Rigol DG3101a Function Gen - No Square / Pulse Output but Sine / Ramp fine

[xyberlink]

Hi,

I have a Rigol DG3101a Function / Waveform Generator with a problem.

Sinewave (max 100Mhz) works fine - 100Mhz - spot on (maybe peak to peak a tad low)
Ramp (max 1Mhz) works fine - 1Mhz - spot on.

Square wave (max 50Mhz) - above 20Mhz I get a rough looking square wave at the correct frequency. Below 20Mhz there's nothing (it's actually permanently at the low limit (~-2.8v).

Pulse wave (max 25Mhz) - nothing on any frequency - again permanently at the low limit (~-2.8v).

Changing functions causes relay clicking, but there's no click changing between square and ramp, which suggests these two share the same signal path, so it's probably one problem affecting both waveforms.

I've not spotted a service manual, hence any help on figuring out which component might be at fault much appreciated.

Main voltages are good... +/-5v and +/-15v all present.

Example outputs attached along with an initial photo of the main board (J302 bottom left is the main output) - I can add extra board photos if something isn't clear.

Thanks, Paul.

[xyberlink]

I've done some tracing - not sure if I'm on the right track or not.

The output signal appears to route through the relay in the attached image.

Red dot = output
Green dot = signwave in
Blue dot = squarewave in

When in sinewave mode (relay off - green dot connected to red dot), there is the expected sinewave on the green / red pins. There is also a matching (frequency / amplitude) squarewave on the blue pin.

When in squarewave mode (relay on - blue dot connected to red dot), the blue / red pins are held low. Still see the signwave on the green pin.

So I'm thinking the issue is in the zone circled orange.

[xyberlink]

After some further tracing / checks I've identified a potential suspect component (circled in orange on the attached image.

Red line shows the path of the signal (either squarewave when another function is selected, or locked low when squarewave function is selected).

It's an SOC-143 package marked up T5V. I can't find any reference to that online, but based on the readings I've seen and it's position I'm thinking it's a Parallel Pair Schottky Diode. Example would be a BAS40-07 from Infineon.

Datasheet at... (https://www.infineon.com/dgdl/Infineon-BAS40series.-DataSheet-v01_00-EN.pdf?fileId=5546d4626d66c2b1016d73f7e0bb20ca)

Reading left (+ve) to right (-ve), I get....
Left T5V top pair: 0.311v
Left T5V bottom pair: 0.311v
Right T5V top pair: 0.312v
Right T5V bottom pair: 0.107v

Reading right (+ve) to left (-ve), I get
Left T5V top pair: open
Left T5V bottom pair: open
Right T5V top pair: 1.227v
Right T5V bottom pair: 0.099v

I can't see anything obvious in the circuit to indicate why I would see different readings on the right hand T5V to the left, hence I currently suspect this may be faulty.

It would be great to know....
- If anyone agrees / disagrees with this theory (and why)?
- If anyone can determine what the T5V's actually are - does a Parallel Pair Schottky Diode seem likely here?

Thanks.