Stereomour II has developed a hum (intermittant) [resolved]

[lskiii]

Hello,

So, my Steremour II has been working great for a couple of months now.  I listen to it pretty much a couple of hours a day.  Yesterday when I turned it on, I noticed a deep hum coming from both speakers that I've never heard before so I turned it off right away.  When I turned it back on, the hum was gone.   Today I turned it on, and the hum was back, but this time, turning it off and then on again didn't fix things.  I've not made any changes recently, so here's what I've done to try and diagnose it:

Hum sound is deep / low - you can hear it even when music is playing.  Also, it does seem to sometimes get louder and starts to make a crackle noise if left on.

Hum doesn't change regardless of the volume knob. 
It is in both channels.
Hum doesn't change if there's an input or not.
I swapped out the preamp tubes - no change.  I don't have a spare set of 2A3s, so can't rule out those.
I moved the amp to a different room and plugged it into a secondary system I have - same hum.  Since this was a different AC power cord, different room different speakers, etc.  I think that isolates things to the amp.

I went ahead and re-tested all of the measurements from the manual.  Note - I have the DC upgrade and the shunt upgrade installed.

Everything tested fine except:

Term 4 and Terminal 17 showed 12.4 and 11.8 respectively, but should be zero.  If this because of the shunt upgrade?
DC upgrade tested fine. 
The shunt upgrade IA and IB tested 301 and 299 but should be 365-410.  OA and OB tested 214 and 209 and should be 300-305.

To deepen the mystery, I put everything back and was going to try to re-tune the hum pots, but guess what, the hum is gone! 

So - I'm concerned that the hum will come back and am unsure what to make of the low readings on the shunt upgrade.

Any suggestions?

Thank you!

[Paul Birkeland]

Term 4 and Terminal 17 showed 12.4 and 11.8 respectively, but should be zero.  If this because of the shunt upgrade?

Are you 100% sure that's 12.4 and 11.8V, or is that 12.4 and 11.8mV? or uV?  What units does your meter show?  12.4V is unacceptable, 12.4mV is believable, 12.4uV is excellent.

The shunt upgrade IA and IB tested 301 and 299 but should be 365-410.  OA and OB tested 214 and 209 and should be 300-305.

Can you post some photos of your build?  I wouldn't run the amp with 300V at IA/IB on the board you added unless by chance you have converted this to run 45s.  If you have 300V at IA and IB on the small C4S board, that would be OK.   Something is definitely not working properly here.  This is not a bad tube or anything like that, there's something touching that isn't supposed to or some kind of build issue.  Did the amp pass its voltage checks after the shunt reg driver update?

[lskiii]

Thanks for the quick reply!    I am having a hard time replying for some reason - perhaps the attachments, so this is my 3rd attempt...

To your questions:

1. Terms 4 and 17, the results are in mV.  I'm attaching a photo of my retesting it (17 came back at 14.2mV this time).

2. So I was testing the small board (oops) - I tested the big (correct) board and IA / OA / IB / OB all came back fine (391 / 299 / 392 / 301).

Attached are some photos.  The only changes I've made since the original build I did was rewire the OTs for 4 Ohms.  I did all of the tests once I did the installs of the upgrades (which I did on different occasions).

Hope that helps?  Seems like only T4 and T7 have issues?

Thanks!

[lskiii]

Additional photos

[Paul Birkeland]

0.014V is plenty close to 0V for these purposes.

Since you have this hum in both channels, that tends to rule a lot out.  Is this 60Hz or 120hz noise? (you may need to listen to a 60Hz and 120Hz tone on your phone to be sure)

I would suggest tightening all the hardware to start.

[lskiii]

Hi Paul,

Thank you for the guidance with issue...  I have an update and a question.

So, the hum definitely sounds like 60Hz.  In troubleshooting, I had already swapped the pre-amp tubes, but figured I would try it again.  And, lo and behold, the hum went away for a week.  Then, this weekend, I was adjusting the volume and the hum came back.  What I noticed, was if I really pushed on the volume knob or (to a lesser extent) the balance knob that I could increase or decrease the volume of the hum.  I then tried wiggling/applying pressure to the right side preamp tube and the hum changed volume and made some crackling noises.  Note - I had unplugged everything but power at this point, plus it was coming from both speakers.

So, I've poured over the wiring again, looking at the volume, balance knobs and also the preamp tubes and I just don't see a "smoking gun".  My hunch is that it's some connection near the pre-amp tubes and/or the volume/balance area.  No other part of the system was effected by bumping/tapping I tried.

Of course, I've put everything back and now it's working again just fine ;-).  I can wiggle the tubes, mess with the knobs, but no hum.

Do you have any tips for how I should track down what I have to assume is some sort of loose physical connection?

Thanks!
Louis

[Paul Birkeland]

You definitely have a loose connection.  I would reheat all the joints.  If you have a disposable pair of speakers, you can hook them up to the Stereomour and then poke around with a wooden chopstick to see if you can activate the noise by pushing on something specific, and that can narrow down your search.

[lskiii]

Thanks for the tip.  As an update (and in case this helps other folks) I was able to isolate the issue to somewhere near the pre-amp tube sockets.  What I believe happened was that when I installed the shunt regulator upgrade, it installs over that section of the board where the preamp tube sockets are.  During the install I must have pushed some of the wires down too close to the board, and/or too close to each other.  Before I went through re-doing solder joints, I just used tweezers to carefully move all of the wires as far away from each other and from the bottom of the board as possible.  Since then, been a few days and the problem hasn't come back.

Thanks again!
Louis