U1620A offset midpoint

[blueskull]

Recently I bought a used Keysight U1620A scopemeter from their official eBay webstore.
It came with ~0.1div CH2 offset, which goes away after warm up. When unit runs hot, the offset goes back on other direction.
CH1 is fine without any temperature dependent offset.

Needless to say, there's something that is temperature dependent, and I need to find it out.

My first attempt was to replace voltage reference chip ADR360, which didn't solve the problem.

I then traced the signal path back from AFE output. The AFE output has an AC path, which went through 2 HV caps to isolated LV side. This meter uses differential capacitive isolation. The DC path went to a differential to single ended converter, then went to an OPAMP to drive a high linearity (feedback-based) optocoupler. One PD output went to LV side, the other went to feedback of optocoupler driver.

I tried to short the input of the optocoupler OPAMP driver, and the result is great -- no offset anymore over cooling and heating. That means the optocoupler driver, the optocoupler and the voltage reference are good.

I then traced back to the differential to SE converter. With its input shorted, the offset went away as well, so either the converter is fine, or it injects offset current to its source.

I then traced further back to the PGA that drives both isolation caps and the DF-SE converter. This alleviates some issue, but the problem persists.

My initial conclusion is the PGA is faulty. To verify this theory, I desoldered its signal source (an FDA pre-driver), and shorted the output of the pre-driver (input of PGA). I cooled and heated the boards, and the offset persists and varies with temperature.

However, I did notice some residual offset from the FDA, so I figure I can replace the FDA as well.

So, I ordered the PGA (AD8370), the pre-driver (LMH6552) and the DF/SE converter (AD8130). Unfortunately I missed today's cutoff time, so I will only get the parts on Friday.

Anyway, the signal path is interesting, and I'm working on tracing the board.

[bitseeker]

Wow, this is becoming a project isn't it?

[TheSteve]

Did you take any screen shots with the offset? As I mentioned before mine has a tiny offset on some volt/div settings but they are gone after a minute or two of warmup. This is what mine looks like after 2 minutes.

[richnormand]

Did you take any screen shots with the offset? As I mentioned before mine has a tiny offset on some volt/div settings but they are gone after a minute or two of warmup. This is what mine looks like after 2 minutes.

Mine recovers too, but as temperature rises, offset goes back.

any possibility of using a dash of freeze mist to find the temperature sensitive component?

[TheSteve]

Any update? Did you run a usercal after it was fully warmed up? I assume you did but figured I'd double check. I ran mine after it had run for 2 hours or so.

[Zucca]

Recently I bought a used Keysight U1620A scopemeter from their official eBay webstore.
It came with ~0.1div CH2 offset, which goes away after warm up. When unit runs hot, the offset goes back on other direction.
CH1 is fine without any temperature dependent offset.

Sorry for asking, but why you did not return it?
Congrats on the repair.

[TheSteve]

Perhaps I see less offset then blueskull as it didn't really concern me. Mine is gone after a few minutes of warmup and is then perfect on all ranges. It is also such a small amount it would never interfere with any measurements I make.
I was actually more concerned that the firmware says Agilent at bootup instead of Keysight even though the firmware release notes specifically say the release I installed was supposed to change the branding to Keysight. I have since unpacked the firmware and do see the new Keysight splash screen but for whatever reason it is never installed/used.

[TheSteve]

Sadly, the unit has to go. I hope the replacement unit is not faulty. Fingers crossed.

Has to go?
Are you actually having Keysight replace it after you opened it up and swapped a bunch of parts?

[TheSteve]

Hah, maybe he wants to send me the broken one then

[bitseeker]

Yeah, I remember those scope issues. About all I can say is at least they're taking care of it. That's more than I can say about some other companies. Nevertheless, it's a bummer that Murphy has been on your case so much when it comes to test gear.

[bitseeker]

Thanks for the update, blueskull.

[analogRF]

I wish I saw this thread an hour sooner 
I just bought one of these probably from the same Keysight auction that @blueskull did
albeit at a much lower value than the asking price. Now they are gonna send that faulty unit to me 

I just emailed them and asked for cancelling the transaction. At this price it looks a good deal though but I prefer to find a 199C...so I'll keep searching...

The offset issue seems to be the similar to the problem that THS7xxA have. I have one and it has a small offset but recently I discovered that the offset disappears slowly when I plug in the adapter and it comes back slowly when on battery even though the batteries are brand new....

Also note that the manual for U1620A says it must be self cal'd if the temperature changes more than 10C from the last cal temperature.


EDIT: In case of THS7xxA many people have fixed the offset issue either by full calibration of the scope (which is very easy in those models) or by replacing the optocouplers after the input attenuator block or by doing both. I have not tampered with the opto's yet since I have a bad input capacitance issue on that scope as well 

[analogRF]

I wish I saw this thread an hour sooner 
I just bought one of these probably from the same Keysight auction that @blueskull did

Do you want mine? I have a perfect replacement unit, and it sees few use.
It was an impulse buy, and I'm happy to get rid of it.

Asking for the same price, but with guaranteed zero offset, plus a battery and two chargers.

Thanks but no, I think I will keep looking for a 199C. It's hard to get these stuff up here in Canada at a reasonable price
so in the heat of the moment I sent an offer for this thinking it will be rejected but suddenly they accepted it. Let's see if they cancel that...

However, I have a couple of questions about the unit if you don't mind answering: How does the Autoscale work? Do you have to push it every time you change the signal? Is it the same as Connect and View in Fluke scopemeters (and a similar thing in THS7xxA)? In these scopes, the instrument keeps adjusting itself by changing the input signal all the time no matter what you through throw at it. So most of the time for regular signals you dont need to touch anything just connect and view. My other question is about the FFT, does it show the frequency scale at the bottom or just a FTT trace? I cannot find a picture of FFT screen that shows frequency scale. 199C however does.