Please don't tell me I need a scope to fix my scope
Please don't tell me I need a scope to fix my scopeI fixed a lot of power supplies with just a multimeter. I don't even have a scope. Most of the time, I can't even get a schematic. I did, however, learn a lot just trying a bunch of different things.
If your fuse is fine, do you get roughly 1.4 x mains AC voltage on the bridge rectifier? If yes, check C8, that might be your SMPS startup cap. It should be in the 16V DC range across the capacitor? The SMPS IC should be around there or on the backside of the pcb? If you can find the SMPS IC, you should be able to find a datasheet.
Also, check the 5V standby rail to see if you have that present.
The dropper resistor/s from HV DC to the IC supply should be checked too. (startup supply)
The dropper resistor/s from HV DC to the IC supply should be checked too. (startup supply)What is scary is that 6 years ago when I started in electronics, I understood none of the above. If it wasn't an obviously bad cap, it went to the "fix later" pile. Now I actually understand it, somewhat. Yes, I'm a slow learner.
Learning how to fix a SMPS power supply and its troubleshooting process is one skill that will help you fix a lot of things throughout life. Plus, it is a lot more satisfying replacing one broken component than doing the board swap that everyone including manufacturers do nowadays.
Please don't tell me I need a scope to fix my scopeI fixed a lot of power supplies with just a multimeter. I don't even have a scope. Most of the time, I can't even get a schematic. I did, however, learn a lot just trying a bunch of different things.
If your fuse is fine, do you get roughly 1.4 x mains AC voltage on the bridge rectifier? If yes, check C8, that might be your SMPS startup cap. It should be in the 16V DC range across the capacitor? The SMPS IC should be around there or on the backside of the pcb? If you can find the SMPS IC, you should be able to find a datasheet.
Also, check the 5V standby rail to see if you have that present.+1 ^^^
Once you've identified the SMPS IC and got the datasheet (get a few different versions if you can find them) look for typical application schematics normally listed near the end. These are so often followed very closely by manufacturers and really help if they are the same as your PSU.
Other failure points are the rectifying diode from the Aux primary winding that supplies the "running" voltage for the SMPS IC.
The dropper resistor/s from HV DC to the IC supply should be checked too. (startup supply)
Without a scope to check that's OK while supplying the IC with a LV external PSU (safest method) be sure to supply mains power via a dim bulb tester to limit the current available and prevent any fry ups.
You'll need to follow the PSU pin outs to the rest of the DSO and attempt to find it.
Hopefully the PCB has an overlay on both the PSU and the DSO mainboard.
Not all SMPS use a standby to start, in some cases the SMPS is just ticking away ready to supply full load when the mainboard tells it to.(turns on)
Strands of the PSU ribbon cable will be paralleled to share the current so looking at the "big" picture will help you understand what's going on there.
You'll need to follow the PSU pin outs to the rest of the DSO and attempt to find it.
Hopefully the PCB has an overlay on both the PSU and the DSO mainboard.
Not all SMPS use a standby to start, in some cases the SMPS is just ticking away ready to supply full load when the mainboard tells it to.(turns on)
Strands of the PSU ribbon cable will be paralleled to share the current so looking at the "big" picture will help you understand what's going on there.
Ok, with the DSO main board plugged in, all voltages on the ribbon connector are zero or floating a little (sub 0.05V DC). No obvious voltage markers on the main board either, just component legends.
My DC voltage reference for all measurements is the -ve side of the HV cap. Not sure if this is good practice...
My DC voltage reference for all measurements is the -ve side of the HV cap. Not sure if this is good practice...
If you've confirmed -ve side of the HV cap is common with the secondary 0V then that should be fine, on P7 of the datasheet it seems that topography is used.
If you've confirmed -ve side of the HV cap is common with the secondary 0V then that should be fine, on P7 of the datasheet it seems that topography is used.
There's enough info in the datasheet to determine the likely fault and without the PSU in front of me the main suspects I have: Opto, diode, cap and resistors in relation to the Control pin or the TOP224P itself.
Not a lot to check with a DMM and if the passives are OK suspect the opto or TOP224P.
You should be able to confirm if the opto is OK with your DMM, that just leaves the TOP224P.
One more thing.....check the transformer primary is not OC.
retiredcaps - same boat, just dipping the toe into smps...
... would anyone have access to the power supply schematic for this beast?
Mains voltage here is 230V AC ish and I am getting 315V DC across the HV filter cap C5 (PCB ref). 0V DC across C8 (PCB ref).
Ok, with the DSO main board plugged in, all voltages on the ribbon connector are zero or floating a little (sub 0.05V DC). No obvious voltage markers on the main board either, just component legends.
My DC voltage reference for all measurements is the -ve side of the HV cap. Not sure if this is good practice...
The attached schematic may be sufficiently similar to be of use:
Ok, with the DSO main board plugged in, all voltages on the ribbon connector are zero or floating a little (sub 0.05V DC). No obvious voltage markers on the main board either, just component legends.
My DC voltage reference for all measurements is the -ve side of the HV cap. Not sure if this is good practice...
Standby Supply
There's no standby supply on that. It's got a real power switch, so there's no need
All the other pins are unmarked on the schematic.