DC offset problem

If you folowed the link I posted, you could see that a 2mHz scope displayed the square wave precisely as well as the 100mHz. That was a true 2mHz scope. There are cheap scopes that claim to be much higher frequency than they are. Virtually any 2ch 20mHz scope in good working order is more than good enough. Leader, BK precision, Kenwood, Phillips... are all decent brands.

Portable (handheld) scopes typically have poor resolution and many perform poorly but some are OK and make checking some signals easier if you refuse to learn to use your scope in differential mode (typically easy with an analog scope).
But it is not a opamp debate it is fixing DC offset on power amplifier output.

Since you have a lot of removed/ replaced
components look for missed cold solder or connections.
or bridges.

DC offset usually gonna happen, with anything controlling
DC offset LOL

So mainly current sources for 1st or 2nd gain stage

could be resistor, transistor or cap
in the constant current source.
or anywhere a possible solder connection
was cold or possible bridge.

or if there is current limiter, with slopes.
there was likely few caps used for the slopes
that were " touched"
or any numerous diodes used to stop DC offset
that were touched

or if its a bridged single supply.
any numerous caps touched controlling
Virtual ground or DC ref voltage at the inputs
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That's okay,

Go for higher bandwidth, it buys you other things too. If you can swing, 100 MHz is the best deal, you don't need more than that. The reason is so that you can see small high frequency details. The price difference is nothing when you consider anything below $1,000. Remember, you will own this for a very long time, decades probably.
I have tendency to use less lead when soldering, i often have hole in it. So I check every solder joint after soldering. I also always check for cold solder, i think it was the one which do not shine, and resolder them. My solder works was several notch below most of you here, but I do try to keep up.

I do learn to solder properly as i sometimes play with 0805 smd. Once I think that shorter trace was better and build preamp using 0805 resistor and 100nf 0805 ceramic capacitor for power filter.

For a project that use many component like this one, i put the resistor first and check the value with a meter. I do this because sometimes I put wrong valued resistor, like 1k instead of 100ohm and 27k instead of 2.7k. I also check a close solder point for potential short. I do this before putting capacitor as the caps seems to charge my meter.

I am trying to get a used analog scope, or if I buy a new scope I want one with better specs. I do not want ended buying another scope in the future as it was not cheap.

For a new one i got two options, goes with the crowd buying a popular brand without warranty or a cheaper one with 1 year warrany.

I want a portable digital as I try to build a custom vfd motor controller using arduino nano, which I want to sell. I also consider replacing the nano for node mcu in the vfd and only use the nano in the central control unit.

I need to look for pwm signal output from this microcontroller, and also to tune inductors that I use. It used for hvac in poultry house. It is impractical to carry a desktop scope around as I have a need to probe the vfd with actual load, which is 50" blower installed on a broiler house.

This is what made it possible to have a budget above $500 for scope. I am not spending hundred of dollars just for audio hobby.
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Hi Widh61,
We are talking about suitable tools for the job.

Okay, so you have a choice. I would argue that the digital scope you are considering isn't good enough for the work you want to do. It might work, but if my work depended on a tool, I would make sure it was up to the task and at least mid-range quality. Obviously whatever you buy for work with have to do for your audio hobby. That's fair. Just be aware of the limitations and live with them.
I'm all for someone trying to bring a new product to the market but...

1. There are a lot of cheap VFD drives on the market.
2. If it's profitable, the chinese will copy it and sell it for less than it costs to produce your product.

Do everything you can do to protect yourself and don't invest any significant money without doing all you can to confirm that the product will be profitable.
Yes, after I watch youtube review I found that that one is unreliable. But I do find a meter and scope combo that is quite good for the price. It was hantek 2c42 40mhz at $167. I do know it got bug with trigger point and did not have differential mode. But, I think it will work for me. I already put an order on it. Today is labor day holiday here, the shop closed and I got time to cancel my order if needed.

It is common and cheap in your area, but not here. Even the one made in china was way too expensive after tax and duty. I think the current policy was made to encourage local product development. It was so expensive so there is none on the market atm, and so far i did not find anyone using vfd here for the application. I know this because I was in the industry several years ago and still in contact with many friend in it.

I also found the several cheapest chinese vfd drive made have active cooling which will be unreliable for the application.

Also, there is a unique feature that i use as my marketing pitch. I use IOT which will be additional selling point, it will be relatively easy for me to integrate the crucial humidity and temperature function into the vfd central control. Imagine, an affordable vfd with IOT on a market which you could not find anyone using vfd. That is an opportunity only found in a developing economy.

Anyway, thanks for your concern. I already have most parts and tools anyway. I have more than 10 motors with various power rating and type for no load test, or maybe simulated load test. I start small and will scale up accordingly. I do have budget and also I am not spending money that I do not afford to loose here. Even if I do fail, the knowledge that I gain will be worth it.

This thread has turned into newbie scope selection help.
I just found out that Hantek DSO2D10 got 100mhz analog bandwidth, is that a digital scope with analog mode feature? It listed for $260. I will look for more info on this particular model.

Hantek DSO2D10
1), 2 channels, each channel has independent knob control;
2), 100MHz analog bandwidth
3), the highest sampling rate is 1 GSa / s;
4), The storage depth is 8Mpts;
5), 2mV / div ~ 10V / div vertical range;
6), Built-in 25MHz signal generator;
7), Vertical resolution: 8bit;
8), Trigger: edge, pulse, video, slope, timeout, window, pattern, interval, runt, UART, LIN, CAN, SPI, IIC;
9), Bus decoding and protocol analysis: RS232 / UART, I2C, SPI, CAN, LIN;
10), Can save settings, waveforms, reference waveforms, CSV, images and other data formats.
11), 3 digit digital voltmeter and 6 digit frequency meter function;
12), Support 32 kinds of automatic measurement and statistical functions, real-time statistics minimum, maximum, standard deviation and other statistical information;
13), Two sets of digital voltmeter functions;
14), Support threshold test to realize free measurement on screen;
15), rich SCPI remote control instructions;
16), Provides several types of peripheral interfaces: Host / USB Device.

The body was quite small, maybe I could remove the psu and put lithium battery with linear regulator in it.
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Well, that one is good for about 20 MHz analogue, 25 Mhz if waveform fidelity isn't too important. Never kid yourself, with sampling you actually do need about 5x the bandwidth that an analogue scope has. A 100 MHz scope good for 100 MHz analogue would be rated at 500 MHz by any reputable manufacturer. So do not believe what they claim as there will be special test conditions used. A 100 MHz digital would capture a 100 MHz sine (a touch lower), but with zero detail. It would look perfect even if there were significant harmonics or distortion. The entire point of examining a signal is to see distortion. Anyway, it would be much more expensive than you quoted for a half decent 100 MHz digital scope. Also pay attention to the input voltage range. The cheaper scopes have very low input voltage ratings.

A 1 GHz sampling is good, but it may be in bursts, or 1 GHz effective which isn't the same thing. In addition, the cheap scopes are blind much of the time, meaning you will miss transients. There is a very good reason the better scopes cost more.
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I ended buying the dso2d10, it was lighter that i anticipate and would be good to be battery operated. I only got one probe with it, but it was a 150mhz probe. I already build a battery pack using 6 2500mah li-ion fed to a pair of lm317/337 and lm7805. The psu supply +/-8v and +5v. I use the battery middle point as gnd, like on some diy headphone amplifier.

The psu have a pin labeled "tr". While looking for info on it, i got interesting stuff. The scope has been extensively modded, from unlocking 150mhz bandwidth to even overocking the cpu/ram. The 150mhz unlock possible because the 100hz model and 150hz have exact same model. Even some cheaper dso2c10 without frequency generator sometimes got the hardware.

In case you are interested for a quick peek, here the forum link discussing the mods:
The even generate a full schematics on the scope including component and even the ic used.

I do not understand about oscilloscope bandwidth, but someone at least attempt to do it. Here is the youtube video on it incase you are interested:
I think i would not use 100mhz amyway. The highest i would need mainly topped around 30hz for square wave pwm signal, or just around 600hz sine wave.

I think this scope was quite decent for an entry level/hobbyist scope considering the price tag.
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Hantek isn't the worst at least.

Overclocking the CPU raises the running temperature and current consumption. That means it will fail sooner (on average). Higher rated speed CPU chips with the same number are rated that way because they run cooler at the higher frequencies. They are graded that way. If you don't need the speed, return it to normal. If you do, buy the rated product. But, whatever, you as you do.

100 MHz will get you good analogue performance up to 20 MHz fundamental. Good enough for most audio work. Certainly good enough for your very low frequency work.

The other thing to look at is sensitivity, and noise level. I'll leave that to you to check out. The other thing most people miss are how well it captures transients. Most scopes are "blind" much of the time, so you miss them many times. They capture, then process (they are blind) and sometimes send info for display (blind again). The better scopes use separate processors to handle acquisition as their only function. Those will catch transients a lot more often, sometimes 100x or 1000x more often. Now that can make a difference to what you're doing.

Digital scopes are a lot more complicated than analogue ones ever were, and there are a ton of things that will impact what you see or capture.
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When I was working as bench tech, I could check CD RF patterns and Mini DV tape playback RF patterns (for tape path alignment) with a digital scope. The trick was to adjust the Persistence of the signal to the LCD, ok maybe not as intuitive as an analog scope but results were adequate. Also as a bonus the ability to save 'scope settings on USB stick was super handy.
Processor binning was the main corcern when overclocking since my amd 486 to early core(i3/5/7) era. The popular topic back then was processor batch hunting for overclocking. Current semiconductor process has improved much on persistency, they graded mostly by putting limitation on driver used or disabling some core block. Similar how hantek/rigol use the same hardware and put limitation on lower model to simplify and cost saving by production volume amd design process. Even my hantek dso2d10 mainboard has dso2d15 written on the pcb.

Elctronics parts mostly degrade because of heat, the molecular structure degradation, usually metal oxidation like on the moafet gate, accelerated exponentially at higher temperature. Another problem is statics/magnetics field which could alter the ion structure on metal. I take chemistry undergraduate major before transfering to economy. We accelerate reaction by heating just below a point where another alternate(unwated) reaction happened.

For the limited noise capturing capability, mostly on cheaper scope, i think from dac resolution and processing capability. Digital conversion use range, which smooth out the result. The 12bit dac used in hantek only provide 4096 steps/resolution far from analog which could be any number limited just by the display which you could zoom in anyway. The time resolution limited by the processing capability, set by the lowest link. Overclocking could help a bit, but would not help much. Only the interface was faster, i already test the temporal overclock script.

The hantek only have single processing unit, just like you explained. If this is a pc, mine got integrated graphic adapter with no pci adapter. Even if i replace the dac ic to 14bit(i have checked the dac datasheet and this was possible) and overclocking, the cpu and memory could not handle the output.

My unit was noisy, at first i think it was from the smps. After i check the dac, i think the noise was the similar pattern with the noise chart presented. So both was the problem.

Comparing digital to analog scope from my standpoint is similar to analog vs digital camera at around 2010s. I know that i need to spend a lot for a good digital one. I am not used to the display of digital scope, back then all of lab instrument except for the digital scale was analog. Anyway, i am putting heatsinks on 3 biggest ic on my scope for longevity. The heatsink was low profile high performance one used for hi wattage leds. While there is a fan, i think it was not a good idea to put a fan near sensitive circuitry.

Thanks for the tips, i just check the manual on my unit that it do have the display persistence options(unlimited, 5s, 10s, and 30s). Before i just thinking of using the saving measurement feature.

I need portable for two reason, first of course for mobility and second because if it was not battery powered i would need differential probe to check ac main voltage. Here, a set of the cheapest differential probe cost more than the scope itself.

I might buy the cheapest analog i could buy, even used, as needed. I think, the digital was enough for a snapshot. At least maybe I could get away with exploding mosfet, lol. I saw a youtube vid on someone investigating burned mosfet on class d amp with diy arduino scope. No actual use for that unit aside from learning how to use a scope. A single probe cost $20, the arduino scope was just $8.
Yeah i could just use substrations.

Finally i could migrate to esp32, the timing implementation was diffrent and the documentation was scarce.

So much i could explore with this scope, like rise/fall time of the pc817 octocoupler i use which is too slow(this is the cause of igbt overheating) and many more. One thing that i hate is the interface bug, when i zoom in and zoom out the wave is missing. Only the wave section that i zoom in avaliable after i zoom out. I hope hantek fix this in the next firmware update, its so annoying.
Mosfet will not work for this, I gave up long ago. Everyone use igbt anyway. Now I use 25n120, way more rugged than mosfet. The vfd should work using single or three phase main and the output is three phase. Three phase induction motor got better efficiency and starting torque, it is widely used as it was cheaper and robust. If i am not wrong, more than 80% use induction motor in industry worldwide.

For the first prototype just between arduino and 3 ir2110(half bridge mosfet driver). Now i switch to esp32 for microcontroller and fan73892(3 ch driver). I ditch pc817 for adum1201(digital isolator), with max 100ns propagation delay and 40ns pulse width distortion. I plan to build delay and maybe pulse width tester using arduino to match all six digital pwm ch. Propagation delay was too close to the current deadtime delay i set.

I also add braking circuit, output voltage sensing circuit, dc bus voltage monitor and shunt, current sensing and limiter in high voltage section. All of them feeding and receiving data from mcu and need to be isolated. I prefer control from mcu than analog, it is easier for me.

The chinese vfd didn't have galvanic isolation, the overvoltage protection did not work either, component packed too close. That thing is dangerous.