VOLTCRAFT DOV series

[goaty]

I'd just like to get rid of the 200 mV/div and 10 V/div ripple at some point.
The other ranges meet my expectations for this device.
1mV/div looks okay to me.

Then it´s simple. Either SL75 mod or removing the buffer results in basically the same result.
I tried both. First SL75, then remove. (And I will probably insert the buffer back in again)

[Martin72]

As my measurements showed, the 39 kHz peak is already present at the main supply voltage(5.3V).
The buffer has, so to speak, unintentionally amplified and passed it on; removing the buffer therefore reduces the problem, but does not really solve it.
One possible approach is the one suggested by SL75: the op-amp “goes into” overcurrent mode because it detects the 10 µF capacitor at its output, which repeatedly draws a high current for short periods due to charging transients.
Hence his suggestion to increase the series output resistance from 1 ohm to 10 ohms—to limit the current.

Basically, we’d need to take a closer look at the sub-voltage generation—that is, everything that’s converted from the 12V supplied by the power supply.
Another phenomenon is the aforementioned 7.62 kHz, which only appears on the 2.5V rail—and only after the scope has finished booting up....
Guys, what are we actually doing here.....

[goaty]

Well for me it's just interest in digging through that hardware. I do not need the scope for any actual work. I'll try out a bit more, but will give up sooner or later. With the 40kHz basically removed, the scope is already very usable.

Edit: Really strange why the interference propagates through the LDOs.
The 5V for example is a TPS7A92
https://www.ti.com/lit/ds/symlink/tps7a92.pdf
Which should have a PSRR of I guess about 20-30dB from the datasheet (5.3V, ~10kHz)

I have to measure that on my device too. I did not find any peaks on the negative rails (-6V (named -5V) and derived)

If you take on the modification, could you operate the reference voltage source (or the buffer amplifier if it’s still present) without load?
It’s interesting to see whether the 7.6 kHz interference is possibly impressed “from the back” or if it might actually be generated by a 7.6 kHz load current from the peripheral devices being powered.

That is a nice idea. Have to try that out myself. But from looking at the layout I guess it´s hard to open up a certain trace to do so.
Removing the 01Y (1Ohm) could work, but that removes the 10+µF also.

Something I noticed: My op-amp is labeled ST3SI instead of ST3SK.

That is just a different production date (month) but same part.

[SL75]

The 10 µF MLCC isn't really that important for the function. However, if the buffer stage (by desoldering the 1 Ohm resistor) is disconnected, the 7.6 kHz interference at the OpAmp output should be gone. If not, the interference would have to be generated either in the reference voltage source or the buffer amplifier, which doesn't make much sense, especially since their power supply is also 'clean'.

[Aldo22]

Can someone give me a few tips on soldering in the DOV?
I’m a little worried about the 0603 components, and I really don’t want to damage the device.

Is it best to remove the components—especially the op-amp—with hot air?
Is there anything I should keep in mind when doing this?

For soldering, I have 0.6mm solder and Amtech flux.
I usually use a 2mm chisel tip.
It might be a bit too big for 0603 components, but it conducts heat well.
Or should I just solder them in with hot air? I don’t have any solder paste, though.

I’d appreciate any tips.

Thank you.

[KungFuJosh]

I highly recommend practicing on something like this before you work on something you care at all about: https://a.co/d/04MmC62V

[Martin72]

Hi,
I'm wary of hot air if you can't apply it with pinpoint accuracy.
Otherwise, thermally insulate the surrounding area; for what will (hopefully) be a brief moment, covering it with Kapton tape should suffice.
Another alternative:
Solder all the op-amp’s pins with plenty of solder.
Then, quickly alternate between heating one side and the other, and pry the chip off.
Disadvantage:
The chip will likely be unusable afterward.

[KungFuJosh]

A better technique for chip removal (that shouldn't destroy anything), is to use some good flux and this low temp alloy: https://a.co/d/0ibZx5ZE

Thanks,
Josh

[Martin72]

My mini soldering iron finally arrived today—at last...

[KungFuJosh]

That's adorable!

Here's a video I made last month using the chip removal alloy:



Thanks,
Josh

[Aldo22]

Thanks!

I highly recommend practicing on something like this before you work on something you care at all about: https://a.co/d/04MmC62V

Yeah, I’m already practicing. 
I have a circuit board from a broken satellite receiver where the spacing is pretty tight and there are even SMD components smaller than 0603 on it.
I removed four 0603 components from there using a heat gun.



Then I soldered them onto another board. They’re still working (511 is a 0603)
Maybe I took a little too much solder, but the connection is there.



But the red PCB seems pretty forgiving to me, whereas the one on the satellite receiver is very tight with short pads.
I’d be interested to hear if anyone has experience with the DOV PCB that they could share.

[KungFuJosh]

It's really easy to use too much solder, especially if your solder is too thick. Thinner diameter is better for smaller components.

[goaty]

I experimented a bit today.
The 5.3V supply has some peaks, but the 5V is totally clean.

The 2.5V reference has the notorious 7.6kHz, which vanishes if disconnected (remove 1Ohm).
-> So the 7.6kHz comes from the load connected (oscillation) and not the ref voltage or buffer itselt.

I then realized that with the SL75 mod, the signal is a bit worse than when removing the opamp alltogether, so I´ll stick with the zero ohms.

I also did the re-route of the reference to the OPA145 for each channel, which I think improved the signal a bit at 200mV.
Did not work. Found out that it shifts vertical offset....

I´ll leave it at that for the moment. Maby someone else has more improvements.

Edit: Also "replaced" the battery which had only 2.7V (fresh delivered unit) and did loose date and time.
I might have done a mistake if this is a rechargable battery, thanks SL75


I also tried the sense line of the REF3425. Re-route it to after the buffer. That results in some really funny oscillations 

[Aldo22]

Hi

I'm wary of hot air if you can't apply it with pinpoint accuracy.

My experience so far has actually been pretty good when desoldering with hot air.
It’s a smoother process than using a soldering iron, with less tugging.

[Aldo22]

My mini soldering iron finally arrived today—at last...

Looks nice! 

Years ago, I bought a “fine-tip” soldering iron cheaply at Conrad, but I was never satisfied with it.
It doesn’t get hot enough, so you have to hold it on the solder joint for way too long.
I’d rather use a coarse-tip one that transfers the heat better.

I hope yours works much better.   

[Aldo22]

It's really easy to use too much solder, especially if your solder is too thick. Thinner diameter is better for smaller components.

Yeah, 0.6 mm is a little too thick for 0603, but I don't have anything else right now, and I think it'll work.
I'm not trying to win any beauty contests—I'm just happy as long as I don't break anything. 

[KungFuJosh]

My experience so far has actually been pretty good when desoldering with hot air.
It’s a smoother process than using a soldering iron, with less tugging.

Did you watch my video in reply #809? It's usually safer than hot air.

I've seen people use hot air and make the PCB get swollen lumps. It also usually takes longer with hot air even if you are good at it.

[Aldo22]

Did you watch my video in reply #809? It's usually safer than hot air.

I've seen people use hot air and make the PCB get swollen lumps. It also usually takes longer with hot air even if you are good at it.

Yeah, I saw that. It looks great, but I'd have to order that low-melt desoldering alloy first.
Actually, I just need to remove three 0603 resistors and this op-amp. I’m not sure if it’s worth it.
That should be a breeze with a heat gun—you definitely won’t have to heat it for very long.
I don’t think much can go wrong there.
I haven’t ruined a PCB doing this yet.
It’s more likely to happen when you have to heat thick stuff for a long time, right?

[Aldo22]

Edit: Also "replaced" the battery which had only 2.7V (fresh delivered unit) and did loose date and time.

Oh no, I guess Owon didn't provide a battery holder?

[Martin72]

The 5.3V supply has some peaks, but the 5V is totally clean.

As is well known, the situation is different with my scope; it would be less than ideal if the interference varied “individually” from scope to scope.

Edit: Also "replaced" the battery which had only 2.7V (fresh delivered unit) and did loose date and time.

The batteries may have been “freshly” installed, but of course no one knows how long they've been in stock, and there's a chance that discharged cells were installed.

Oh no, I guess Owon didn't provide a battery holder?

No, there's no holder; the battery is soldered in.

[Martin72]

I hope yours works much better.   

I'm not worried about that.
The Weller WXMP series has soldering tips with built-in heating elements, which means the heat-up time is just a few seconds, and the time needed to maintain heat while soldering is just as short.
The same goes for my WXMT SMD soldering tweezers.
I also have the 120W WRMP soldering iron, which looks much slimmer than its power output suggests, and the whole setup is controlled via the WX-1 station.

[goaty]

I’d be interested to hear if anyone has experience with the DOV PCB that they could share.

Mostly experience. Use a blob of solder over the resistor and it comes off at once.
Grab it with tweezers if it sticks to the iron.
Also use solder wick and flux.
I used leaded solder first on the opamp then hotair.
Most important is to have flux and isoprop alcohol for cleaning.
Magnification is also important if you´re over 50.
Either loupe with 3x (not 3 diopters! 3x is 8diopters) or better stereo microscope.

Dont need fancy  1000€+ Weller for that

[elettram]

Hi everyone, this is my first post on this forum.
I also purchased this oscilloscope and have encountered the same issues regarding ripple in some ranges.
Beyond that, I have a more serious problem saving waveforms:
the images are saved without problems, but when I save the data (.csv, .mat, .zip), the files contain a correct header, but with truncated or incorrect data, or voltages that are constant at zero or a random value.
Has anyone encountered these errors when saving?
Thanks for your attention.

[SL75]

The 5.3V supply has some peaks, but the 5V is totally clean.

As is well known, the situation is different with my scope; it would be less than ideal if the interference varied “individually” from scope to scope.

Edit: Also "replaced" the battery which had only 2.7V (fresh delivered unit) and did loose date and time.

The batteries may have been “freshly” installed, but of course no one knows how long they've been in stock, and there's a chance that discharged cells were installed.

Oh no, I guess Owon didn't provide a battery holder?

No, there's no holder; the battery is soldered in.

I would say that the appearing interference signals are largely determined by the components used and the board revision. On the PCB Rev 1.0 in a pristine state (I assume yours hasn’t been modified yet?), the main disturbance comes from the oscillating reference buffer (f ≈ 38 kHz), which, because it is effectively short-circuited on the output side by the low-impedance 10 µF MLCC, generates a noticeable AC current draw from the connected power supplies.
On Goaty's modified PCB, this disturbance has, as expected, completely disappeared.

[SL75]

I experimented a bit today.
The 5.3V supply has some peaks, but the 5V is totally clean.
(Attachment Link) (Attachment Link)
The 2.5V reference has the notorious 7.6kHz, which vanishes if disconnected (remove 1Ohm).
(Attachment Link) (Attachment Link)
I then realized that with the SL75 mod, the signal is a bit worse than when removing the opamp alltogether, so I´ll stick with the zero ohms.
(Attachment Link) (Attachment Link)
I also did the re-route of the reference to the OPA145 for each channel, which I think improved the signal a bit at 200mV.
Did not work. Found out that it shifts vertical offset....

I´ll leave it at that for the moment. Maby someone else has more improvements.

Edit: Also "replaced" the battery which had only 2.7V (fresh delivered unit) and did loose date and time.
(Attachment Link)

Interesting measurement results. So it seems that some part of the circuit is drawing energy from the reference voltage source at a rate of f≈ 7.6 kHz. 
What do we know about this frequency? 
• It seems to be controlled by the firmware, because it doesn't appear during booting. 
• It seems to be crystal-stable, or derived (divided) from a crystal-stable source. 
What could it be?