Yamaha A960 audible background noise

[spaghetti]

I am trying to fix this amplifier, I've already been through the most common issues such as the exploded X2 cap (it took me about 1 day to remove all the crap and the smell), the burnt lamps that turned the amp off when pressing the DIRECT switch and I duly checked/set the voltages and DC offset according to the service manual specifications.

So, it basically works now, the sound quality is very good, clean and detailed except for a background hiss that is ruining everything. The hiss goes almost completely away in direct mode; the high filter button makes it worse. As a side note the transformer makes a light buzz when the amp is on, the buzz resembles the background noise produced by the amp.

Is there anything obvious I should check/replace to get rid of the noise?

Thanks,
Andrea

 

[avionic]

The hiss goes almost completely away in direct mode; the high filter button makes it worse.

Both channels ?

 

[spaghetti]

Yes, both channels.

 

[gpounce32768]

Have you done any recapping? I just did an A550, I measured the caps as they came out- many had drifted though none were obviously bad. Before the recap the sound was OK but kind of lifeless, the recap perked up the unit considerably. Theres always the possibility of leaky transistors as well.

 

[spaghetti]

No recap yet. I just measured a few caps and they looked fine. Also, the noise doesn't change with volume.

 

[gpounce32768]

The 550 doesn't have a tone control bypass, OTOH it does have the tone controls after the volume, thus volume would not affect the loudness of a hiss originating in the tone circuit. If the 960 is similar then I'd go for recapping just the tone circuit- generally a few caps. I don't know what active devices the 960 uses there- but not likely to be many.

 

[spaghetti]

Today I replaced the caps on the tone boards, but there was no improvement.

 

[spaghetti]

Well, the amp now works fine. What happened? I really don't know.

As I wrote earlier, I did replace all the caps on both tone boards with no improvement, so I decided to put the amp aside for a while and close it.
I put back the control circuit boards (PSU boards) inside their lodge, closed the amp, and the noise is now gone, for good I hope.

What happened? My guess it that the problem was already cured after replacing the caps, but the control circuit boards were still delivering some interference/noise because they were not properly installed in their place. Does this makes any sense?