Alright, since my previous thread was juggling multiple issues, I've closed that one down and started this new one to deal with the current issue at hand. This picks up a little after where that one left off, as described below.

Pin: 1977 Bally Bobby Orr Power Play
Problem: Consistently blowing rectifier board fuse F4.

TL;DR Summary: Something on Voltage/Solenoid Regulator board linked up with connector J2 is causing the fuse to blow immediately when turning on power. Need help pinpointing exact culprit(s), e.g. how to test each transistor, etc.?

Troubleshooting completed thus far:
Using the procedure outlined at http://techniek.flipperwinkel.nl/ballyss/rep/index2.htm#flash , in the section "The MPU Diagnostic LED Flash Sequences Explained" --> "Problems/Solutions with the Seventh Flash", I've determined the cause of the problem is definitely something which connects to connector J2 on the voltage/solenoid regulator board. With J2 connector on V/SR board connected, Rectifier Board fuse F4 blows immediately upon turning on the power. With J2 disconnected, the F4 fuse stays intact.

Note, my backbox knocker and cabinet coin door lockout solenoids are disconnected (wires cut at solenoids).

Due to the condition of the machine when I bought it (ref. https://pinside.com/pinball/forum/topic/need-help-troubleshooting-1977-ssbally-pin-bobby-orr-power-play/ if you want the full history), Transistor Q3 (knocker) on the voltage/SR board was already toast; Vid recommended I replace Transistor Q3, Diode CR3, and the 120 & 330 Ohm resistors that I believe were R15 & R16, respectively.

During my preparation to perform this surgery on the V/SR board, I looked closely at the other components, and found a few others that were suspect (a few resistors with burns, a few transistors with discoloration). I replaced all of the following components:

Transistor Q3 (melted) / Diode CR3 / Resistors R15 & R16
Transistor Q6 / Diode CR6 (markings 'faded', maybe from heat?) / Resistors R11 & R12 (R12 burned during previous troubleshooting)
Transistor Q2 (discolored)
Transistor Q5 (linked with Resistor R10, which was previously burned -- and I replaced -- when I bought the machine)
Transistor Q16 (discolored) / Resistor R34 (burned)

After surgery was complete, I reinstalled the board with all connectors (including J2). When I turned the power on, the F4 fuse held during the startup sequence, until it got to cycling the solenoids, and then it blew.

I then disconnected the J2 connector, replaced the fuse, and turned it back on (to simulate the earlier test, to verify it's something on J2 that's blowing it.) As expected, the fuse stayed intact.

I reconnected J2 (intending to shoot a video for reference, to show the exact moment the fuse would blow) ... but this time the fuse blew immediately upon turning on the machine. I left the power on for a short while just to see if anything would happen (like when R12 started smoking, ref. end of previous thread) ... nothing noticeable occurred.

.... so .... where do I go from here? Is there a way to test all the components that link up to the J2 connector? Obviously I know how to check the resistors, but how do I check the transistors and diodes and whatever else?

Here are some pics of the components before I replaced them:

VSR-1.jpg

VSR-2.jpg

VSR-3.jpg

FYI: Here is a current photo of the board, after replacing the components as described.

20150822_231717.jpg

Thanks very much in advance!

Great tip. (I'm still new to this; this type of guidance is exactly why I need you! lol)

Ok so here's what I see on the electric wiring diagram for J2:

20150823_104738.jpg

I'll visually check all of these, but a few questions/comments:
1) What is "TST. SW. RET." ?
2) My coin lockout coil is already disconnected.
3) Is there a test (e.g., resistance across terminals) I should use to check these? What are the expected results for good coils at these locations?

Thanks!

Let me say it if I haven't already:

DAMN I LOVE THIS COMMUNITY!

With johninc's guidance, I started checking those J2-linked coils, and lo and behold:

20150823_114508.jpg

Chime-1000 is burned out. I checked resistance on the other chimes, they're all ~50 ohms. This nasty one reads 0.

I desoldered the wires from it, and the game starts up through the entire boot sequence and the fuse doesn't blow! I'll replace that coil then will have all the lovely chimes running again.

Since I already had the soldering iron out, and since I did the surgery on the board to fix the knocker, I soldered those wires back in, and the knocker works properly now too!

I love you guys. Thanks vid and johninc. Your guidance led me to good troubleshooting and repair. I've learned much on this machine with your help, so hopefully these simple ones will be resolved in-house from now on.

THANKS AGAIN!!!

(Though I would still like to know what "TST. SW. RET." is, for future reference.)

Man I gotta get a new coil ASAP, this game is so quiet without the 1000 chime, lol.