Repair of Lecroy AP020 Active Probe (TearDown Photos Included)

[riyadh144]

Hi,
A couple of weeks ago I came across a good deal on a Lecroy LC584AM, and I couldn't miss out on a 1Ghz Scope for $400, so I picked it up, of course it came with no probes, what is the use of a 1Ghz scope with 200MHz probe that I already had, I started looking for probes to get, I bought sever HP 10441A 500Mhz mini probes from an ebay seller 30 something $ each and man they are wonderful  (Check them out), and I bought a 1Ghz probe claimed to be new.

Enough Waffling, I get the probe and for some reason I have a 38V dc offset on the screen, I thought maybe I didn't know how to operate an active probe, but yeah that probe was bad, anyway the seller agreed to get me another one. But for now I decided to fix the probe. I believe it is on the input side on one of the FETs.

I came to that conclusion by tracing the 3.8 volts (10:1 probe)  to its source, I started from the output of the scope tracing the input all the way back, and I figured it is coming from the probe input, at first it was the caps but I removed and checked them for ESR and they are fine and spot on then I figured that the first time that 3.8V occurs on the signal path was the V2 labeled FET on pin 1. I tried measuring the internal diodes, I only found one and it was behaving weirdly, so I figured this must be the culprit.

Even if I cannot repair it I thought I would post the teardown photos maybe someone can benefit from them

Some Photos are repeated just in case one of them doesn't show a component value.

https://imgur.com/a/T1y4d8X

Regards
 

[dmderev]

These probes are apparently using split-processing architecture. There is a high-pass FET AC buffer in the probe, and there is a low-frequency op-amp amplifier in the probe box. The offset is commanded to DAC (you see 16-bit DAC in your pictures), and the DAC feeds the low-frequency channel. You can see the datasheet of BUF802 by Texas Instruments to get explanation of the architecture.
If you see no high-frequency (edges) of the signal, it may be failing FET. However, the offset is commanded by DAC and op-amp in the box; the scope is sending digital code to DAC when you are rotating the offset knob. I explained how it works in another post on the forum (ProBus reverse engineering).
These probes are difficult to break, but, if you want to troubleshoot it, you will need to check some signals in the box, and to do so, you will need a 6-pin extender for ProBus.
If you did not fix it yet, try to measure voltage at the output if DAC's output amplifier (see its datasheet). When you rotate the offset knob, then the voltage there should change. If you see that and the output of the probe does not change, then check the low-speed path opamp. It sums the DAC and 1/10 of the voltage on probe pin. 

[dmderev]

P.S. The low-speed channel is built around LT118 opamp (near R30). The trimmer pot adjust the balance of gains between high and low frequency path. Thanks for pictures, by the way...