Help identifying component on 858D rework clone KSD858DP

[rsumner]

I'm new to the forum and just starting my journey into hobby electronics, so please excuse any of my ignorance in advance 

I bought a cheap 858D hot air station clone from Amazon about a month ago.  After using it a couple of times, I noticed the gun was no longer blowing air.  I got a new air gun unit from the seller on Amazon only to find the same problem.  The handheld will heat-up. but will quickly start to melt the plastic because it is blowing now air.

I had a look at the circuit board and noticed I had a fried component.  I looked at schematics online of other 858D clones, but none seem to match my layout. It seems to the CN manufacturer has switched to nearly 100% SMD components.  The identifier on the board is KSD858DP-F/C REV D with a date marking of 2021-05-27. 

Can anyone help me identify the 6-pin SMD chip marked "U1" in the picture so I can replace it?  In addition, I need the size of the cap at C17 and the resistor right above it in the pick, which is R30. 

I think I'm going to need to replace most of the resistors near this chip because R2 reads as a 1.003k resistor, but it's reading 3.5k. 

[bob91343]

I don't see any U1.

[rsumner]

It's in the 3rd pic to the left of the C7 cap.

[TheBay]

I can't believe a company is still making these clones with a removable handpiece, they are so dangerous as they have live mains voltage at the din socket. 

[rsumner]

@TheBay, oh, they're definitely dangerous.  The chip that is blown on my board is clearly involved in temp control of the heating element and fan control (both of which are contained in the hand piece).  What's crazy is that the fan control is completely broken in my case, but the heating element will still stay on.  If I were to leave it unattended, the hand piece would likely catch fire, but would definitely melt the entire casing of the hand piece. 

I've already replaced the air station with something more reliable, but interested in seeing if I can get this unit running as a backup.

My problem is the component that's in the U1 position on the circuit board is too badly damaged for me to determine what it was. Looking at the reverse engineered schematics for the 858 (https://blog.enbewe.de/posts/images/858D/858D_Schematic-big.png), I was thinking it was the SOT-223 package format of the 2N3904 transistor, which would be a PZT3904.

[TheBay]

U1 looks like a MOSFET to me, that part of the circuit looks like some form of DC-DC voltage regulation.
What part is U5?

[wraper]

Looks like buck converter IC with built in switch. Though we cannot tell which exactly without the marking.

[rsumner]

@TheBay, U5 is a LM358 dual-channel opamp.

Q1 is a P104T (couldn't find anything about it). but I snagged a pic of it. 

[wraper]

I was thinking it was the SOT-223 package format of the 2N3904 transistor, which would be a PZT3904.

It's sot-23-6. If input and output voltage is known, you can find a suitable replacement buck converter with the same pinout. There should be plenty of similar. Even if replacement has a different feedback reference voltage, you still can use it. But will need to adjust feedback voltage divider (R2, R3). https://antenna-dvb-t2.info/mark_smd_dcdc.php

I think I'm going to need to replace most of the resistors near this chip because R2 reads as a 1.003k resistor, but it's reading 3.5k.

R3 is 100k https://www.hobby-hour.com/electronics/smdcalc.php#smd_resistor_code

[rsumner]

I tested some of the pads around the burned IC and the first two pads on the bottom left are 41V.  The blog post that goes along with the schematic I posted earlier is here: https://blog.enbewe.de/posts/2018-06-21-saike-858d-original-schematic/.  They explain fan control control with influence from the front-panel potentiometer is controlled with three transistors, but clearly aren't that many here, so maybe @wraper is right and this is basically an adjustable buck converter.  Again, from the blog post above, the fan inside the hand held piece is rated for 24v, so the buck converter is giving us linear voltage based upon the potentiometer.

[wraper]

I tested some of the pads around the burned IC and the first two pads on the bottom left are 41V.  The blog post that goes along with the schematic I posted earlier is here: https://blog.enbewe.de/posts/2018-06-21-saike-858d-original-schematic/.  They explain fan control control with influence from the front-panel potentiometer is controlled with three transistors, but clearly aren't that many here, so maybe @wraper is right and this is basically an adjustable buck converter.  Again, from the blog post above, the fan inside the hand held piece is rated for 24v, so the buck converter is giving us linear voltage based upon the potentiometer.

No, output voltage is fixed. Fan speed is adjusted by Q1 on negative side of the fan.

[TheBay]

I tested some of the pads around the burned IC and the first two pads on the bottom left are 41V.  The blog post that goes along with the schematic I posted earlier is here: https://blog.enbewe.de/posts/2018-06-21-saike-858d-original-schematic/.  They explain fan control control with influence from the front-panel potentiometer is controlled with three transistors, but clearly aren't that many here, so maybe @wraper is right and this is basically an adjustable buck converter.  Again, from the blog post above, the fan inside the hand held piece is rated for 24v, so the buck converter is giving us linear voltage based upon the potentiometer.

Wrapper is right, it does look like a buck converter IC.
I originally thought it was a SOT-23-5 but looking closely it is a SOT-23-6.
I think you may have more issues than you anticipated even if you replace that IC and the other passives, when that IC blew it probably took out other things.

[rsumner]

Yeah, I agree that it looks like SOT-23-6 package.  Here's one that looks like it will fit the bill:  https://www.mouser.com/datasheet/2/268/MCP16331_Data_Sheet_20005308-1989955.pdf

It has input up to 50V and output at 24V. 

[TheBay]

I wouldn't just buy any random IC, you need to know the exact pinout and specifications of the one that was fitted.

[rsumner]

thanks @TheBay. I did remove the chip and the pads are pretty shot.   Maybe someone on this forum will have this 858D model board and they can have a look at the chip on their board.  I'll stick on the shelf until then.   

[wraper]

Yeah, I agree that it looks like SOT-23-6 package.  Here's one that looks like it will fit the bill:  https://www.mouser.com/datasheet/2/268/MCP16331_Data_Sheet_20005308-1989955.pdf

It has input up to 50V and output at 24V.

Seems suitable, even V_FB is right for 24V output.

[wraper]

I wouldn't just buy any random IC, you need to know the exact pinout and specifications of the one that was fitted.

I don't see something going wrong with such substitute. Pinout does match. Feedback divider is right for MCP16331 to get 24V out, output inductor is about right for it too. The only concern may be a crappy PCB layout for 500kHz.

[rsumner]

I usually use Mouser to by all my ICs and they won't have this one in-stock until next year.  The MCP16331 looks pretty versatile as a buck for 1/2 amp, so I'll probably snag a dozen or so of them for the bin to use in some DIY stuff.  Thanks to both @wraper and @TheBay for the feedback today -- I really appreciate it.

[TheBay]

I wouldn't just buy any random IC, you need to know the exact pinout and specifications of the one that was fitted.

I don't see something going wrong with such substitute. Pinout does match. Feedback divider is right for MCP16331 to get 24V out, output inductor is about right for it too. The only concern may be a crappy PCB layout for 500kHz.

Yes I agree looking at the feedback divider and inductor.

[bob91343]

This thread may turn out to be valuable to me.  I also have the 858, not sure who made it.  It has worked well for several years, although I don't use it much.  Should the unit fail I will see how closely it resembles the unit here.

[wraper]

I also suggest checking if D1 and D7 are not shorted. Measuring inductance of L1 (if you have an LCR meter, should be ~47uH). Maybe checking the fan from external power supply.

[ygi]

I wouldn't just buy any random IC, you need to know the exact pinout and specifications of the one that was fitted.

I don't see something going wrong with such substitute. Pinout does match. Feedback divider is right for MCP16331 to get 24V out, output inductor is about right for it too. The only concern may be a crappy PCB layout for 500kHz.

According to the above datasheet, it needs a 100µH inductor for 24V output (see p.19). Just to make it clear, I'm not qualified to discuss this, it's what's written in the chart. I figured it might be important and if not, well, better safe than sorry...

[DavidAlfa]

I recall there's a temperature fuse inside the heater, even on my $20  858D, so it won't catch fire so easily.

[wraper]

I recall there's a temperature fuse inside the heater, even on my $20  858D, so it won't catch fire so easily.

Often there is none in these cheap stations. If you search for replacement heater some do have it but most of them lack a visible thermal fuse.