Part identification needed Rigol DG2041A

[byoungblood]

Output on my Rigol signal generator has gone out, last month I knocked managed to short one of the protection diodes and foolishly kept using the instrument after removing the shorted diode from the circuit and not replacing it. So now something else has gone kaput.

When the output relay closes, the output waveform is shorted to ground. I have traced the issue down to this part, as it is measuring a short across all 4 terminals, but I have no clue what this part is. It is the one at the left of the photo with 4 solder pads.  It is just ahead of the passives that are the last components before the coax going to the front panel BNC output connector.

Obviously replacement of this part and the protection diodes are in the works to get this working properly again.

For scale, the part is 4.7 x 3.5mm.

[R-1125F]

Is the Rigol model number correct?

[byoungblood]

Is the Rigol model number correct?

Fixed it, it is a 2041A

[fzabkar]

The sig gen has an output impedance of 50 ohms. How does this relate to the components in that area?

[byoungblood]

The sig gen has an output impedance of 50 ohms. How does this relate to the components in that area?

The output impedance is set just prior to the output relay and these components in the picture. 4 resistors in series/parallel.

[fzabkar]

Does the ground trace connect the top two pads?

Would it make any sense for it to be a common mode choke???

https://pccomponents.com/datasheets/STEW-CM3312.PDF

[byoungblood]

Does the ground trace connect the top two pads?

Would it make any sense for it to be a common mode choke???

https://pccomponents.com/datasheets/STEW-CM3312.PDF

I know the top right one does, I measured continuity between it and the BNC shell. I'll have to pull the component and measure the other when I get my hot air station tomorrow.

Looking at the measurements on the datasheet thats the first thing I've seen that would make sense there. Someone else on FB suggested that it may be a 90 degree hybrid coupler, but I can't find anything that matches the footprint of the component.

[byoungblood]

Finally got the hot air rework a couple of days late.

Pulled the part and I do believe it is a common mode choke. BNC shell would go through this component to the chassis ground. With this part removed the output is no longer shorted at the pad where this component was placed. Now just to place an order to Mouser and get the replacement parts on the way.

Looking at datasheets, I assume that the actual inductance probably isn't awfully important here? Looking at TDK one, looks like the insertion loss is about nil at the frequency range this device operates at.

I never use the DC offset on this generator, would there be any downside to permanently installing a capacitor in the output to serve as a DC block?

[coromonadalix]

I'm no expert

A capacitor would block the dc voltages, but upon certain frequencies, i think it would not be signal efficient for a full frequency coverage, or different wave form types 

I did saw in the past  2 or 3 capacitors of different values / specs  trying to cover a wider frequency range, but it as it's practical limitations


Not sure if an external amplifier or an follower (zero gain)  with output protection exists ??

[byoungblood]

I'm no expert

A capacitor would block the dc voltages, but upon certain frequencies, i think it would not be signal efficient for a full frequency coverage, or different wave form types 

I did saw in the past  2 or 3 capacitors of different values / specs  trying to cover a wider frequency range, but it as it's practical limitations


Not sure if an external amplifier or an follower (zero gain)  with output protection exists ??

Looks like most of the commercial ones are just a single capacitor, one from Pomona is a single 1000pf (I assume) ceramic cap.

This generator goes from DC to 40 MHz, not particularly demanding specs. I sometime use a .1uf cap clipped in line with the alligator clip leads I use with this. For my uses (troubleshooting audio amps and doing AM/FM radio alignments) attenuation of the signal isn't a real problem.

I'd just like to have something more permanent so if I connect it to something that has an unexpected source of DC on it I'm not having to go through this again.

[byoungblood]

Ok, I replaced the common mode choke and protection diodes on the output, and now I have a signal at the BNC connector again.

But the signal level is still low. I think I have traced this issue back to an AD8009 op amp. I calculated the gain in the feedback circuit to be about 1.7, but the output from the op amp is actually attenuating the signal. My guess is that one of the FETs/transistors in the output of the op amp was damaged when the common mode filter shorted to ground.

The suspect op amp is a couple of stages back from the output, at lower signal levels (below 2v) it appears to be the only op amp in the output. As you increase the output level it switches in other op amps via relays.

Has anyone had any success with obtaining schematics or general service info on signal generators from Rigol? The part in question is in the price range where I don't want to be swapping it out just to find it isn't the problem.