Lexmark 41X1202 Powersupply 24v rail at 15v - Page 1
Forum section: electronics / repair

author: Der_M
author_rank: Newbie
timestamp: January 05, 2021, 08:36:19 pm
content: Hi,I’m currently troubleshooting a Lexmark 41X1202 Printer Power Supply. Problem, the +24V rail is at 15,09v.Well, first glance, feedback, but I measured the resistors in this area, found no problems. As their is no / too low voltage for the optocouplers I went to primary side. I already replaced the NCP1246 and as I blow the fuse during troubleshooting I also blow the main 15J65 Mosfet which is also replaced. Oh and also replace both optocouplers (I’m still not sure why they use two). After this I am back at the 15,09V output. Of course there is no schematic online.NCP1246https://www.onsemi.com/pub/Collateral/NCP1246-D.PDFVCC=fluctuating between 10-11v = OKFeedback Pin = fluctuating between 0,192-0,196vLatch = 1,141vFeedbackTop PC817 LED side=0vTop PC817 Feedback side = 1,141Bottom PC817 LED side= 0,3 – 0,7vBottom PC817 Feedback side = 0,194LM359GVCC = 0v is connected to 9561AGH mosfet, so most likely OKAnyone an idea?Many thanks!Matthias

author: m k
author_rank: Super Contributor
timestamp: January 12, 2021, 07:20:17 pm
content: Why VCC = 0v is most likely OK?Two optos are usually two different feedbacks.If top side led is 0V it's not feeding back very much.

author: fzabkar
author_rank: Super Contributor
timestamp: January 12, 2021, 07:38:48 pm
content: ISTM that the Vcc for the NCP1246 is too low. I would suspect C1 in the application example on page 2 of the datasheet. IME that is the usual culprit in these types of circuits.

author: Der_M
author_rank: Newbie
timestamp: January 12, 2021, 07:50:58 pm
content: Hi both,many thanks for your replies!@m kthink the LM359G is not directly connected to the feedback circuit. The mosfet switches the "+24-SW" rail, where the Opamp's VCC is connected to.@fzabkarIn the datasheet on page 5 I found:ELECTRICAL CHARACTERISTICS (For typical values TJ = 25°C, for min/max values TJ = −40°C to +125°C, VHV = 125 V,VCC = 11 Vunless otherwise notedThis is why I think it is ok. But nevertheless I will test / replace it.Will come back to you.Many thanks again!

author: fzabkar
author_rank: Super Contributor
timestamp: January 12, 2021, 08:49:11 pm
content: The 18-pin connector has a 24V pin and several 24V-SW pins. The latter appear to be switched by the 24V-SW ON/OFF pin. Are you measuring the correct pin?The datasheet is suggesting to me that your IC is stuck between VCC(on) and VCC(min). This in turn suggests that the auxiliary voltage source (D3 and C1 in datasheet) is not coming up to spec. Could it be that this is the way that the circuit behaves in standby mode

author: Der_M
author_rank: Newbie
timestamp: January 13, 2021, 07:05:05 pm
content: You are absolutely right with your message.So first, NCP is at VCC(Min). I checked the VCC cap for the NCP, it is fine.But I connected the ps to the printer, which is bit scary because of the construction (have to turn the printer 90° and place the PS on a paperbox) but anyway.After connecting it the, +24 rail (which was 15v) was at 0V.and the "+24V-SW" as well, which is of course swichted via the mosfet.So in connected state, there no voltage on any pin, what so ever.The power button of the printer (tactile switch) is connected to a micro, so the printer definetly needs, I assume, the +24V rail to function.So right now I have no idea what to check next.

author: fzabkar
author_rank: Super Contributor
timestamp: January 13, 2021, 10:02:16 pm
content: Could D3 be open or leaky? Is the transformer tap open?

author: m k
author_rank: Super Contributor
timestamp: January 14, 2021, 04:16:09 pm
content: My guess is that +24V is a standby voltage and 9651 connects it fully when button is pressed.On/off pin seems to be going elsewhere also, could be an indicator of some sort.Have you tried plugging it in while button is pressed?It should try something.

author: Der_M
author_rank: Newbie
timestamp: January 15, 2021, 09:17:22 pm
content: Hi both,thanks for your posts.Just checked D3 = D107 has forward voltage of ~0,5v, reverse nothing.Reistance between blue = 0,1 Ohm, black =0,2 Ohm.Btw, the transistor for second NCP VCC storage cap is on, so both caps are at the same 10-11V. FB5, R113 is also good.@ mkVoltage drops directly to 0V once connected, no matter if power is pressed.Thanks again!Matthias

author: m k
author_rank: Super Contributor
timestamp: January 16, 2021, 01:11:28 pm
content: What kind of instruments you have?I just bought a cheap battery powered mini scope, no need to worry so much how hot side is measured.(unluckily it was a kit, own fault I guess)When you had it unloaded you got that 15V.Continue from there.Use small dummy load, a watt or two, so that you can see how +24V operates.Add load if voltage goes up.Typical, if possible, operation of LM358 is VCO, maybe you should see some pulsing opto leds.Input of that other opto was 0V, maybe it's from on/off switch, or one outcome of your problem.

author: Der_M
author_rank: Newbie
timestamp: January 22, 2021, 08:22:46 pm
content: Hi,I'm very sorry for the delayed response. I totally missed the mail notification about the reply.Hope I got your message right.I had a look at the optos with the oscilloscope.First picture top opto, second picture bottom opto (the spike was me disconnecting the probes).Many thanks!

author: m k
author_rank: Super Contributor
timestamp: January 23, 2021, 04:52:12 pm
content: Don't worry about the time.Have you said somewhere that LM358G in the picture is wrong?Anyway, if its power is from switched 24V you can forget it for now.What situation the picture is, mains connection, extra load?How 15V behaves during that time?If you do a picture of it the horizontal time is so long that you can trig it like you did with the opto, two signal channels are not needed.I understand the circuit so that upper opto changes 2nd primary winding sourced voltage by activating ZD106 but can't say where it goes, though it seems to finally go to NCP1246 Vcc.Lower opto seems to go to NCP1246 pin 1 or 2, if it's FB then fine, it's not objecting and latch was measured earlier as fine.Have you checked that HV is ok?Hot side measurements would give you more info but don't try anything fancy, your possible second patient will make you sad.You have those original optos, use them and make a probe extension, use both and make it "AC" coupled.