Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

Return to Tube DIY Asylum


Message Sort: Post Order or Asylum Reverse Threaded

If the sine wave isn't pretty???

174.238.2.183

Posted on March 14, 2017 at 11:29:41
TomWh
Audiophile

Posts: 629
Location: Tucson Az
Joined: August 7, 2003



Well maybe Michael will kick in? Well we know one thing for sure pretty sign waves are not on that scope.

OK I got the new scope to check phase angles on some motor controllers I have been building. So I decided to go after some hum on the phono stage. So I hooked up the scope to the right channel output before the avc. Not sure where I should be testing and what that picture tells us. Also the fluke 87 give about 23mv RMS and the scope is 150 pp those numbers do not equal after doing the math.

I have done very little scope work in the past. So anyone who wants to give me a step by step lesson on where they would check and what it should or should not look like I would be great full. Hum is a tricky thing.

Thanks Tom

 

Hide full thread outline!
    ...
If I didn't know better... it kinda looks like V-tach, posted on March 14, 2017 at 12:09:31
FenderLover
Bored Member

Posts: 12533
Joined: July 31, 2007
Contributor
  Since:
May 17, 2009

A potentially life threatening cardiac rhythm.

 

RE: If the sine wave isn't pretty???, posted on March 14, 2017 at 12:47:12
gusser
Audiophile

Posts: 2402
Location: So. California
Joined: September 6, 2006
A meter will tend to average out all that noise. I would trust the scope over any meter.

You also have to be careful and not introduce a ground loop via your test equipment - especially on something like a phono stage with it's low signal levels and high gain.

For example is does your scope have a three wire power cord? The scope electrical ground can easily carry noise into the system under test and corrupt the measurement. This is one of those cases where it's often necessary to lift he scope safety ground just for a test. Ideally in that case the scope should be operated through a 120v/120v isolation transformer

 

RE: If the sine wave isn't pretty???, posted on March 14, 2017 at 17:37:19
Michael Samra
Dealer

Posts: 35847
Location: saginaw michigan
Joined: January 30, 2005
Contributor
  Since:
June 24, 2005
You remembered my old signature moniker eh? Anyway,your waveform simply looks like an audio signal whether it be music or voice going thru the preamp.You need to tell us more about your hum,like is it in both channels and when did it start.. Nice scope BTW.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: If the sine wave isn't pretty???, posted on March 14, 2017 at 18:16:20
TomWh
Audiophile

Posts: 629
Location: Tucson Az
Joined: August 7, 2003
Well it is from the tubes,transformers, interstages, ac lines etc. That is just the right channel on the phono amp turned on. It is a great sounding phono amp but it has a lot of iron. So the hum is there not enough to freak out but I thought I might use scope as a detective tool.

I have gone after hum before by all the normal means but I thought it would be fun to play with the scope to see if the finger gets pointed to certain area.

So if you have a certain procedure you would use I am all ears. The MV tells us we have unwanted ac but the picture is confusing at best. It is common to both channels.

Thanks Tom

 

RE: If the sine wave isn't pretty???, posted on March 14, 2017 at 19:02:45
Michael Samra
Dealer

Posts: 35847
Location: saginaw michigan
Joined: January 30, 2005
Contributor
  Since:
June 24, 2005
Tom
If the hum wasn't always there,and this unit has one power supply per channel,it is probably a filter cap.It can also be a bad ground on the input or chassis.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: If the sine wave isn't pretty???, posted on March 15, 2017 at 06:58:53
Triode_Kingdom
Audiophile

Posts: 5993
Location: Texas
Joined: September 24, 2006
Your scope is showing 120Hz hum plus a small amount of noise. Additional power supply filtering should reduce it.


--------------------------
Buy Chinese. Bury freedom.

 

RE: If the sine wave isn't pretty???, posted on March 15, 2017 at 08:54:23
Hornlover
Manufacturer

Posts: 2107
Joined: March 8, 2002
The math only works with a sine wave. This is not a sine wave, so both are most likely correct. The meter is giving you an RMS value of the somewhat random signal. The scope is giving you the p-p value of the signal.

 

RE: If the sine wave isn't pretty???, posted on March 15, 2017 at 09:35:44
TomWh
Audiophile

Posts: 629
Location: Tucson Az
Joined: August 7, 2003
Hey Triode

How did you see that? I am really trying to learn how to use this thing.

Thanks Tom

 

RE: If the sine wave isn't pretty???, posted on March 15, 2017 at 09:42:25
Michael Samra
Dealer

Posts: 35847
Location: saginaw michigan
Joined: January 30, 2005
Contributor
  Since:
June 24, 2005
Where are you seeing 120hz hum in that blur? I can't see the number data.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Do you have the ground of the -scope probe hooked up?, posted on March 15, 2017 at 09:44:45
Ralph
Manufacturer

Posts: 3293
Location: Minnesota
Joined: April 24, 2002
Contributor
  Since:
April 1, 2002
If not you'll get something like this.

 

RE: If the sine wave isn't pretty???, posted on March 15, 2017 at 11:02:16
Triode_Kingdom
Audiophile

Posts: 5993
Location: Texas
Joined: September 24, 2006
The scope is set to 10 mS per division. Each cycle of the waveform is slightly less than one division. That means the frequency is a little above 100 Hz, so I'm sure it's 120 Hz hum.





--------------------------
Buy Chinese. Bury freedom.

 

RE: If the sine wave isn't pretty???, posted on March 15, 2017 at 11:07:03
Steve O
Audiophile

Posts: 6762
Location: SE MI
Joined: September 6, 2001
The lower level underlying "sawtooth" has a period of ~ 8ms. f=1/p=1/.008=125...close enough to assumed mains f = 60Hz X2=120Hz.......I think?

 

Page processed in 0.035 seconds.