppression for the outlets and for the TV coax cable. I presume it also does something with the ground for the coax, perhaps ground isolation, but I don't know. A cheaper terminal strip I tried didn't solve the issue, though. It can be hit or miss I guess.

 

[Agvo]

Does the A-760 have an exposed metal chassis or is totally enclosed in plastic?

It's got exposed metal. In mean, the outside of the case seems to have a layer of black spray-paint, but nothing inside is coated in plastic if that's what you mean. Does this mean it's not double-insulated but floating chassis?

 

[Agvo]

What are the sources? Can you describe the full setup? All inputs, all outputs. How they are connected, how they are grounded.

Can you try each connection one at a time to find out which connections introduce the hum?

Hi, yes I've done this—any external line-level source that has a 3-prong AC cable, when connected to the amplifier's line-level inputs or outputs introduces the hum/buzz. Any line-level source that's got a 2-prong AC cable or running on DC (battery, like an iPod/iPhone) doesn't introduce the hum.

The amp is being used in a studio setup, so the 3-prong devices in question are I/O interfaces, samplers, things of that nature.

Because there is no hum problem when substituting other identical Yamaha A-760 amps into the exact same setup, I think what I'm looking for here is something wrong with the internal circuit boards/components of this particular unit.

 

[Agvo]

The OP started a thread for the same problem on the diyAudio website. My post there correlates with your's. We both have encountered the same problem.

FYI, I solved mine with a terminal strip that included surge suppression for the outlets and for the TV coax cable. I presume it also does something with the ground for the coax, perhaps ground isolation, but I don't know. A cheaper terminal strip I tried didn't solve the issue, though. It can be hit or miss I guess.

Hi, yes I posted there too...the amount of response has been amazing...I feel I'm getting closer to understanding this...thanks to all!

There's no TV coax connected to this particular setup however. It's being used in a recording studio rather than a home audio/video setup.

 

[antcollinet]

Does this mean it's not double-insulated but floating chassis?

They are both (basically) the same thing. To have a floating (ungrounded) chassis, you must also have double insulation. The lack of a ground pin on the mains connection tells us there is both.

 

[Agvo]

They are both (basically) the same thing. To have a floating (ungrounded) chassis, you must also have double insulation. The lack of a ground pin on the mains connection tells us there is both.

I see. Okay. I think part of what confuses me in these discussions is the language 'the chassis is floating' could mean it was meant to float by design (and the goal is to make sure it is), or there is a fault and it's floating when it shouldn't.