HP3325A Function Generator - Troubleshooting Modulation on Output

[TGroulx]

Edited content:

Yes, it is an HP3325A model.

I am getting modulated distortion on the output sparatically. I can not trace where it is coming from. I would like to fix it. If anyone has dealt with this error (which is not common and unheard of by most of my peers) please let me know what the error was so that I may fix it.

I have a feeling I am dealing with a short (or other inductance) from the amplifier into the frequency oscillator output. As it is amplitude modulation.

Thank you for any help.

Terry

 

[Jim Wynne]

Re: 3325A Troubleshooting Modulation on Output

Trying to find where this could be inserted if anyone has dealt with this.

Looks like AM

pretty funky, have never seen anything like it, it is not constant and in fact at the moment I can not duplicate it. Yesterday it was failing today it is fine

I have a feeling that most people here will have no idea what you're asking. Could you explain the issue in more detail?

 

[Al Rosen]

Re: 3325A Troubleshooting Modulation on Output

Is that an HP Function Generator?

 

[TGroulx]

Re: 3325A Troubleshooting Modulation on Output

Is that an HP Function Generator?

Yes, it is an HP3325A model.

I am getting modulated distortion on the output sparatically. I can not trace where it is coming from. I would like to fix it. If anyone has dealt with this error (which is not common and unheard of by most of my peers) please let me know what the error was so that I may fix it.

I have a feeling I am dealing with a short (or other inductance) from the amplifier into the frequency oscillator output. As it is amplitude modulation.

Thank you for any help.

Terry

 

[Jerry Eldred]

Re: 3325A Troubleshooting Modulation on Output

I have a couple of thoughts...

1. When it appears, demodulate it and see what the modulation frequency is. That may be a clue.

2. Beware of ground-loop-induced RFI/EMI.

3. Beware of RFI/EMI in general. Could be you have interference coming from somewhere else. I have seen numerous instances of RFI/EMI causing extra junk to intermittently ride on generator outputs, and oscilloscope inputs.

4. Check line voltage (low probability; but I have had crazy problems due to insufficient line voltage). It is at least worth a quick check.

5. Definitely check power supply voltages and ripple while the problem appears. If you have a flaky cap on a supply voltage, that MIGHT cause that sort of problem.

6. Press CAL and see what it does.

7. Change functions and see what it does. Does the problem appear in triangle, square, sine, or just sine? That could narrow down it's source.

Those are a few thoughts.

 

[TGroulx]

Re: 3325A Troubleshooting Modulation on Output

I have a couple of thoughts...

1. When it appears, demodulate it and see what the modulation frequency is. That may be a clue.

2. Beware of ground-loop-induced RFI/EMI.

3. Beware of RFI/EMI in general. Could be you have interference coming from somewhere else. I have seen numerous instances of RFI/EMI causing extra junk to intermittently ride on generator outputs, and oscilloscope inputs.

4. Check line voltage (low probability; but I have had crazy problems due to insufficient line voltage). It is at least worth a quick check.

5. Definitely check power supply voltages and ripple while the problem appears. If you have a flaky cap on a supply voltage, that MIGHT cause that sort of problem.

6. Press CAL and see what it does.

7. Change functions and see what it does. Does the problem appear in triangle, square, sine, or just sine? That could narrow down it's source.

Those are a few thoughts.

Thanks!!!!!


1. I cannot believe I did not check that.

2. Checked

3. Checked

4. Good Idea

6. Good Idea

6. No change

7. No Change

 

[wmarhel]

Re: 3325A Troubleshooting Modulation on Output

1. I cannot believe I did not check that.

It was amazing the number of times people would write up test equipment for failure to operate, only to find out that:

1) It wasn't plugged in
2) The batteries were dead
3) the fuse was blown (which the operator was usually allowed to repair themselves if they would have checked it in the first place)
4) The unit wasn't turned on to begin with

Some of the people writing the problem up would have used the equipment to repair and maintain an aircraft. Everyone is a allowed a brain fart now and then I guess. Better to write the equipment up, then to pencil whip the repair of the aircraft.

As for being sporadic, does it seem to have a pattern to the occurrence? Does it happen when somebody might be banging on, or bumping into the test bench? Does it do this when the case cover is on/off/both? How about trying to isolate the ground of this unit?

I didn't read the particular specs for this unit, but what frequency ranges are we talking about, and does it do this across the entire frequency span of unit?

Have you cleaned the board and connections and make sure there is nothing which might be bridging somewhere?

It's been over 10 years since I've worked on anything like that, but keep us posted for a root cause.

Wayne

 

[Hershal]

I agree with the others, most likely a bad cap.....also take a good look at the circuit board for cracks or other signs of solder joints degrading especially with caps in the power supply, or if a hair or similar might have shorted something just enough to allow bleed through to your output....

Hershal

 

[TGroulx]

eh, contacted customer, we set it off to the side temporarily. No bad caps, no other locatable problems, we have decided it is now a spare time project in order to keep costs down for the customer.