Modifying my broken monitor power supply (WITH SCHEMATIC!) - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: August 25, 2023, 06:33:01 pm
content: This is somewhat a followup to this post:https://www.eevblog.com/forum/beginners/why-is-my-power-supply-broken-(with-schematic!)/Schematic attached.Background of previous post: Essentially with the previous post, I had found the fault on the monitor but damaged other components in the process.What I want to do: I've decided that instead of trying to further repair the monitor PSU, I instead want to cut it in half between the primary and secondary (image B1), but retain the PFC circuitry on the primary side (for use with other projects) and retain functionality of the 5V buck converter (on the secondary side).My understanding of the circuit: By cutting the board, I will be disabling the 5vSB power rail, but I need to leave transformer TB801s untouched to retain functionality of the PFC circuitry. Based onimage S20, it looks like I can simply disregard the PC803s associated circuitry, but that I do need to retain the functionality of PC801s and PC802s (both are optocouplers, circled inimage S20).Solution?: I'm thinking that I can simply replace the PC801s and PC802s transistors with resistors, to enable the PFC circuitry. Is this correct, and if so, what value of resistors should I use? I understand that by replacing PC801s and PC802s transistors with resistors that the PFC will always be active, wasting energy, which I understand.Further, to use of the secondary side, I simply plan to use an external 19v laptop power supply connected directly to the secondary side of the TM801s transformer (the one creating the 24v and 5.3v bucked supply rails). I'm thinking that the monitors other components (mainboard, CCFL backlight) will be perfectly content with getting 19v instead of the original 24v.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: August 25, 2023, 06:39:37 pm
content: I've already desoldered the components coloured in green on the attachedimage B2, to make it easier to cut the board in half. I'm pretty sure these are not required for the PFC associated circuitry. I have not yet removed anything else (specifically DB871 (5vSB diode), CYB805s (primary/secondary capacitor for TB801s), ZDT871 (TL41 for the 5vSB rail), PC801s + PC802s (optocouplers)).

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: August 26, 2023, 04:48:43 pm
content: Bump?Can I simply replace the PC801s and PC802s (optocouplers) transistors with resistors, to enable the PFC circuitry. Is this correct, and if so, what value of resistors should I use?

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 26, 2023, 05:12:50 pm
content: Don't remove the 5VSB parts, they also provide regulation to the auxiliary voltages.Just use as it is, or you'll cut it down, break it and later start another thread asking how to fix it.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: August 26, 2023, 05:31:28 pm
content: Quote from: DavidAlfa on August 26, 2023, 05:12:50 pmI have no interest in trying to fix this PSU board; I want to repurpose it for what is outlined in the main post.

author: magic
author_rank: Super Contributor
timestamp: August 26, 2023, 07:05:10 pm
content: PC801s is feedback from the 5Vsb rail to its primary side PWM controller. Simply shorting the output will look like 5Vsb is always too high - this should disable the controller and it should be harmless (look at the datasheet, there is probably a few mA current source feeding the FB pin internally). Or just remove the controller altogether.For PC802 pick something that delivers enough base current for QB801 to work without excessively decreasing emitter voltage. More resistance will cause emitter voltage to drop under load, which maybe could be useful protection from output short or other overcurrent. Pick something that works and don't overthink it too much.That being said, if you can repair it to the point that PFC works, perhaps you could get the 24V rail going too...

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 11, 2023, 09:31:06 pm
content: Bit of an update.I removed the RB802 resistor (it goes to the ICB801S IC VCC) and left the PC801s-transistor side unpopulated. Having a 4.7k ohm resistor + switch at the PC802s-transistor location allows me to turn on-and-off the 400V DC active PFC (measured at the bulk storage capacitor BP803). When the 4.7k ohm resistor is not connected (therefore QB801 doesn't turn on), I still somehow get 160v DC (measured at the bulk storage capacitor BP803).I have 0 idea how I'm getting 160v DC when both ICB801S and ICP801S are seemingly disconnected (per the schematic)? ?? How??? Is there just more errors in the schematic?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 11, 2023, 09:58:22 pm
content: Doe, I'm dumb. I now see how I'm getting 160v DC... It's from the bridge rectifier.Anyway, I removed the ICB801S IC and I (obviously) still saw 160v DC, however I was no longer able to active the 400v DC of the active PFC. Not entirely sure how ICB801s works without VCC, maybe from RB812 (but that's a 68k resistor?)?Anyway I think this may be the end. I'm going to do some thermal camera shots and see if anything gets abnormally hot with my mods.Edit: Nothing overheats.I'm now considering removing ICB801S entirely and simply powering the ICP801S IC via a high voltage linear regulator. I do also plan to tinker with the RP809-RP814 voltage divider so that I can vary the DC output voltage. What will I do with a non-isolated 90w 200-400v active PFC power supply? No idea, but I think it's pretty cool to have. Ideas on what I can do with this, aside from converting non-PFC power supplies to ones with PFC?

author: magic
author_rank: Super Contributor
timestamp: September 12, 2023, 05:06:27 am
content: Okay, I haven't looked at that schematic in a few weeks.That being said, getting some voltage on active PFC output is normal. The APFC is a boost converter, with a switched inductor to ground and a diode going to the output capacitor. The diode works like a diode even without power to the PFC controller.

author: alligatorblues
author_rank: Regular Contributor
timestamp: September 14, 2023, 04:05:49 pm
content: Quote from: LooseJunkHater on September 11, 2023, 09:58:22 pmDisassemble the display panel and install LEDs. Then you can forget everything over 12VDC. Conversion also cuts power consumption by 2/3. You can get conversion kits on eBay. I've done about 10 conversions.