Sigilent SDS 1052DL no AC coupling

[GeorgeLG]

The AC coupling on this scope does not work, I have to use a discrete capacitor at the end of the probe to keep AC waveforms vertically centered on the scope when there is a significant DC voltage component. I work on guitar tube amps which routinely have 500V of DC in the signal path. Does anyone have guidance for me on how I might fix this. I'm good at board level repair. My FW level is 5.06.02.19. I tried to fins any available FW update but they only list them for the plus version = SDS 1052DL+.

Thanks,

George

[tautech]

Welcome to the forum.

Your FW is the most current I can find here in an old Siglent site that's luckily still online for older products:
http://old.siglentamerica.com/gjjrj.aspx?id=15&page=1

What probes are you using for such elevated voltages ?
Even using stock 1x/10x probes on 10x is getting dangerously near their rated voltages and a moments inattention may have been used on 1x and damaged the scope's input that is only rated to 400V.

[GeorgeLG]

I have 3 probes:

P4100, 100x, 2KV

6100, 1;1, 10:1, 300V/600V

PB470 300V/600V

If I have damaged the input is that repairable on a scope like this?

George

[tautech]

I fished out a SDS1102CML+ that should have a very similar UI to your DL.
Sanity check, can yours show the probe comp signal in DC and AC coupling like these.
I'd like to see screenshots if I may but the forum doesn't permit .bmp files so we need convert them to .png which is piss simple with MS Paint. Open the .bmp file then Save As a .png back onto your pendrive.

[GeorgeLG]

I took the scope apart last night to check the AC coupling capacitors. When I saw your post I buttoned it back up before I got finished and checked the probe comp signal and the AC coupling worked. I then tried it on my circuit and it still works. Not sure whats going on but for now I do not have to use a separate coupling capacitor on my probe to center my AC waveforms and filter out the very large DC offset voltages. I used my 100x probe for this check.

Thanks,

George

[tautech]

I took the scope apart last night to check the AC coupling capacitors. When I saw your post I buttoned it back up before I got finished and checked the probe comp signal and the AC coupling worked. I then tried it on my circuit and it still works. Not sure whats going on but for now I do not have to use a separate coupling capacitor on my probe to center my AC waveforms and filter out the very large DC offset voltages. I used my 100x probe for this check.

Thanks,

George

Good, and for your own safety and that of your scope continue to use a 100x probe on elevated voltages.
I fish mine out for anything 150V+.

I checked the CML+ for relay clicks when switching from DC to AC coupling and didn't hear any so I believe the switching is done with silicon but there should be some schematics of the input stages of 1000 series Siglents here in the forum that we can refer to if you still have a fault. Of course it shouldn't be too hard to work it out if we can find the DC/AC coupling control signal and trace it to whatever part changes the input coupling path.

When I have a moment I'll have a hunt through the forum for circuits that have been posted.
If you still have an issue do you have hot air for SMD rework ?

[GeorgeLG]

I have a Hakko FR-301 desoldering tool and low temp rework solder/braids. One of these days in need to get a hot air station.

That schematic would be great.

Thanks,

George

[tautech]

That schematic would be great.

It's likely to be a CNL or CML or even earlier CN or CM 5'' display versions or even an old CA Atten DSO if you want to go looking yourself. Use the Search button and not the Search box.

I won't get onto having a look for some hours or maybe even tomorrow when we are due to get some rain as I'm busy trying to get some paint on the house while we are confined to barracks due to the beer virus.

[tautech]

From this old thread a pic of the SDS1000 front end and looks identical to a SDS1102CML+ PCB I have here:
https://www.eevblog.com/forum/reviews/siglent-sds1000-7_-oscilloscope-anatomy/



The input BNC is under the cover and the signal path runs past CA1 through RA1 then onto the relay common connection then right to the 9533 resistor and can then to the 4 pin U42.
AFAIK the relay is only used for switching another attenuation range in.
The large R's and C's are 500V rated to provide for the scopes 400V input rating.

It's not at all hard to signal trace the analog input with one working channel or another scope with just the Probe Comp signal and follow its path. I had a poke around on the CML+ PCB I have in the manner just described while unpowered and you can certainly get far enough to find a fault....if there is one.