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FG 502 Adjustment and Possible New Issue


Stephen
 

I’m trying to adjust my FG 502 to factory specs as per the manual.
So far there are a couple of things that have popped up as being of concern to me:

1- R170 (Sym) has absolutely no visible effect whatsoever.
2- C287 (Triangle Peak) has absolutely no visible effect either.
3- There is a -25mV DC bleeding when no DC offset is engaged.
4- When the switch is set to triangle, settings 10(-6) and .1 on the dial, this is what I see on a 466:

https://groups.io/g/TekScopes/album?id=251651


Roger Evans
 

According to the manual R170 compensates for small offsets in the op-amps that control the positive and negative constant current sources that control the timing. The offsets are most noticeable for small voltages into the op-amps (ie the 0.1 end of the frequency dial. So R170 should compensate for item 4. in your list but have very little effect when the frequency dial is set to the upper half of its range.

My FG502 also has a small DC offset when the offset control is switched out, looks like R250 should adjust that, I will try it later on.

C287 (0.25 - 1.5pF) is only going to affect the very highest frequency components, ie the shape of the tip of the triangle at the switching point. Make sure the cable to the scope is terminated in 50ohm for best high frequency response.

Roger


Stephen
 

Rectifications:

1-R170 has much effect. I wasn’t looking for the right thing.
2- C287 has still no visible effect.
3- -25db bleed is still present
4- The picture above is because I messed with R170 without understanding they meant horizontal “Symmetry”. Much better now.

Addendum:

5- More often than not, for a brief moment, the trace gets taller as if more voltage was input...
That happens pretty often, (too often if you ask me), and the signal doesn’t seem to be extremely stable. Flickering vertically just a tiny bit too. It’s hard to explain, but my overall impression is that it’s not very healthy.

On a side note, all pots were cleaned several times over, both with IPA and a good, non lubricating, contact cleaner that doesn’t leave any residual stuff anywhere.


(OFF TOPIC) PS: I bought this contact cleaner once at an all-purpose hardware store to test It, because it was dirt cheap.
I never went back to the much more expensive stuff. It’s from the U.K., so some of you may have seen it or even used it. Here it’s part of the “3-EN-UN” TECHNIQUE (“3-IN-ONE“ TECHNIC), line of products.


Stephen
 

On Tue, Aug 4, 2020 at 10:14 PM, Roger Evans wrote:


According to the manual R170 compensates for small offsets in the op-amps that
control the positive and negative constant current sources that control the
timing. The offsets are most noticeable for small voltages into the op-amps
(ie the 0.1 end of the frequency dial. So R170 should compensate for item 4.
in your list but have very little effect when the frequency dial is set to the
upper half of its range.

My FG502 also has a small DC offset when the offset control is switched out,
looks like R250 should adjust that, I will try it later on.

C287 (0.25 - 1.5pF) is only going to affect the very highest frequency
components, ie the shape of the tip of the triangle at the switching point.
Make sure the cable to the scope is terminated in 50ohm for best high
frequency response.

Roger

I actually figured out (I think) what R170 was doing. It adjustS the horizontal symmetry of a square wave signal. You adjust for the same length (pos and neg sides). And that also affects the triangle shapes as well... Hence my weird looking triangles when I messed with it.


Stephen
 

On Tue, Aug 4, 2020 at 10:14 PM, Roger Evans wrote:

Interleaved answers.
My FG502 also has a small DC offset when the offset control is switched out,
looks like R250 should adjust that, I will try it later on.
I will definitely check that out. Thank you Roger!


C287 (0.25 - 1.5pF) is only going to affect the very highest frequency
components, ie the shape of the tip of the triangle at the switching point.
Hummm, interesting... Will check that out too. The interaction is not very obvious with you touch it

Make sure the cable to the scope is terminated in 50ohm for best high
frequency response.
It always is (Albeit with my homemade through terminators).

Thanks again, Roger.


Roger Evans
 

R170 actually controls the slope of one half of the triangle wave by adjusting the strength of one half of the constant current source into the timing capacitor. You want to make the positive and negative currents equal so that the positive and negative slopes are equal and the positive peak is half way between the two negative peaks. The square wave switches when the triangle generator switches between the charge / discharge cycles so that makes the square wave 1:1 as well. It is easier to adjust the way you did it!

Roger


Stephen
 

On Tue, Aug 4, 2020 at 11:04 PM, Roger Evans wrote:


R170 actually controls the slope of one half of the triangle wave by adjusting
the strength of one half of the constant current source into the timing
capacitor. You want to make the positive and negative currents equal so that
the positive and negative slopes are equal and the positive peak is half way
between the two negative peaks. The square wave switches when the triangle
generator switches between the charge / discharge cycles so that makes the
square wave 1:1 as well. It is easier to adjust the way you did it!

Roger
What I actually did, is set the switch to 10(-6), the frequency to .1, and triangle waves. Scope to 1us. That way I have quite a very clear view of the symmetry of the shape. And, as far as I can tell, it works. I find it much easier, and it allows for a more precise, and finer adjustment than what the manual says.


Roger Evans
 

Just had a quick read of the Theory of Operation and Calibrations setions of the manual.

R250 is to set the DC level of the triangle wave before it goes into the triangle to sine converter. It needs to be set with a distortion meter, so don't touch it for now.

The DC level at the output is set with R485 (DC Bal).

Roger


Stephen
 

On Wed, Aug 5, 2020 at 04:02 AM, Roger Evans wrote:


Just had a quick read of the Theory of Operation and Calibrations setions of
the manual.

R250 is to set the DC level of the triangle wave before it goes into the
triangle to sine converter. It needs to be set with a distortion meter, so
don't touch it for now.

The DC level at the output is set with R485 (DC Bal).

Roger
You’re right. I don’t have a distortion analyser. Not a good idea to mess with that at the moment. It’s only 25mV. I can live with that until I get a descent analyser.