A question for other users of this scope:
I'm slowly getting back into electronics after a 3-year break. My scope was stored (in "normal" room conditions) unused for a while and now it developed a series of issues which are probably inter-related
.
First thing I've noticed is that the MC34063A is getting quite hot.
The stock Chinese part had issues back when the scope was new. Basically, it would start switching, then stop at around 50% of transition, take a short break, drink a bit of coffee and then complete the switch. Changing the chip to an Onsemi part fixed the issue. For some reason I don't remember now, I also replaced the R33 current-limiting resistor with a short. Back when I was using the scope, things seemed to be working fine for at least 1 year if not more.
Now, on the other hand, the chip is getting really hot and is untouchable when the scope is connected to the PSU. After a while, it will stop working and the voltages on screen drift up to infinity. If the scope is disconnected from the PSU, the chip remains cool. A while later, the Onsemi regulator died completely, so I replaced it with another one. At the same time, I removed the short and put pins for a multimeter in its place. One thing I did notice is that when I have multimeter in series, it stabilizes the MC and the rest of the scope is using a varying amount of current that seems to drift between 3,5 mA and 5 mA. Of course, with such current, the regulator stays cool. If I replace the multimeter with a regular short, regulator gets hot again. The power supply is PCB-T115-J rev4. Unfortunately, I don't have another handy source of negative voltage, so I can't measure how much current the scope would like to take if there's no limiting from the MC.
The other issue I have is auto-calibration. Basically, the scope won't properly auto-calibrate and on some voltage ranges, the zero volts line has offset. I'm not really sure how to go around troubleshooting this thing. Also the rechargeable battery on the ADC board is at around 0.8 V. This is with the multimeter connected to the MC34063A's current limit pins. The ADC board is SDS1209 version.
The third issue is that vertical position adjustment is a bit jumpy. Basically, I'd adjust it in one direction and the line would go up and down and sometimes multiple steps for one step of encoder rotation. My gut feeling is that it's most certainly a bad encoder, but I thought I'd just mention it here for completeness's sake.
So does anyone have any ideas what could be causing any of the issues?