Hantek 5000 series, correction of AC coupling issues

[steveod21]

Many people seem to be interested in hacking their Hantek scopes to get 200MHz when they were bought as 70 or 100MHz.  My problem concerned the bottom end of the frequency range.

I've just received a Hantek DSO5070P scope and in testing it out I found that the AC coupling was rather strange. The cutoff (-3dB) was a rather unusual 17Hz although the spec says 10Hz or 5Hz depending on where you look! 
Neither of these is what I'd call normal and most scopes have typically had a 100nF capacitor which with the I/P Z of 1MR will give a -3dB point of about 1.6Hz.  Also on the Hantek, strangely, I noted quite strong peaking (+1.5dB) at approx 100Hz when AC coupled. Both beams were pretty much identical.

I searched for any 'previous' on this and found that it had been noticed some time ago; I think the user names were 'Snoopy' and 'RF-Loop'  . (Other issues were raised at the same time and I need to take time to see if these are present in my machine). I couldn't see a solution posted to the AC coupling issues although it is always possible use DC coupling and connect an external 100nF capacitor if required.

I thought I'd look a little closer and happened on some excellent schematics posted by 'Tinhead' (thank you!) which helped enormously. Looking at the first image the coupling component for channel 1 is C01-8. This has a value of 15nF which looked suspiciously low. As a crude test I fitted a 100nF in parallel for channel 1 only.  Not ideal as it was a leaded component.  I thought it would help the -3dB point, but had no idea of the effect, if any, on the peaking problem.  As it happens, the peaking was much reduced too - my luck was in 

Image 1 is the I/P stage. Note the coupling cap which has a solid state relay to short it for DC.
Image 2 is the scope screen where both channels are AC coupled with a signal of 14V p-p @ 1kHz.
Image 3 shows peaking at 100Hz on the unmodified channel 2. Channel 1 doesn't peak significantly at any frequency.
Image 4 is at 2Hz. Channel 1 is circa -3dB. Compare to channel 2 which has all but given up.
Image 5 shows the 100nF 250V cap in place. Later I modified channel 2 similarly (though SM caps would be neater).
Image 6 is shows both channels AC coupled with the -3dB point at a more acceptable 2Hz.

[snoopy]

Well done !! Yes the AC coupling has always been a bane in this scope so I always switched it off The leaded components you used is probably the best solution as trying to find an NPO ceramic or MKT smd cap of sufficient value and voltage is probably impossible. They should have used a suitable leaded component in the first place but they compromised with a small value SMD and this resulted in an unusable feature for low frequency work. This is a mod I will look at doing if I keep the scope

[steveod21]

Hi Snoopy! -  I name checked you in my text and I think your post was 5 years ago - maybe more.

You're probably right about an SM cap being too large to sit over the existing device so that excuses my bodge! 
Please let me know if you do the mod if it has the same rewards as I gained.

(I've only had the scope 2 days(!) but I do hate imperfect products.  I often risk breaking things by taking them apart to 'fix' them. Still, too late to change now, I've been doing it for years!).

[snoopy]

Hi stevo

Last night I had some time and opened up the Hantek and added 0.47nF 100V MKT caps across the existing 15 nF smd caps. Now the AC coupling is flat down to 1Hz. No problems

cheers