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

[antcollinet]

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.

Does it matter which source is selected as the input for the amp?

For example, if you only connect input 1 to a source, but then set the amp to input 2 (which has nothing connected) do you still get hum? If you do, what happens if you then short the input to input 2?

Are the inputs balanced or not?

Does the hum vary with the volume control?

Is there anywhere you could beg or borrow an isolation transformer from to try between the mains and the problematic amp?

 

[suttondesign]

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 had an ATI Amp some years back whose magnetic field caused buzzing in a nearby analog signal processor which fed the speakers. I had to physically move the ASP (and later some other things, too) away from the ATI Amp. That solved the problem.

 

[Agvo]

Does it matter which source is selected as the input for the amp?

For example, if you only connect input 1 to a source, but then set the amp to input 2 (which has nothing connected) do you still get hum? If you do, what happens if you then short the input to input 2?

Are the inputs balanced or not?

Does the hum vary with the volume control?

Is there anywhere you could beg or borrow an isolation transformer from to try between the mains and the problematic amp?

Hi Antcollinet—

The inputs are unbalanced (standard RCA), the hum doesn't vary with the volume control and it exists regardless of which input is selected when a 3-prong device is connected to any input (even if it's the Record Out from the amp going to an input on the outboard 3-prong device).

I'm not sure whether I can get my hands on an isolation transformer, but the key thing here is when I put either of my other 2 same-model amps into this setup, I get no hum/buzz. So something's going on internally with this particular amp.

 

[antcollinet]

So something's going on internally with this particular amp.

Based on what you've said, I agree. I suspect some common mode noise coming in from the mains (hence the suggestion of trying an isolation transformer) that when another grounded device is connected causes the common mode currents to flow to earth. I'm guessing due to a problem in the amp (eg dried out capacitor) that current is passing through a part of the amp it wouldn't normally, and causing the hum.

 

[mcdn]

Not that it helps with your problem, but I am fascinated by the question of why it’s so important to use the is specific amp model. To the extent you have 3 and have waited years on repairs that didn’t work? Is it for a museum or replica of something?

 

[AnalogSteph]

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.

A bad internal ground connection still remains the most likely culprit - or, in fact, one too many, i.e. an internal ground loop that may have been inadvertently created at some point. (Imagine there was a a bad ground, somebody bodged a fix, and then it was fixed properly without removing the bodge.) Going over both a good unit and the bad one in comparison with a fine tooth comb may be required to find the issue. Also consult the wiring diagram.

 

[Speedskater]

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?

The AC power supply is double insulated from the metal chassis.

So scrape the paint under a screw off, and run a wire to the metal chassis of another component (with a metal chassis) that has a 3 wire cord.