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Trying to find the source of a low hum

Sergimm

Sergimm

Active Member
I am experiencing a 60hz low hum in a Marantz 1050 amp I recently recapped. It sounds very much like a ground loop, but I’m not sure anymore. This is how it behaves:
  • Unaffected by volume level
  • Present in both channels, although it hums louder on the right channel
  • Unaffected by tone controls, although lower bass control makes it slightly louder
  • Unaffected by input selection
  • Noticeable both in headphones and speakers
  • Him present regardless of whether there are inputs connected
  • The hum appears a second after turning on the amp.
Here’s what I’ve tried to get rid of it so far:
  • Tried different plugs in the house
  • Turned plug upside down
  • Used a “cheater” plug
  • Plugged to a power strip
  • No other appliances are connected along with it
  • No dimmers, fluorescent lights
  • Did a first pass on cold solders, Redid a couple of suspicious solder points but haven’t found anything yet.

I assume the next step is to revise circuit components. What’s the best way to tackle this, or to narrow it down?

I’m a bit discouraged at the moment; having discarded the most basic mistakes, finding the source of hum sounds like a daunting task.
 
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So, I've been replacing resistors that were slightly out of spec with metal film 1%. It hasn't helped, the hum is still there. It seems that the next thing to check are transistors. I really didn't want it to get to that :)

This unit has mainly 2SC2390 and 2SA1039. In the 70s Service Manual they recommend to eventually substitute them with 2SC1400 and 2SA750, both of which are in the black list of transistors that cause trouble. My plan is to replace originals (if necessary) with 2SA1039->KSA992 and 2SC2390->KSC1845, based on the responses in this forum about replacements for 2SC1400 and 2SA750.

Are those good replacements? I'm not good yet at reading transistor properties, and since replacements are based on the recommended substitutes for the original transistors, I feel like they could be too different.

2SA1039
2SC2390
 
I’ve just noticed that the bass fader actually influences the hum. With volume at 0, all the controls in neutral position, bringing the bass all the way down does lower the hum noticeably. I’ve cleaner the faders again, and went over the solder points in the tone control board.

Anything else I may be missing? Does the fact that the bass fader influences the hum mean that the problem arsonists that area, rather than the power supply one?
 
If this is happening post recapping, I recommend you do a careful check for any electrolytic capacitors inadvertently soldered in the reverse position.

Rgds
Mayank
 
Mayank said:
If this is happening post recapping, I recommend you do a careful check for any electrolytic capacitors inadvertently soldered in the reverse position.

Rgds
Mayank
Click to expand...
Thanks for the suggestion. I did a re-check and all capacitors seem to be correctly positioned. At this point I’m close to throw the towel, I feel like I don’t have enough knowledge to solve this one. Although I’m convinced it’s a ground loop of some sort, it could really be anything.

I haven’t checked transistors, but I don’t know how to start. Check them all one by one? I’m not sure the work is worth it.

It could also be the rectifier diodes...but I don’t know how to check those either to confirm that something is wrong.

Very tempted to put the amp back together and live with the hum. The sound is otherwise pretty good.
 
I'd make a mental list of things you did and you don't trust 100%, wires you had to bend to access to some board, corners hard to reach, some soldering under a hidden place, any loose wire that you resoldered, anything you remember it could be wrong. Usually the fail is in those things you don't trust and you are not telling.

If the amp was working ok before, obviously the hum is being introduced by some mistake or loose connection after the work done.
 
I'm picking this up again, and I'm determined to find the source of hum.

Facts so far:
- Hum is very low frequency, sounds like 50Hz
- It is independent of volume
- Happens in different plugs around the house
- Happens when nothing is connected to the amp
- Happens with or without the case on
- Replaced all electrolytics
- The amp was not really working before I repaired (the original cause was a bad resistor), so not sure if the hum was there then
- Replaced rectifier diodes
- DC bias is adjusted as per SM
- Unaffected by tone controls, although lower bass control makes it slightly louder
- Unaffected by input selection
- Noticeable both in headphones and speakers

I'd like to know what methodology people more experienced than me would follow to find the culprit. At this point I'm out of options, except for replacing every single component in the amp...which I really would like to avoid.

The only strange thing I can think of is the fact that increasing the bass slider makes the hum louder.

Thanks!
 
There's a complete amplifier stage between the volume control and the tone controls, I would be checking out that.

Craig
 
I had a hum in my mc,c26 and c28, I had the covers off monkeying around inside them.
Today I got back inside them and took a careful look around and put the covers back on securely.
Now the hum is gone in both, just like that.
I remember watching a video on a grat site that shows electronic repair of vintage gear.
He was working on a mcintosh, he said it was very important to make sure the cabinet screws were very secure and metal to metal contact for the complete grounding of the cabinet and frame.
I would mention his name but it might break a rule, but this guy is super smart and professional.
Could be that simple.
 
Have you checked all ground connections carefully? And have you cleaned all pots and switches thoroughly with a switch cleaner like Servisol?
 
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