Hey All - I have a thread going about a Firepower II I'm working on, but it was suggested by another Pinsider that I start a separate post about a related question that is more general to System 7/11 Power Boards and transistors.

I'm rebuilding the power supply, and I've run into a question with the transistors Q1 and Q3. Based on research, my understanding is that most System 3-11 boards (aside from the late series 11b and 11c power boards) used SDS201 and SDS202 Transistors. With those unavailable, the replacements frequently used are MJE15030 and MJE15031.

Due to the different pinouts, per the instructions online like here: https://www.greatplainselectronics.com/Downloads/W3-11_HV_Kit.pdf , on most boards you need to "cross" the legs on the transistor to get the proper flow. As noted in the instructions, however, on late model boards this was NOT needed, as they already used the 15030 and 15031 transistors.

Now, here's my issue: When I pulled the board and prepared to replace the transistors, I noticed that they're already 15030 and 15031. See attached image of my board. But they are NOT cross legged as instructed. This leads me to believe either (1) The original board was replaced with a late model System 11 board or (2) Someone else replaced the transistors on the original board, but did it wrong.

Part Number on the Board: 5765-09466-01 (no revision letter)

The game was actually playing fine for a while before I started having issues with fuses blowing (solenoid fuse, then eventually the display fuse). It's hard for me to believe it would have worked at all with improperly installed transistors, but I'm not sure.

Thoughts and suggestions welcome. I'm ready to replace the transistors, but want to do so properly! Thanks in advance.

FP2_PS_Trans (resized).jpg

No, good idea, I can take a look and see how it matches up. I really do think this is a non-late model board, just going by my gut, so I have to wonder if these are simply installed wrong. But again... wouldn't that cause all sorts of hell? So strange.

Okay, so using the schematics I have from my FP2 manual, and testing all of the traces so I know where everything goes, here's what I found:

All trace checking from Q1 and Q3 matches the original schematic and diagrams, meaning that my board must require the original setup for the transistors (EBC). Just for reference, based on the schematics (if someone wants to double-check me), here's how the positions trace out:

Q1
E -> C2, R3, 3J5 Pin 4 (100VDC)
B -> Z2, Q2 (collector)
C -> D3, Z1, C1(+), R2

Q3
E -> C4, R6, 3J5 Pin 3 (-100VDC)
B -> Z4, Q4 (collector)
C -> D4, Z3, C3(-), R5

The replacements that have to be used are BCE, and need to be wired as shown in the link from my original post. As far as I can tell, that means that whomever last replaced these with 15030 and 15031 Transistors did them wrong, and never crossed the legs. I can't believe the game even functioned like that, but it did it seems.

Anyone object to my logic before I go ahead and replace with the cross-leg method?

FP2_Trans_Schem (resized).jpg
FP2_Trans_Dia (resized).jpg

FP2_GPE_Install (resized).jpg

I don't see any jumpering or altering of the original traces, but maybe I missed something? If the traces were changed or altered, wouldn't the tests I did not match the schematics? Here are some pics, I apologize for the back not being pretty/clean, but I snapped these quick without cleaning it first. The previous work done on it wasn't pretty. I marked Q1 and Q3.

FP2_Power_Back1a (resized).jpg
FP2_Power_Back2 (resized).jpg

I checked the board/traces with a meter, nothing seems to be bridged that shouldn't be, but I'll clean it off some more and see if that reveals anything unexpected. Assuming nothing is bridged, should I go ahead and cross the legs of the MJE15030/MJE15031 as directed?

I pulled the existing Q1 MJE15030 off the board and tested it. Definitely no good. Here are the results (marked with positive or negative for the test lead used):

Base (Pos) to Emitter (Neg) = Shorted
Base (+) to Collector (-) = 0.439
Emitter (+) to Base (-) = Shorted
Collector (+) to Base (-) = 1.450
Collector (+) to Emitter (-) = 1.432
Emitter (+) to Collector (-) = 0.388

I guess that short allowed the game to somehow work based on your comment? Guessing if I pull Q3 I'm going to find the same issue, or similar.

Following up - pulled the Q3 (MJE15031) PNP that was on the board and it seems to test good:

Base (Pos) to Emitter (Neg) = OL
Base (+) to Collector (-) = OL
Emitter (+) to Base (-) = 0.552
Collector (+) to Base (-) = 0.555
Collector (+) to Emitter (-) = OL
Emitter (+) to Collector (-) = OL

I expected both to be shorted or test bad, but this one seems to be okay if I'm reading things properly. I will still replace it with the new GPE one I have, and I assume cross the legs as well?

Still amazed it was working like that for a while, but who knows. Okay, I'll go ahead and install the new transistors and see how it goes. Fingers (and transistor legs) crossed.

Success!!! Just finished everything up, reinstalled, and it's working. Including the pop bumper that was dead thanks to the new transistors on the driver board.

Thank you all for chiming in on the Transistor issue. While it's interesting to know that a shorted and improperly installed MJE15030 in Q1 and incorrectly installed MJE15031 in Q3 can somehow allow the game to work temporarily, it's clearly not a good thing and causes a lot of headaches.

Make sure you get the legs correct when replacing these! Whomever did this HV rebuild before I got the game failed that effort. Glad you all helped me correct it. Thanks!

My understanding is that the very late series boards were indeed different and used the newer transistors, without need for leg crossing. What game are you working on?

As noted above, and easy check is to compare the schematic to the board and check/trace connections to make sure everything matches. When I did this, it verified I had an older board.

You can post a pic too, might make it easier to tell.