Random issue with GW Instek GDS-1074B

[MacMan72]

I recently picked up a used GDS-1074B that has a weird random issue. The scope works perfectly most of the time but occasionally it will start to randomly change settings and screens, like there is a ghost in the machine or someone is remotely operating it. When it does this, the buttons become unresponsive and I have to turn it off and back on again to get control back. I’m thinking maybe there’s a problem with the front control panel or maybe excessive ripple from the power supply. Has anybody come across this type of behavior before or have any suggestions on where to start troubleshooting it? I’ve looked around on the internet for a service manual but I can’t find anything. Does anyone happen to have one or know where I can get one from?
Thanks in advance for your help

[tautech]

Welcome to the forum.

Check for a sticking membrane button.

[MacMan72]

Thank you, long time listener, first time caller… that’s a great idea, thank you, I’ll start there

[nctnico]

Maybe try another mains power cable as well. My GW Instek GDS2204E would behave erratic or even reset itself and it turned out the mains power cable didn't make good contact. But your symptoms look like the buttons are the first thing to check.

[MacMan72]

Yeah, I had that same thought so I did replace the mains power cable which I thought fixed the problem but it came back the next day. I’m wondering if maybe one of the solder joints for the mains plug to the PS is a cold solder joint, or maybe cracked?

[MacMan72]

I took the unit apart and cleaned all the button membranes and the contacts on the front panel so we’ll see how that goes. I also checked all the caps on the power supply board with a de-5000. Some of the capacitors were reading double the rated capacitance with the ESR value reading 0.0 (used 120 Hz). Does that mean there are other nearby components that are getting turned on and messing with the reading or that the caps are going bad? Either way, I ordered new low esr Nichicon capacitors and I’m going to recap the whole thing since there are only a few and they are all generic questionable quality caps. Is there any reason not to use low esr caps? I’m a mechanical engineer by trade so I have basic circuit theory knowledge but some of the more advanced concepts I’ve yet to learn.

[tautech]

I took the unit apart and cleaned all the button membranes and the contacts on the front panel so we’ll see how that goes. I also checked all the caps on the power supply board with a de-5000. Some of the capacitors were reading double the rated capacitance with the ESR value reading 0.0 (used 120 Hz). Does that mean there are other nearby components that are getting turned on and messing with the reading or that the caps are going bad? Either way, I ordered new low esr Nichicon capacitors and I’m going to recap the whole thing since there are only a few and they are all generic questionable quality caps. Is there any reason not to use low esr caps? I’m a mechanical engineer by trade so I have basic circuit theory knowledge but some of the more advanced concepts I’ve yet to learn.

SMPS, right ?
Use low ESR caps. Only the HV DC cap sees ~100 Hz while all the others are subjected to 20+ kHz !

[MacMan72]

Yeah, pretty sure it’s smps. (I’m an EE noob, be gentle)
So, should I be testing the caps at a higher frequency? I gotta admit, Dave confused me in his video on this concept.

[nctnico]

I took the unit apart and cleaned all the button membranes and the contacts on the front panel so we’ll see how that goes. I also checked all the caps on the power supply board with a de-5000. Some of the capacitors were reading double the rated capacitance with the ESR value reading 0.0 (used 120 Hz). Does that mean there are other nearby components that are getting turned on and messing with the reading or that the caps are going bad? Either way, I ordered new low esr Nichicon capacitors and I’m going to recap the whole thing since there are only a few and they are all generic questionable quality caps. Is there any reason not to use low esr caps? I’m a mechanical engineer by trade so I have basic circuit theory knowledge but some of the more advanced concepts I’ve yet to learn.

If you measured the capacitors in circuit, then you won't get a valid reading. My strong advise is to leave the power supply as it is. You'll likely do more damage than good. IIRC the PSU is a single sided board so it is easy to damage. And you'll want to make sure the board gets soldered properly as to not create a fire hazzard. In your situation, I'd check the solder joints on the mains connector using a magnifier glass and if these are OK, then just close the unit back up. IOW: if it ain't broken, don't ruin it.

[MacMan72]

Yeah, that’s good advice, thank you. If the glitch doesn’t happen again, I think I’ll follow that rule. I’ve gotten burned before “fixing” stuff that wasn’t broken.