REVIEW - Owon SDS7102 - A look at the SDS series from Owon

[TomC]

First, sorry for the confusion. My readings were made using battery only! Next, my L2 is L3 on this new schematic and is same as L2 on TomC's reverse-engineered schematic! It's the coil used by the MC34063A to generate negative voltage. Compared to the new schematic, some components are missing on my board. First, there was only one green 470  µF 16 V electrolytic capacitor right next to the IC. There are (originally unpopulated) foorprints for another electrolytic and ceramic capacitors next to the connector.

Anyway, I found components that to me look like the snubber for the diode (but I didn't use that, since power is coming from the battery, right?). They are marked C4 and R11. C4 shows 2.287 nF, which is pretty close to expected value, from what I can see. R11 gives 21.7 ohm reading on my multimeter, so that too looks OK. The soldering of R11 was bad in my opinion, so resoldered it.

My problem with the capacitors is that I don' have an ESR tester at the moment and my multimeters can't measure large electrolytic capacitors. I'm making a crappy one from schematics of an old Poptronix kit, so maybe that will help and maybe not.

As for the ferrite, I'll post a picture of it when I get a camera. It's about the size posted maybe 20 pages back.

I'm off to get a camera now, so hopefully my next post will have photos of the PSU itself. Thanks for all the ideas!

If you took the readings using the battery only, then the 8.4V, which is the majority of what's in the PSU board, is probably not involved. The only thing left is the -7.6V in the PSU board and all the DC/DC converters in the adapter board. I would concentrate on the filter components for those in this case.

To check electrolytic capacitors you need an ESR meter, in all the years I've been troubleshooting electronics, 99% of all electrolytic troubles were diagnosed with an ESR meter. The capacitance usually checks OK even if the capacitor has a very high ESR. The alternative, which works maybe 50% of the time, is to visually check under the capacitors (if radial) for signs of leakage or corrosion. If you see that you should replace the electrolytic capacitor.

Just for a simple idea of how to build an ESR meter, I attached a schematic of the one I built back in the 70's. Actually I was still using this a couple of years ago when I upgraded to a microprocessor based LC & ESR meter. Unfortunately I don't have specs for the transformer, I think I used what I had on hand and then altered the meter's scale to match it.

[lemon]

These attached photos have full resolution but with some compression for less Mb...

As I told you, there is same like yours but with other revision PCB-T113-J Rev.6 (through-hole components). These photos are without any mods.

[AndrejaKo]

These new pictures are much closer to what I have. One thing that I immediately noticed it that the ground connections on the top of the board are cut while they are connected on my board! They exist only on the bottom. I'll experiment with isolating top ones and report back what happens.

Here are two pictures of my PSU. I've marked components on the top side whose markings were hard to determine from the photo. All components on the bottom were added by me. If you need close-up of any specific area, please let me know. EC8 is normally soldered and had additional SMD ceramic capacitors, but I removed them for debugging purposes.

Once again, this is PCB-T115-J rev4.

[AndrejaKo]

Sorry for double-posting, but I had to do this!

BAM!!!!!

It's the top ground connection that's making most of the problems!!!! I just placed a piece of paper between the top of the PSU and the mounting holes and I got results in calibrated.png! I think I have some insulating washers in my junkbox, so I'll try them out. They should probably make a better permanent solution to this problem.

The problem of 80 ms pulses still remains to be identified, so I'll focus on it now. To me it looks like ground noise this large is within usual levels, unless I'm mistaken.

Also those huge voltage spikes that were intermittent and above the usual are now gone.

Update: I added the insulating washers under the top screws and it solved that problem. Another interesting issue us that now, when the battery is supposed to be charging, the LED keeps blinking. Perhaps some of the modifications I did upset the charging circuit? Also the remaining battery indicator keeps changing values pretty quickly as well. Just to be safe, I removed the battery until I can investigate this. Without the battery, the green LED is on, as expected.

[TomC]

Sorry for double-posting, but I had to do this!

BAM!!!!!

It's the top ground connection that's making most of the problems!!!! I just placed a piece of paper between the top of the PSU and the mounting holes and I got results in calibrated.png! I think I have some insulating washers in my junkbox, so I'll try them out. They should probably make a better permanent solution to this problem.

The problem of 80 ms pulses still remains to be identified, so I'll focus on it now. To me it looks like ground noise this large is within usual levels, unless I'm mistaken.

Also those huge voltage spikes that were intermittent and above the usual are now gone.

Update: I added the insulating washers under the top screws and it solved that problem. Another interesting issue us that now, when the battery is supposed to be charging, the LED keeps blinking. Perhaps some of the modifications I did upset the charging circuit? Also the remaining battery indicator keeps changing values pretty quickly as well. Just to be safe, I removed the battery until I can investigate this. Without the battery, the green LED is on, as expected.

AndrejaKo, Just to let you know my thoughts after your last update.

The ground that you have isolated on the PSU board corresponds to the GND-C on the PSU schematic I reverse engineered. That ground is isolated on that PSU version. However, on the newest PSU that Owon is shipping now, they have gone back to grounding it locally to the Z plane, as in your board. In any case, this ground doesn't remain isolated from the Z plate for long any way, because it connects to the adapter board via pins 7 & 8 of the adapter connector. As you know, on the adapter board there are several pads that connect this ground to the Z plane when the board is attached to it with it's screws.

So in view of the above, I think the reason you are getting better results now is because GND-C, before it gets to the adapter board and the rest of the scope, has to go through the ferrite you have on the adapter cable. To me, that indicates that some of the noise is being generated on the PSU board. Since (based on you previous post) you get the noise when operating on battery only, it seems that the best candidate for a noise maker is the -7.6V DC/DC converter circuit.

The problem with the LEDs acting funny when the battery is charging is probably an entirely different problem, possibly related to one of your mods as you said. I think you can probably troubleshoot that with just a multimeter by checking some voltages. In the Theory of Operation I previously posted there is an explanation of how this circuit works on pages 3 and 4. I'm going to attach a copy of it to this post in case you want to take a look at it.

Good luck and keep us posted!

[lemon]

AndrejaKo, I don't understand what top ground you talk about, you said "It's the top ground connection that's making most of the problems"

Anyway, I upload some suggestions mods to your psu board. It is something like yours mods but they based more to changes to new Owon board and some other mod from previous messages.

I have note over the photo the marks that I believe to put some smd capacitors. At the output that the ribbon cable was fixed you can mod like the new Owon one, this is the blue line that I draw.

[AndrejaKo]

Lemon, take a look at the picture you edited. The "top ground" is term I used to refer to the two ground connections on the right side of the photo.

Anyway, about the blinking LED: From what I can see, it's blinking correctly. The PSU seems to be switching on and off.

Did some measurements with a multimeter and here are the results (I used component markings from TomC's reverse engineered PSU schematic):
When the PSU is on, scope is off and battery is disconnected I get 7.5 V on the D9, EC3, R11 joint and I get 8.5 V on the battery jack and LM324 power pin. Green LED is shining.
When the PSU is on, scope is on and battery is disconnected I get 8.4 V on the D9, EC3, R11 joint and I get 8.3 V on the battery jack and LM324 power pin. Green LED is shining.
Rail marked -7.6 V reads -7.55 V on the adapter board power cable and voltage marked 8.4 V(sw) is 7.6 V to 7.7 V


When the PSU is on, scope is off and battery is connected I get 1.5 V on the D9, EC3, R11 joint, jumping up when LED blinks and I get 7.73 V on the battery jack and LM324 power pin. Green and red LED is blinking.
When the PSU is on, scope is on and battery is connected I get 1.5 V on the D9, EC3, R11 joint, jumping up when LED blinks and I get 7.4 V on the battery jack and LM324 power pin. Green and red LED is blinking.
Rail marked -7.6 V reads -7.55 V on the adapter board power cable and voltage marked 8.4 V(sw) is 6.7 V to 6.8 V, jumping up when the LED blinks. I can also hear a tick whenever the LED blinks. I guess this is probably from the movement of the coils.

I believe that when the power is on and the battery is in, the scope is actually running from the battery and not from the mains power. PSU also seems to be resetting all the time for some reason and will not enter current limiting mode to charge the battery. It's behaving as if the output is shorted. I'm currently charging the battery by hand and testing out how the PSU behaves with different charge levels. It's voltage is at 8.17 V now and there are no changes to the way PSU is responding. I'll do some more checking on the first tho op-amps on the LM324. To me it seems as if the problem could be somewhere close to them.

[TomC]

AndrejaKo, I gave  a quick read to your post, and it seems to me the 8.4V is running in hiccup mode. This may be caused by overvoltage protection. I think you mentioned you replaced all the diodes with UF4007, maybe you should try changing D6 back to 1N4007. Without further modification to this circuit, the UF4007 may be providing too much voltage to the R7731A PWM controller.

I didn't have too much time to analyze what you just posted because I have to run some important errands. When I come back I'll look at it in some more detail and give you more feedback if you haven't yet found the problem.

Good luck, and keep us posted!

[lemon]

AndrejaKo, I understand now what was the "top ground"...OK

Just I made a quick read to your post. Very carefully with the Richtek R7731A, because it is dip8 and there isn't nothing everywhere (only smd version).
Before time ago I destroyed one R7731A and cost me like a new psu board! There isn't available in dip8, only to few dealers to China with 5-8$ each and 25$ cost of shipping!

[AndrejaKo]

@TomC
I replaced D6 with 1N4007 and the power supply isn't resetting itself any more when the battery is in. On the other hand, the battery was by then almost charged by hand and I of course forgot to test the PSU with the battery before changing the D6. I'll discharge the battery now and report back what happens. Thanks a lot for the assistance!

@lemon
I'll be careful with it! Thanks for the tip! I'll try to keep my modifications as far away from it as possible.

Update: I discharged battery back to one half and now it seems to be charging correctly.

[TomC]

@TomC
I replaced D6 with 1N4007 and the power supply isn't resetting itself any more when the battery is in. On the other hand, the battery was by then almost charged by hand and I of course forgot to test the PSU with the battery before changing the D6. I'll discharge the battery now and report back what happens. Thanks a lot for the assistance!

@lemon
I'll be careful with it! Thanks for the tip! I'll try to keep my modifications as far away from it as possible.

Update: I discharged battery back to one half and now it seems to be charging correctly.

That's great news!
Now you can concentrate on the noise. Maybe you can test the GND noise again using Lemon's post as a guideline and post the results. Personally I would like to see Average Mode 16 at 10M added to that to also get an idea of overall scope performance.

[TomC]

AndrejaKo, I understand now what was the "top ground"...OK

Just I made a quick read to your post. Very carefully with the Richtek R7731A, because it is dip8 and there isn't nothing everywhere (only smd version).
Before time ago I destroyed one R7731A and cost me like a new psu board! There is available in dip8 only to specific dealers to China with 5-8$ each and 25$ cost of shipping!

That's pretty outrageous! While researching the PSU circuits I ran into datasheets for several other IC's almost identical to the R7731A. I'm not sure if they are completely interchangeable without circuit modifications. But switching to a different manufacturer may be an alternative if they stop making the Richtek or the price becomes prohibitive.

[lemon]

TomC this chip was my nightmare.
I couldn't to find it, I search all the knows sources but nothing, only smd version (sot 23-6) from China (that is including from your version and after).
For these reason I sent a request for sample to Richtek Company. Finally they send me (command from Richtek USA) sot 23-6 version not dip8! 
I had to make a prototype micro board sot 23-6 to dip8 but I have only explotion... all the time the resistor R9 (0.67-0.68 Ohm), the mosfet Q1 and NTC-10D-11 booom!
I don't know why, I done many times check to paths, the characteristics are identical btw sot and dip version (only the legs differs), it dosen't work! Probably the send me other IC, who knows with funny marks of smd.
Finally, I paid a lot of money to bought dip version direct from China at prices that I said!

AndrejaKo very well! We'll waiting for the noise now!

[TomC]

TomC this chip was my nightmare.
I couldn't to find it, I search all the knows sources but nothing, only smd version (sot 23-6) from China (that is including from your version and after).
For these reason I sent a request for sample to Richtek Company. Finally they send me (command from Richtek USA) sot 23-6 version!
I had to make a prototype micro board sot 23-6 to dip8 but I have only explotion... all the time the resistor R9 (0.67-0.68 Ohm), the mosfet Q1 and NTC-10D-11 booom!
I don't know why, I done many times check to paths, the characteristics are identical btw sot and dip version (only the legs differs), it dosen't work! Probably the send me other IC, who knows with funny marks of smd.
Finally, I paid a lot of money to bought dip version direct from China at prices that I said!

AndrejaKo very well! We'll waiting for the noise now!

I don't know why that happened, like you said, the function is identical. You would think the adapter board you made should have worked. The only difference I can think of is the power rating, the SOT version is lower, maybe the adapter board didn't provide enough heat dissipation. But you would think it would at least work for a while until the heat was too much. Hope is a mystery you won't have to deal again with.

[lemon]

You have right, it should have worked for a while, but just switch-on the ac...boom.. like a short-circuit. Maybe take a capture of adaptor board for what marks have it?

[TomC]

You have right, it should have worked for a while, but just switch-on the ac...boom.. like a short-circuit. Maybe take a capture of adaptor board for what marks have it?

Yeah, if you still have it, I'd like to see a picture of it.

[AndrejaKo]

Sorry, I'm unable to do the recommended mods at correctly at the moment. I'm out of 220 nF and 1.5 nF capacitors (I usually don't use them), so I'll have to get some more tomorrow.

Images made with scope running on AC with no battery.

First attached image are implemented capacitors. Capacitors in red ellipse are ones I managed to place. I'm not sure how t implement ones with a question mark next to them. I'm still looking for a suitable bar to use.
Second image is scope in average mode with 10 meg memory.
Third image is scope in peak-detect as described previously.
Fourth image is scope in scan mode where large peaks can be seen. Looks like the big peaks are back.

I'll report back when have needed components.


Also how well are your scopes handling the experimenting in the mechanical sense? My scope is starting to show some signs of wear. For example, the screwdriver slots on the M type screws of the PSU are getting worn (I'll probably replace them soon) and screws holes for the back cover are getting loose on my scope. I also added two non-populated screws that hold the screen attached to the front cover on the left side, since I'm not using back cover screws at the moment to decrease the wear of the mechanism.

[TomC]

Sorry, I'm unable to do the recommended mods at correctly at the moment. I'm out of 220 nF and 1.5 nF capacitors (I usually don't use them), so I'll have to get some more tomorrow.

Images made with scope running on AC with no battery.

First attached image are implemented capacitors. Capacitors in red ellipse are ones I managed to place. I'm not sure how t implement ones with a question mark next to them. I'm still looking for a suitable bar to use.
Second image is scope in average mode with 10 meg memory.
Third image is scope in peak-detect as described previously.
Fourth image is scope in scan mode where large peaks can be seen. Looks like the big peaks are back.

I'll report back when have needed components.


Also how well are your scopes handling the experimenting in the mechanical sense? My scope is starting to show some signs of wear. For example, the screwdriver slots on the M type screws of the PSU are getting worn (I'll probably replace them soon) and screws holes for the back cover are getting loose on my scope. I also added two non-populated screws that hold the screen attached to the front cover on the left side, since I'm not using back cover screws at the moment to decrease the wear of the mechanism.

I haven't taken apart my scope that many times, so wear and tear is OK for now. If you have a chance, can you capture the same tests running the scope from the battery and post them. It would be helpful for comparison.

[TomC]

AndrejaKo, The last capture, with the high amplitude pulses, roughly comes out to be 25Hz bursts about every 140ms. Resembles something like some sort of telephone ringing cadence. Are you by any chance in the vicinity of telephone switching equipment. Just a wild guess! The pattern reminded me of a telecommunications course I used to teach, so I thought I'll ask, just in case.

[lemon]

You have right, it should have worked for a while, but just switch-on the ac...boom.. like a short-circuit. Maybe take a capture of adaptor board for what marks have it?

Yeah, if you still have it, I'd like to see a picture of it.

The marking is DP=JCM. Look at the attachment captures.

...
 I'm not sure how t implement ones with a question mark next to them. I'm still looking for a suitable bar to use.
...

Because the middle foot of mosfet is away fron the gnd, you can put the smd like the schema and joined it by a short cable.
Generally, the values of smd capacitors is from major works of rf-loop and Siri (both of them are member of EEVBlog) adapted to ours versions. I don't know exactly what values of smd caps Owon used to the last version, I supposed that used the rf-loop's suggestion.
AndrejaKo, I estimated that your noise, after a good decoupling work on psu, falls at 100mV level about. The next is adapter Ver3.0 board...

By the way, which is the scan mode? I remember that have 3 modes: Sample, Peak, Average (Acq. Mode)

[AndrejaKo]

There was no telephone switching equipment near when I made the ringing images.

The scan mode has to do with triggering. If you set the s/div value low enough, the scope will stop triggering in the classical way and will switch to scan mode. In it, the waveform will "flow" across the screen from right to left.

[lemon]

AndrejaKo, thanks for the info. 

[TomC]

[The marking is DP=JCM. Look at the attachment captures.

That should have worked if you had the correct chip. The chip they gave you probably had a permanent positive voltage on the gate pin and turned on Q1 all the time. That's the only explanation I can think of for it to fry all the components you mentioned.

[TomC]

I don't know exactly what values of smd caps Owon used to the last version, I supposed that used the rf-loop's suggestion.

While I had the scope apart I measured the SMD caps on the new PSU board. Most of them didn't give me a true reading because of the surrounding circuits, but the few I was able to measure were around 220nF. These were of the thicker type. There are also thinner caps I couldn't measure, I suppose those are the 1.5nF ones.

[TomC]

There was no telephone switching equipment near when I made the ringing images.

Do you get that same ringing image when running from the battery only?