0

To give a bit of background, I'm trying to fix a friend's NeoGeo CD console which doesn't read disc: The disc doesn't spin at all, the laser assembly and lens are moving, the diode turns on. But it seems it didn't detect any disc and so never make it spin. The spinning motor seems to work, applying 1 V makes it turns. The driver IC seems Ok too (it drives the carriage motor correctly).

My friend thought the laser assembly was bad, and order a new one and so asked me to replace it. This replacement comes from Ali Express, and even if it uses the same SHARP IC, the PCB and wiring on it doesn't seems to be the same as the original one. So it might be bad, as long as the original one. But with either one or the other, there's no spinning at all.

But after making some research on google, I found an old thread post of someone having similar issue (disc not spinning), and the guy said he founded a bad transistor (didn't say which one) on the cd drive controller board (he did have another working NeoGeo and could swap controller board to confirm that the issue comes from it).

So I went ahead and try to test (in circuit ...) with a DMM (in diode mode) the 5 transistors on the PCB, and found one suspicious, in a SOT23 package (sorry for the picture quality, I should have clean the board):

enter image description here

With the diode tester, I find only a connection between 2 pins of the transistor. And I couldn't find what kind of transistor this one is (I know it can be hard with smd marking).

Fortunately, there's another one on the board, which I could test (and which seems to be OK):

enter image description here

I'm absolutely not an expert, so when I tried to identify the transistor and its pinout, so I'm not sure at all that this is correct. It seems it's an NPN, but the pinout seems uncommon (as far as I know most SOT23 transistor has a common pinout?). Also what I think it's the base is connected to ground (on both transistors), which also seems uncommon (common base?).

Considering it's an SOT23, I don't think I can easily unsolder it from the board (I do have a cheap hot air desoldering station), test it, and put it back if it's OK. I would preferred to replace it with an equivalent TO92, if it's worth it. Also I don't want to do any useless damage to the PCB.

toolic
  • 5,637
  • 5
  • 20
  • 33
Pierre-Laurent Chambert
  • 1
  • 1
  • 1
    Welcome! _”Considering it's an sot23, I don't think I can easily unsolder it from the board”_ What makes you think that? It can be unsoldered from the board within seconds with a soldering iron and a pair of tweezers. What makes you believe this device is defective in the first place? – winny Feb 26 '23 at 11:54
  • From the "64", there are several items it could be listed in [THE SMD CODEBOOK](https://www.sphere.bc.ca/download/smd-codebook.pdf) (PDF). – Andrew Morton Feb 26 '23 at 12:28
  • IMO, it'll be *harder* to replace it with a TO92 package. If you remove the solder from the pads with some solder wick, clean with a bit of isopropanol, and tin one pad, you can tack the new transistor onto that and then solder the other pins, then go back to the first one if it needs a bit of tidying up. Maybe have a practise first on a different PCB so that when you do it for real you don't heat up the pads too much and damage them. – Andrew Morton Feb 26 '23 at 12:33
  • Most SOT-23 BJTs, but not all, have the collector as the pin on its own, with the base and emitter on the side with two pins. The base is usually pin 1 (bottom left pin in your second picture) and emitter pin 2, but "flipped" pinouts, with pins 1 and 2 swapped, are fairly common--sometimes you can get the same exact device in both pinouts. Pin 3 (the one on its own) is almost always the collector (or drain, for MOSFETs), though, because of the geometry of the device. (The collector/drain is the backside of the silicon wafer, which is directly bonded to a metal pad connected to pin 3.) – Hearth Feb 26 '23 at 16:08
  • start by cleaning the board and testing ... the top picture shows some contamination – jsotola Feb 26 '23 at 19:09
  • My fear is to damage the pcb (which seems fragile) and/or the transistor (which might be good) if I try to unsolder it and test it out of the board. I am focusing on it it's because of the diode test,I'm not sure if it's reliable. I looked at the SMD codebook, the 64 can reference a digital transistor. What I think was the base, would be the emitter (to ground). I have to measure voltage at the input and ouput, as it can be a simple inverter. I'll also clean the board with some isopropyl alcool and see if it makes a difference (could it be that easy?) Thank you all! – Pierre-Laurent Chambert Feb 27 '23 at 07:53
  • @Pierre-LaurentChambert You can get an SMD practice board for €10 or less so that you can get good enough before trying soldering/desoldering on the actual board. – Andrew Morton Feb 28 '23 at 18:10

0 Answers0