2325 hum in both channels

[runningdog]

I have a 2325 with a hum in both channels. It starts 5-10 seconds after the relay engages, unless the receiver is already warm, then it starts sooner. The hum is there when the Pre Out jacks are connected to another amp. The hum goes away when I touch the receiver.

The receiver is newly recapped and working well except for the hum. I've checked all the grounding points where I've caught hums before with no luck this time.

Is it more likely to be a grounding issue or a power supply issue? Or? No transistors or diodes have been replaced on the power supply board.

Thanks in advance for suggestions.

 

[catrafter]

So, if I am understanding correctly, the hum goes away when you touch the receiver case? Does flipping the AC plug over do anything?
Does it hum with nothing connected but the speakers?

Tom

 

[runningdog]

The hum goes away when I touch the receiver case. Flipping the AC plug has no affect. It hums with nothing connected but speakers. But if it has not started humming yet and a CD player is plugged into Aux or Tape 1 it immediately starts humming. At first it seemed like something had to be connected to Aux for it to hum.

Thanks Tom!

 

[rBuckner]

Well ain't this strange! How about with no speakers but with headphones? I'm not even sure what this may tell us right now. It would be great if you had another 2325 to open up and compare the insides. I'd suspect someone did a mod to try to fix some other issue and left it there. Is it an international unit? Did you try using an external ground wire like a scope probe ground to the bare chassis? This is with the outer covers in place? Hmmm.....

 

[runningdog]

I'd suspect someone did a mod to try to fix some other issue and left it there.

Heh. I'd like to blame it on someone else. I believe the unit was untouched.

It would be great if you had another 2325 to open up and compare the insides.

I do have a working 2325. I plan to compare the two power supplies today with a tracker.

Is it an international unit?

No.

This is with the outer covers in place?

Top and bottom metal covers off.

Did you try using an external ground wire like a scope probe ground to the bare chassis?

I just tried this. Grounding to the scope kills the hum.

How about with no speakers but with headphones?

Same result with headphones.

 

[rBuckner]

There's usually some hum with the bottom cover off but that's not what you are hearing with that delay. Has it been recapped? If not I'd do that power supply before poking around too long. Keep us posted, these weird ones are interesting!

 

[runningdog]

The entire unit has been recapped by me. But the power supply has all its original transistors and diodes. I don't like to replace if I don't know they are bad but if there are any that are no brainers to replace I'll try that.

 

[runningdog]

Some clarification: It hums with nothing but speakers attached, and touching it gets rid of some or all of the hum.

It hums much louder when a CD player is plugged into Aux or Tape. Touching it reduces the hum but does not get rid of it.

And finally, I can make it hum without connecting any input. If I touch the unplugged RCA cables from the CD player, the 2325 hums like they were plugged into the Aux jack. The CD player is plugged into the same wall circuit.

I compared the power supply components with a working 2325 using a tracker and found no anomalies.

This sounds to me more like a bad ground than a power supply issue?

 

[petehall347]

try shorting plugs into an input and see if hum goes away ..also try putting the covers back on

 

[ConradH]

Take a DVM and measure the resistance between the chassis (different parts) and the RCA connectors and the speaker ground terminals. Measure to various circuit grounds as well. Something has a high resistance ground path or no ground. At least that's my guess.

 

[runningdog]

All good suggestions, thank you. I tried them all and Mr Buckner and PeteHall347 called it...put the bottom cover on with all the screws and the hum is completely gone.

It was discovering the hum went away if I tilted the entire chassis back or it it was on its side that convinced me. Since the weight distribution mattered that said that not all the chassis sections were completely grounded to each other when it was siting horizontal. And the hum was changing each time I tried it because every time the receiver was shifted the hum changed too.

At that point the benefit of the bottom cover tying the chassis sections together became clear. Instant fix. Should've done it the first time I was told but it was so much fun working through the other suggestions first...

 

[rBuckner]

Cool! I always check all hardware when I'm done with a unit, I've found loose screws here and there. It seems the boards also compress a little or just dry out over the years and shrink and so the hold down screws can need snugging up.

 

[KutzlerTrans]

If i would check hardware, i would have found that missing esotec knob that vanished. I think its inside the unit.
It was not packed right and the plastic dial cover got smashed in, so while taking the front off, to repair it, the knob disappeared, only to be found the next time i take the top off. Good thing its plastic.