Left channel hum - rack + Speedball

[jj4001]

After instal of the Speedball, I now have a low volume hum (110 -120 hz approx). Mostly in left channel. It is there regardless of volume position, inputs connected or not, tried different location, etc.

It is pretty low level but noticable between tracks and very quiet music passages.

I've done the chopstick test, reflowed a few joints, and tightened all screws/bolts. Any ideas for next things to check?  Odd to me that the volume control has no impact at all.

Thank you!!

 

[Paul Birkeland]

What are the DC voltages on each terminal of each Speedball board?

 

[jj4001]

Well I did something wrong for sure. When I went to test the voltages (black lead on 12U) I got a loud spark when I went to measure at 1A.

The fuse is not blown, but it won't power up now so I could have caused a big problem.

 

[Paul Birkeland]

If you got a big spark, perhaps your meter wasn't set to read DC voltages?

If the amp won't power up, the fuse likely is blown and you'd want to pull it out and test it for continuity with your meter.

 

[jj4001]

Ok - the fuse was good and amp powers up fine..  Whew...  Here are the voltages

OA 77
IA 180
BAB 0
IB 180
OB 72

OA 106
OB 106
G 0.1
B+ 180

 

[Paul Birkeland]

Those are working voltages.

 

[jj4001]

Good to hear. Any thoughts on what might be causing the low volume hum I described?

 

[Paul Birkeland]

Do you get this noise with nothing plugged into the RCA jacks and the volume pot turned all the way down?

Is there a wifi router sitting next to the amp by chance?

 

[jj4001]

Position of the volume control has no impact on the Hum. Rca's connected or not also has no impact. I've also moved it into a different room and still the same.

 

[Paul Birkeland]

Can you post some build photos?

 

[jj4001]

https://photos.app.goo.gl/JRRznj9BZFjryLPz8

Here's a link to 5 build
pics in my Google photos

 

[Paul Birkeland]

I would give this a try:
https://forum.bottlehead.com/index.php?topic=11676.0

 

[jj4001]

Thanks, I will give this a try. Parts have been ordered.

 

[jj4001]

I inserted the 2 opposing diodes today and removed the bare wire per the instructions. The left channel low hum is still there. No change in the level at all.

 

[Paul Birkeland]

The next step in isolating is to pop off the front C4S board, then reinstall it 180 degrees from how it was installed previously.  You won't be able to screw it down in this orientation, but you should be able it leave it flipped up and get the wires reattached.  Once you've done this, observe if the noise has switched sides or stayed put.

 

[jj4001]

Ok. So that's the small board between the volume Pot and the output jack, is that correct?

And if I understand you correctly, the white wires at OA and OB would reverse places. Red wires at 1A and 1B would reverse places. And black wire would stay in BAB.  Do I have that right?

 

[Paul Birkeland]

Yes, that's all correct.

 

[jj4001]

Thank you for your help.

I will try this today or tomorrow and report back.

 

[jj4001]

I flipped the board, rewired, and the hum has stayed in the left channel.

 

[Paul Birkeland]

Now try doing the same thing with the big board (which is a bit easier.