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Old 1st August 2010, 07:31 PM   #1
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Default Troulbleshooting BOZ-J

Hi

I need help troubleshooting the simples circuit ever.

Very load hum when driving an amp and test speakers.

I used a meter and scope, but don't know how to use a scope properly.

9v DC across bias resistor 2.2k.

Readings (no signal):

22v DC pre-regualtors
20v DC post-regulators

+- 1mV DC at output. It varies, not steady. It goes negative and positive.

Right:

D- .04v DC
G- 0v DC
S- 10.9v DC

Left:

D- .03v DC
G- 0v DC
S- 12.43v DC

The scope showed this pattern with a 1k computer generated sine wave. Probably not the cleanest waveform.

When turning up the volume pot, the sine became more like a sine wave, but it had take the form of smaller sines composing the larger sine wave.
Like a large single carrying a smaller signal.
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Old 1st August 2010, 08:43 PM   #2
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Are we talking about the attached circuit?

Assuming we are, isn't your supply voltage too low then? What regulator are you using?
I'd guess that when the supply is 35V, the bias resistor should drop around 10V and that the drain should be maybe .5V over ground potential. As the gate is not biased it should sit at 0V, just like the AC-coupled output.

And concerning the scope: The tip of the probe is where you pick up the signal, the reference (ground) is usually the small alligator clip (or the shield of the BNC connector). But BEWARE, it's usually internally connected with earth-ground, so it is not floating (like your battery operated DMM)! If you f.e. would want to look at the signal across the bias resistor, and connect the tip and the clip over the resistor, you'd be grounding one side and most likely release some magic smoke. A scope is best used to read signal in reference to the circuit's ground.

For troubleshooting you usually inject a sinewave (1k, 1Vpp or so) in the circuit and look for the part of the circuit where it gets screwed up. The fault will often be close to this point. Square-waves are also highly useful.

And post some pics please!
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Old 1st August 2010, 10:17 PM   #3
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Hi Rodeoodave,

I'm using 19v, because I read 16v was suppose to be optimum, but 16v wasn't working. I'm starting in to think I made it up somehow.

Some of the stuff you said about using a scope went over my head in first reading.

Might just bump it up to 35v.
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Old 1st August 2010, 11:13 PM   #4
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Quote:
Originally Posted by vdi_nenna View Post
...
Some of the stuff you said about using a scope went over my head in first reading.
...
Maybe it was too fast, and certainly confusing.

Okay, what a scope does: It shows a time-resolved signal. A DMM doesn't do this, it it shows an averaged signal (DC or RMS-AC).

A DMM is usually battery operated, so the contacts of the probes are floating. This means that there is no fixed or defined potential between the probes and mother earth, they're just floating in mid-air. When you measure a voltage drop over a resistor, the measured potentials ideally don't get disturbed because they don't get pulled somewhere.

With a scope it's usually different. It's mains-powered, and older types have CRT displays and thus employ high voltages. For shielding and protection these scopes usually have a metal case which is connected to earth.

I like to distinguish between ground and earth-ground. Ground can be floating (=not having defined potential ), earth-ground is connected to mother earth (literally). A battery f.e. is floating: Between the poles there's say 1.5V, but no fixed potential to anywhere else. We usually call the minus-pole ground, and the plus-pole hot.
The same goes for a Galvanically isolated secondary of a transformer: The primary is usually connected to Live and Neutral. Live is referenced 110V or 240V (RMS) above/below Neutral, 60 or 50 times per second. Neutral is basically Earth, Earth itself is connected to the soil via a conductor. That's why you can touch N or E, but usually get zapped when you touch L while not being perfectly isolated from earth. Current will flow through you to mother earth (or the other way, depending on polarity). Once there is a Galvanic isolation, the secondary is floating again (assuming there's nothing connected to the secondary).

Now the important part: The DMM usually has two probes. When measuring voltages you can put the probes anywhere, you won't produce a short. Think of it as a 10M Ohm resistor you put over stuff (in parallel). You can put it anywhere, no excessive current will flow through it. (When measuring currents it's like putting a wire over stuff, that's why you should measure current in series with stuff).

Now it comes: A scope is usually different. It's like the DMM with two probes, but one is connected to earth! Try it, take your DMM in continuity tester mode and measure from the scope's BNC-connector to the earth-pin of it's power plug. It will likely go beep.
The way to NOT use a scope is to measure over components as you would do with a floating DMM. You will short out things and they will blow up.
The way to use a scope is to connect the ground of the scope with the circuit's ground, and then measure voltages with the tip (assuming you use a scope probe).
Your probe will look like to one attached. The alligator-clip is usually earth-ground, as is the shiny metal part and the outer part of the BNC-connector. With the shiny metal part you can easily short out stuff, so put heatshrink tubing over it. The tip is used to prod around.


When using a scope you should be aware of two things:

You earth-ground your circuit, potentially causing ground-loops resulting in hum.

When prodding around in live equipment, earth-connections are as dangerous as the present voltages. Why? Because they deliver a low-resistance path for currents. Touching a live wire while your other hand, elbow or pinky finger touches earth may result in a nasty current from one arm to the other.
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Last edited by Rodeodave; 1st August 2010 at 11:22 PM. Reason: Forgot attachment
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Old 2nd August 2010, 10:11 AM   #5
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Quote:
Originally Posted by vdi_nenna View Post
Right:

D- .04v DC
G- 0v DC
S- 10.9v DC

Left:

D- .03v DC
G- 0v DC
S- 12.43v DC
Should be:
D ~ 10V
S ~ 0.5V

Have you turned those lovely SK170's the wrong way?
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Old 2nd August 2010, 01:49 PM   #6
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Rodeodve, thanks for taking the time to explain. I will sit down and try to make sense of your explainations.

I should have mentioned that I did not have the proper scope test leads. I had a couple of BNC to female RCA adaptors. I plugged in the output of the preamp to the female RCAs/BNCs of the scope. Sounds like maybe I should not have done it this way.

Quote:
The way to NOT use a scope is to measure over components as you would do with a floating DMM. You will short out things and they will blow up
Even in my elementery thinking, I decided to use the scope for viewing the signal waveform only. I used the DMM to read voltage across components.

Quote:
Have you turned those lovely SK170's the wrong way?
Yggdrasil,

I don't think I have them reversed. I checked and re-checked this against the 2sk170's spec sheet.

Facing the flat part of the jfet: D G S. Is this correct?

Thanks,

Vince
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Last edited by vdi_nenna; 2nd August 2010 at 01:59 PM.
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Old 2nd August 2010, 07:09 PM   #7
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Facing the flat part, from left to right, it's D G S, yes.
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Old 2nd August 2010, 08:29 PM   #8
Magura is offline Magura  Denmark
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Quote:
Originally Posted by vdi_nenna View Post
Hi


22v DC pre-regualtors
20v DC post-regulators
I take this is the PSU regulator you've measured.

A drop of 2V over the regulator is not enough. You have most likely 50 or 100Hz hum, and that's from the PSU.

Try adjusting the regulator down to supply like 17V, and see how things change.


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Old 2nd August 2010, 08:38 PM   #9
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That won't be a problem because I have the adjust resistor set on test leads.
Got tired of soldering and de-soldering.

Will know later tonight.

thanks!
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Old 4th August 2010, 01:49 AM   #10
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OK,

I am running the JFETBOZ with 14.05V rails - matched 2SK170BL's

All other resistor values exactly as Nelson posted on the 1st page of the JFETBOZ thread.

#1
D=2.02
G=0
S=57.4mV

#2
D=2.00
G=0
S=55.0mV

The psu is a RCRC with 0.47 3W power resistors and 2200 uF audio grade electrolytics
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