Help with Hum issues.

Find out if there are any fuses In the circuits inside  the amps or preamps, not in the main back.  If so you may have one out in the preamp. I got a hum in my right channel last week. I’m running BAT monos each amp has 4 fuses, one was out in one amp. Change fuse no more hum. 

Cold solder joints of Amplifier might cause a 50-60Hz hum.

If after all of the suggestions here are exhausted, with the amplifier unplugged and cooled down, carefully with a multi-meter check the resistance on the left side and right side channel inputs. Between signal and grounds, dry solder or cold solder joints can create hum issues. You could also Ohm it out, check resistance between the input contact and the solder joint on the next solder joint past where the connector is soldered to ensure continuity, and also Ohm to ground. I only suggest inputs because they are physically stressed by installing and removal of connectors.

This isn't fun, it takes a bit of time to pull it down and check it. I am only suggesting the input as it's easier than say a filter cap, or one along signal path. Do you know if it has a separate power supply for each channel in the amplifier?

This is all becoming too confusing and hard to follow.

Let me state (To simplify this hopefully) how this all started and where I am now:

I started out connecting an Onkyo large Amp and matching pre-amp.  Had a hum in the R side.  Grounded it every which way, sometimes it is better than others but the hum is never gone.

I noticed the Phono stage on the Pre-amp is often faint or fades away.

So I replaced the Onkyo pre with a Phase linear pre.  I still have the hum.

So I removed the Onkyo Amp and replaced it with a Mitsubishi Amp.  The hum is still there (NO matter what source or NO source).

This is also true with the Original set up (ALL Onkyo)

So just now, I did the following:

To complicate things further or maybe not:

I removed the Phase linear pre amp and connected the Onkyo pre amp instead.  Connected to the Mitsubishi amp.   I also connected a Tuner as a source.

The hum is gone but the Onkyo pre amp has issues with the phono stage and the L, R, Mono switch....as I mentioned above.  I have cleaned the switches with the spray but the issue is not resolved.

And when I had the Onkyo amp connected to it I had the same hum issue.

So what does this mean ??

I guess the Phase Linear pre amp is causing a Hum ?

And the Onkyo Amplifier is also causing a hum ?

And I need to fix the Onkyo Pre-amp as a Minimum and maybe the Onkyo Amp if the hum doesn't go away.

Ok, I hope things are clearer now.

So I tried these different setups:

1- All Onkyo (Amp, pre-amp).  Hum in R. side.

2- Onkyo Amp, Phase Linear pre-amp.  Hum in R. side.

3- Mitsubishi Amp.  Phase linear pre-amp.  Hum in R. side.

4- Mitsubishi Amp.  Onkyo pre-amp.  Hum is gone.  But there are the issues with the pre-amp that I stated. 

I eliminated the signal source (Tuner) since it is not the issue here.

All cables and speakers were changed so those are not the issue either.

What are the odds that the Onkyo amp and P. Linear pre-amp needs repair in addition to the repairs needed on the Onkyo pre-amp ??

And would any of this cause the Hum ?   I don't understand.

Thanks Tvad 🇦🇺

Hey Customerfirst, it would seem you have tested all the configurations. Your conclusion would seem unlikely, but possible. BOTH preamps with problems?

What models are the amp/preamps? How old are they?

I would also agree with rixthetrick that there may be a dry joint or component failing in each (you need to be unlucky).

If you were talking about some sort of room treatment of a dubious source, you might attract more notice and help.

If I can think of anything further I’ll pop my head in and see how you are
doing. Good luck from me. 🇦🇺