DS1054z Square Output Problem

[m.m.m]

I lent my DS1054z scope to a friend and they apparently used the square wave output which is for probe compensation calibration, as an input for an opamp. I do not know what their circuit was, but after returning, the square wave output is limited to a 50mV voltage.
My guess is that they drew too much current and the output buffer of the square wave generator (if there's any) is blown up. 
I would appreciate your comments, or your experience if you had a similar accident.

[modoran]

Just open the scope and see what's the problem there. And never lent to "friends" again.

[TurboTom]

It seems there's one SOT32 device directly connected from the test signal output to ground, presumably a protection device (Z-Diode, transient suppressor) and then some RC stuff to the test signal generator "silicon" output. So the first item I would look at, especially if there's still "a little" of the signal present, is the protection device for a short. See here for a series of photos of the "later" PCB version.

I don't expect the test signal output can be damaged by shorting it to ground externally. But application of a reverse voltage would surely be able to cause some problem... 

[m.m.m]

It seems there's one SOT32 device directly connected from the test signal output to ground, presumably a protection device (Z-Diode, transient suppressor) and then some RC stuff to the test signal generator "silicon" output. So the first item I would look at, especially if there's still "a little" of the signal present, is the protection device for a short. See here for a series of photos of the "later" PCB version.

I don't expect the test signal output can be damaged by shorting it to ground externally. But application of a reverse voltage would surely be able to cause some problem... 

I just had the time to open the scope (I've been quite busy  ). My multimeter reads a ~30 Ohm resistance from the test signal output to the ground. So I think the protection diode is short.
The diode has the marking "1UW". It appears that the part number is PESD5V0U2BT. Please correct me if I'm wrong.
I'll try to order a replacement and report the results.
Without the diode, the resistance is ~25k so I hope the rest is good.

[TomKatt]

I don't expect the test signal output can be damaged by shorting it to ground externally. But application of a reverse voltage would surely be able to cause some problem... 

+1

Sounds like you are on the right track.

[MAS3]

I see some brownish stuff between the trough hole solderings, and the SOT32 part on your picture.
And all 3 trough hole solderings look a bit dodgy to me.
So i would resolder those (remove old solder and replace it) while you're at it, and clean up afterwards.
You might loose the lead free solder certificate, but reliability is nice to have.

[radiolistener]

probably some voltage was applied to calibration output. Also check if probes are alive, it is possible that it was used to measure voltage on RF transmitter output and probe was burned out, because 1:10 probe rated for 600 V DC can be damaged with >25 Vpk RF on the input. If probes are burned, it may show lower amplitude.