PC Engine LT with Bad Power

[NFG]

My PCE LT sat on a shelf for many years after the capacitors died and it stopped working.  When I finally got around to recapping it I found that there was damage done to the corner of the board where all the power is handled. 

Unfortunately this is a four-layer PCB with traces that are nigh impossible for me to follow.  I've been poking at it, using the excellent (but unfinished) guide to the power section on the [[https://www.retrosix.wiki/power-regulators-pc-engine-lt|Retrosix Wiki]].

According to Retrosix, power from the power switch goes through an M5291 chip, which drops the 9V input to 6.5V.  (I'm measuring ~16V at this point, which seems correct for an old wall wart PSU that's not under load.  All my other PSUs do similar, 16V where 9V should be, so this isn't part of the problem.)

I get V coming out of the M5291, but that's pretty much where it ends.  The Retrosix wiki is a bit vague about where, exactly, the power is going.  It seems to be going through the M5291 and then Q500, which is a PNP regulator which supplies power to the 24M05 regulator, which is where the V drops to a stable 5V and powers the rest of the system.

Here's an image from the R6 wiki:

And here's an image from my LT.  The yellow dots have V, the blue dots are where I think V should be, but there isn't any measured there.

There's no V drop on pin 1 of the M5291 (center chip), which is where 6.5V should be coming from, which makes sense because pin 5 (bottom right blue dot on the 5291 chip) is where it gets the comparison V from, and makes adjustments to the output on pin 1.  If it has nothing to compare, it can't drop the power to where it should be, is my theory so far.

And so I've got power right up until just before the 24M05 (the top-right-most blue dot), but I cannot figure out where the power is disappearing. 

Any ideas?

[NFG]

I got some help from Zaxour, and checked the v-drop on Q500, and it was well within spec (0.55V drop on pins 2 & 3).  While checking the next thing I stupidly did it with the power still connected, and I fried something. 

Before I worked my idiot magic it looked like this:



And after:



Zaxour guessed It might be a BFN21.

If anyone knows more specifically what this is, I'd love to hear from you. 

[Fix_Metal]

Just trying to help here, I don't have any insights nor I possess a PC Engine LT. A brainstorming post.

That N29 might be 82N29 (2SD1628?)

• which would make sense as far as I understand the circuit portion: basically, a glorified npn to activate the other one, which would make perfect sense activating the BFN21 [1].

On a side note I've recently discovered that Gemini AI can provide good hints and advices when troubleshooting, of course it hallucinates, and fails sometime, but it gave me pretty good kickstarting theories when I was done already with mine. I'll paste the conversation about it when asked "
Is there any reference for BN29 or BFN21 transistors (in the PC engine LT PSU)?"

Finding information on these components can be tricky because "BN29" is an SMD marking code rather than the full part number, and "BFN21" is a high-voltage part that is less common in modern consumer electronics but critical for the PC Engine LT's power and display logic.
1. BN29 Transistor (Marking Code)
The component marked "BN" or "BN29" on the PC Engine LT motherboard is typically the 2SD1628 NPN Bipolar Transistor.
 * Type: NPN Silicon Transistor
 * Package: PCP (SOT-89 compatible)
 * Key Specs: * V_{CEO}: 20V
  * I_C (Continuous): 5A
  * V_{CE(sat)}: Very low (typically 0.5V at 3A), making it ideal for the LT's switching power supply.
 * Role in LT: It is part of the high-current switching circuit that regulates the main power rails. If this transistor fails, the console will usually stay completely dead or fail to provide enough current to the backlight.
2. BFN21 Transistor
Unlike BN29, BFN21 is the actual part number. It is a PNP High-Voltage Transistor, often manufactured by Siemens or NXP.
 * Type: PNP Silicon High-Voltage Transistor
 * Package: SOT-89
 * Key Specs:
  * V_{CEO}: 300V
  * I_C: 200mA (0.2A)
  * f_T: 60 MHz
 * Role in LT: Because of its high voltage rating, this transistor is primarily involved in the LCD backlight inverter circuit or the video output stage. It handles the higher voltages required to strike and maintain the CCFL (Cold Cathode Fluorescent Lamp) in the original LT screen.
Comparison Table for Reference
| Component | Full Part Number | Type | Main Use Case in PC Engine LT |
|---|---|---|---|
| BN29 | 2SD1628 | NPN | DC-DC Step-down converter / Power Regulation |
| BFN21 | BFN21 | PNP | Backlight High-Voltage / Video Signal Logic |
Troubleshooting Tip
If you are replacing these because of a power failure, check the 1.25A fuse and the Matsushita TK1-6V relay first. These transistors usually only fail if there has been a major short circuit caused by leaking capacitors or if a "Center Negative" power supply was used by mistake.
Would you like me to help you find suitable modern equivalents for these parts if you are having trouble sourcing the originals?
PC Engine LT Repair: Complete Teardown and Recap
This video provides a complete teardown and repair of the PC Engine LT, which can help you locate the transistors on the board during a restoration.

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• https://www.alldatasheet.com/datasheet-pdf/view/413446/UTC/82N29.html

[1]

https://www.alldatasheet.com/datasheet-pdf/view/44539/SIEMENS/BFN21.html

[NFG]

Just trying to help here, I don't have any insights nor I possess a PC Engine LT. A brainstorming post.

Much appreciated.

That N29 might be 82N29 (2SD1628?)

• which would make sense as far as I understand the circuit portion: basically, a glorified npn to activate the other one, which would make perfect sense activating the BFN21 [1].

I like where you're going with this.

On a side note I've recently discovered that Gemini AI can provide good hints and advice

I hate, hate hate it that you've done something useful with AI.  =)

Now, can it tell you where to buy the BFN21?

[Fix_Metal]

I hate, hate hate it that you've done something useful with AI.  =)

Can't argue, I blame myself any time I use it now, lol.

Now, can it tell you where to buy the BFN21?

Well, yes, it seems in the SOT-89 packaging these seems pretty similar. You should be double checking datasheets to confirm, and source it on Aliexpress (or whatever it's most convenient for you).

Direct Equivalents (SOT-89 Package)
These transistors share the same physical footprint and electrical characteristics:

BF721: This is the most common direct equivalent. It is a PNP high-voltage transistor in the same SOT-89 package with a 300V rating.

BFN23: Also a PNP high-voltage transistor in the same package family, often used in similar circuits.

PBHV9040T: A high-voltage PNP transistor from Nexperia that can often serve as a modern replacement.