Please help identify this IC. Turned to help with narrow down the problem. - Page 1
Forum section: electronics / repair

author: grusus
author_rank: Regular Contributor
timestamp: March 29, 2025, 04:34:12 pm
content: Hi.Can someone help identify this IC? It is working fine when warmed with hotair. When in room temperature it is faulty.Here is picture. 1.2V is output from transistor and it powers microprocessor. Input voltage is 3.3V. When malfunctioning frequency is jumping between 10 and 50 kHz. When working fine frequency is stable and is approx 22kHz on pin 1. Also here is schematic that I manage to draw ( may not be 100% accurate). Thanks in advance!

author: fzabkar
author_rank: Super Contributor
timestamp: March 29, 2025, 06:23:44 pm
content: I think the IC could be a dual op amp, something like an LM358. The first amp is a voltage follower with a MOSFET pass transistor in the feedback loop. The second amp is an unused voltage follower with grounded input.The reference voltage is set by a 750 / 470 potential divider:Vout = 470K / (470K + 750K) x 3.3V = 1.27VFQD20N06 is an N-channel MOSFET:https://www.mouser.com/datasheet/2/149/FQD20N06-109180.pdfYou have drawn it with Drain and Source reversed.Edit:I don't understand why there would be a "frequency", unless the oscillation is symptomatic of a fault. As far as I can tell, this is a linear regulator. I'm assuming that pin #2 is connected to the 1.2V output. If not, then my guess is wrong.

author: jwet
author_rank: Frequent Contributor
timestamp: March 29, 2025, 07:32:35 pm
content: I agree with fzabkar- this is a linear regulator. The fact that its oscillating at 22 Khz probably means that a bypass is open- its a linear circuit.I looked at it a while and couldn't find an inductor assuming it was buck- its a weird low cost linear.

author: fzabkar
author_rank: Super Contributor
timestamp: March 29, 2025, 07:50:39 pm
content: Quote from: jwet on March 29, 2025, 07:32:35 pmI'm thinking that the circuit "works" at 22kHz but not at lower frequencies because the ripple would be lower at 22kHz.

author: grusus
author_rank: Regular Contributor
timestamp: March 29, 2025, 08:10:59 pm
content: Hi,Thanks for all replies.I've redrawn the schematic + double checked all connections (no vias under elements). Also attaching larger area picture for reference.I'm more like a electronics hobbyist. What should I check while searching for replacement? VCC ofc but sth else to be extra careful?BTW this is cheap chinese audio processor so I am not surprised that it is made cheaply :-)I think this 22kHz leaks from 3,3V as it is there also.Again thanks for help!

author: fzabkar
author_rank: Super Contributor
timestamp: March 29, 2025, 08:18:38 pm
content: Your circuit still doesn't show any voltage-sensing feedback from Vout back to the IC, but pin #2 is actually sitting at the correct voltage.

author: grusus
author_rank: Regular Contributor
timestamp: March 29, 2025, 08:24:33 pm
content: Yes. Right. My bad. I was redrawing schematic from another quickly made one and forgot one connection. Updated one in attachment.

author: fzabkar
author_rank: Super Contributor
timestamp: March 29, 2025, 08:34:01 pm
content: I would add a small ceramic capacitor in parallel with the existing capacitor between pins 1 and 2. Does the oscillation stop?If the 3.3V supply is the source of the oscillation, then the linear regulator is working correctly.

author: grusus
author_rank: Regular Contributor
timestamp: March 29, 2025, 08:42:44 pm
content: You suggest more like pF or nF? I have some through hole ceramic ones. I do not have any smd capacitors.

author: fzabkar
author_rank: Super Contributor
timestamp: March 29, 2025, 08:44:46 pm
content: Quote from: grusus on March 29, 2025, 08:42:44 pmActually, I reread your previous reply and edited my own. It looks like you need to troubleshoot the 3.3V supply. The linear regulator is faithfully following the input at pin #3, so if there is oscillation at the input, the same oscillation will appear at the output.

author: grusus
author_rank: Regular Contributor
timestamp: March 29, 2025, 08:50:25 pm
content: Hmmm, okay I will check the 3,3V supply but why if i heat up this Ic 1.2V supply starts working, and when it is working, I cool it down with Freezer spray and then it stops working.

author: fzabkar
author_rank: Super Contributor
timestamp: March 29, 2025, 09:03:16 pm
content: Quote from: grusus on March 29, 2025, 08:50:25 pmYes, that's odd, but the 3.3V supply should be clean. Where is it coming from?

author: grusus
author_rank: Regular Contributor
timestamp: March 29, 2025, 09:10:29 pm
content: Directly from this mosfet p20ne06L + electrolytic capacitor (picture). I will check 3.3V tomorrow and get back with some useful informations hopefully.

author: fzabkar
author_rank: Super Contributor
timestamp: March 29, 2025, 09:25:54 pm
content: Quote from: grusus on March 29, 2025, 09:10:29 pmThat appears to be another linear regulator with a TL431 as the voltage reference.https://www.ti.com/lit/ds/symlink/lm431.pdfhttp://markingcodes.com/search/c/n1f

author: jwet
author_rank: Frequent Contributor
timestamp: March 30, 2025, 12:33:58 am
content: On the first circuit- I can't see any reason why an LM358 wouldn't work fine here. After you pull the amp, check the caps for value and ESR if possible.That last schematic revision show 15 on the opamp and sensing on the 1.2 clinches it. Try freezing the caps rather than the part.

author: grusus
author_rank: Regular Contributor
timestamp: March 30, 2025, 07:07:22 pm
content: Hi. I have looked into the issue more today. Looks like main psu is generating 22 and -22 (stabilised to +15 and -15 by 7815 and 7915) and 5V -> 3.3V -> 1.2V. PSU is based on top234Y. +5V is sourced from MBR745 on secondary. Fluctuation of frequency is observed even on the hot side of optocoupler (and slight variation of voltage also).https://www.tme.eu/Document/465410bb8dcf836c6906cc86ffedaaed/top232.pdfI've tried to narrow down the broken element in 1,2 V supply and looks like FQD20n06 might be broken. when i touch it with my finger it starts working. even little drop of freezer spray on it makes it stop working. Tried to desolder and resolder it back - no change in behaviour. Capacitors in that area looks good.I've ordered FQD20n06 to swap it just in case.Any ideas?Attached some more pictures of psu.

author: fzabkar
author_rank: Super Contributor
timestamp: March 30, 2025, 07:25:20 pm
content: "I think this 22kHz leaks from 3,3V as it is there also."Doesn't this mean that the 3.3V supply is the source of your problem?Measure the amplitude of the oscillation on each side of the 750 ohm resistor.

author: grusus
author_rank: Regular Contributor
timestamp: March 30, 2025, 07:29:30 pm
content: Yes but this is everywhere, on every voltage rail that psu is generating (+-22 and 5V) looks like frequency of PSU. I am just thinking that maybe PSU is struggling to regulate when FQD20n06 is cold? or this TOP234Y is marginal (however cooling it down or heating has no impact)Oscillation is about 0.005 V AC

author: fzabkar
author_rank: Super Contributor
timestamp: March 30, 2025, 08:37:19 pm
content: OK, I would think that 5mV isn't anything to worry about. I was thinking that the ripple may have been substantial enough to affect the MCU. Sorry for the distraction.

author: grusus
author_rank: Regular Contributor
timestamp: March 30, 2025, 08:48:05 pm
content: No worries, Every hint is appreciated. :-) I am just thinking if swapping this FQD20n06 won't help, I will disconnect this 1.2V supply from 3.3V and inject 3.3 directly into it from bench supply? That way I can measure current of cold/warm 1.2V PSU and check if it affects or not main psu stability on cold start. What do you think?

author: fzabkar
author_rank: Super Contributor
timestamp: March 30, 2025, 08:54:42 pm
content: It's a 1.2V regulator, so does the output voltage drop when it misbehaves? If so, then compare the IC voltages when working and not working. If touching a component causes the circuit to stop working, then I would suspect a dry solder joint.

author: grusus
author_rank: Regular Contributor
timestamp: March 30, 2025, 09:00:08 pm
content: When it misbehaves voltage is jumping +/- 0,05V, on 1.2V rail. Similar on 3,3V 5V, and +/- 22V rails. As I said, it look like psu is failing to regulate for some reason. I doubt it is dry solder joint because i have reflowed everything in 1.2V supply area few days back and today again - I was thinking same - touch sensitive = dry solder joint. Or there is microcrack between pad and trace on pcb.

author: fzabkar
author_rank: Super Contributor
timestamp: March 30, 2025, 09:13:34 pm
content: It seems to me that the ground reference must be shifting rather than the supply rails. :-?

author: grusus
author_rank: Regular Contributor
timestamp: April 05, 2025, 05:30:38 pm
content: Hi.After replacing FQD20N06 it started behaving much better. But still sometimes it run into problem. I've replaced all 3 ceramic caps + 10uf tantalum one. After one hour in almost freezing temperatures it is starting right away. No more odd behaviour. Thank you all for your help!