My Beepre has developed a hum [solved]

[pvannest]

After months of PERFECT operation, my Beepre has developed a hum on the left channel.    My first thought was perhaps a tube was going bad.    I swapped the tubes around and the hum did not follow.  I did notice though that if I nudged the EL84 for the left channel the hum would go away for a few seconds.  With that bit of information I proceeded to rewet the solder joints to the tube socket.  While I was at it I noticed that the solder joint to pin heating wires on PT-7A looked a little ratty so I did those as well.  After that I took some basic measurements and all appeared fine on both sides.  I put the preamp back into my system and turned it on.  Viola, the hum was gone.  I proceeded to listen to a few albums and after about 45 minutes I noticed the hum had come back.  That was yesterday.  Today when I got home from work, I rewetted anything that looked suspect on the left side.  At first it appeared that may have done the trick and after warm up there was no hum.  Then just like yesterday the hum came back but after maybe 10 minutes of play time.  I went in again tried to reheat anything that wasn't shiny.  Now the hum is actually a little worse.  It really sounds to me there may be a thermal issue or a part going bad.  Right now I don't even know where to begin.

What it is doing right now is as soon as you turn it on you get the low level hum on the left side.  One strange thing though is it is not really constant.  It will be there for maybe 20 to 30 seconds and then stop.  Then a few seconds later it picks right back up again..

Obviously some guidance is needed here.  I am already have withdrawal issues.

Thanks in advance.

 

[Paul Birkeland]

I'd try warming up the preamp and measuring the voltages.  I would wonder if the hybrid shunt regulator on one side simply isn't working.  This would show up as high voltage on the "IA" pad on that particular side, as well as a Kreg voltage that is either below 2.5V or strangely high.

Let us know what you find and we can help get you sorted out.

-PB

 

[pvannest]

Believe it or not, after I posted this message, I went back for another try at isolating the problem.  What I discovered last night makes sense to me a bit, but doesn't explain why only the left channel was affected.    I was getting ready to take the  measurements that you suggested but decided to check to make sure that none of my other components could be the culprit.    With that in mind, I turned everything off and disconnected the CD player leaving the Eros and Old cassette player in the system.  After turning the components back on the hum was still there.    I then disconnected the cassette player.  Still there was a hum.  All that was left connected was the Eros.  I then disconnected that and the hum disappeared....  After that I reconnected the CD player and cassette deck.  No Hum....  Finally I reconnected the EROS.    The hum magically came back.  Now with that information, I realized that there was more likely and issue with the Eros and not the Beepre.    On a hunch, I decided to swap to a different set of interconnects.  Low and behold, that made the difference.  I think I may have had a ground loop with one or both of the interconnects.    Since they are home made I am sure it was my own quality control.  I will rip them apart tonight and see if there are any issues.

What this doesn't really explain to me though is why the issue would come an go like it did.  I suppose there could be some environmental issues.  I live in southern California and it was in the 90's yesterday.    We don't run the air on at the house during the day so it was pretty toasty in the living room where my system is.

The upshot of this is I do believe I had a few minor issues with the Beepre's tube socket on the left side that had gone unnoticed.  That has now been repaired.

If I have any further issues or if the hum comes back for some reason, I will again request your thoughts.

 

[Paul Joppa]

Check that the Eros and BeePre are both on the same outlet, and if they are then keep their power cords close to each other for as long as possible.

Be aware that our preamps (except Quickie) switch ground as well as hot at the preamp input. If you left the selector on Phono, then naturally the CD and cassette would not make any ground noise, even if they did make noise when selected.

 

[pvannest]

Good advise.  I will make sure that they are on different outlets

 

[2wo]

I brought a set of my home made interconnects to a Bottleneck meet a year or so ago. When they were used to connect the amps on test, there was a horrible noise that was resolved by replacing them.

Returning home, careful test and inspection revealed no issues and they continue to work fine in my system.

Just saying, when looking for noise, suspect everything, even "known good"...John   

 

[Paul Joppa]

Good advise.  I will make sure that they are on different outlets

Uh-oh  -  perhaps I was not clear: You want everything to be on the SAME outlet. Most of us use a power strip to do this. That way all the safety grounds have as little voltage difference as possible.

All our line powered products have the signal ground (and chassis ground) connected to safety ground, and if there are differences in the safety ground voltage there will be currents in the ground (shield) of the interconnects, which will come out as hum. That's why you want everything to share the same safety ground as closely as possible.

Some sources have 2-wire power cord with no safety ground, and they can induce currents into the shields of interconnects. In those cases, there are some ways to minimize either the currents or the hum voltage due to the currents. As always, these tricks are solutions to some situations and irrelevant in others. Hum is a sneaky devil!

 

[pvannest]

Yes I did miss read your post.    I was really busy yesterday and need to slow down a little bit. 

Thank you 

 

[johnsonad]

I had something similar happen with my Eros and Otari tape deck.  When plugged into different outlets there was a horrible hum.  When plugged into the same outlet it disappeared....

 

[pvannest]

Well I had some time last night to sit down and do some critical listening.  Plugged both into the same outlet.  No hum, dead silent...

Put on a Sheffield Labs, Direct to Disc, Vivaldi from the 70's.    Got goose bumps......