I don't think a Q100 has a "PC board"...but anywho. Before you start taking too much apart and making many adjustments many problems are actually caused by a build up of old grease, often a good cleaning and the use of the CORRECT oil in the CORRECT places will solve many issues.
The following is from the late great Tony Miller's page for your issue:
http://www.west.net/~jay/fmillera/fmillera/index.htm
Ron Rich can be found on the yahoo groups and phonoland, he is an awesome guy and can help you fix your problem, consider buying one of his guides first so you know the terms
The following is the beginning of the instructions from Tony's page:
Symptom 3: Scans twice, plays nothing
The classic 'scans twice, plays nothing' problem can be caused by the Tormat Write-In, Read-Out, or Trip circuits. We must eliminate two of the three possibilities, to determine which circuit has the problem. First, check the Write-In voltage (usually labeled 'Write-In Source' on the control center or selection receiver test terminal strip). For the digital machines (black/gray or red boxes), the voltage measured at this point will be about +125 VDC. For the solid-state machines, this voltage will be +395 VDC or +150 VDC depending upon model and number of selections. For the tube-type machines, this voltage will be -300 VDC. The Techie Stuff section of this website talks about how the Tormat works, so you should check it out before proceeding. If you measure significantly less than the correct voltage, check the Write/Read supplies in the control center or selection receiver. Given the symptom, it is probably reasonable to assume that the other supplies are functioning properly, but it doesn't hurt to check them, too. There should be test terminals for these supplies on the same terminal strip. Note that the machines using the TCC1 (LPC1 and LPC480) use a pair of test plugs instead of a simple terminal strip. Refer to that machine's schematics for details.
Disconnect the RCA plug sense loop connector from control center or selection receiver. Connect the negative side of a C or D cell battery to the center pin of the plug, while grounding the positive side of the battery to set all toroids to the selected state. (Note that if you use this procedure on a tube-type jukebox, you must reverse the battery connections.) Plug the sense loop connector back in and manually scan the mechanism. It should now trip at each record, transfer, and play it. Upon reject, the mechanism will move to the next record, and repeat the cycle. If this does not occur, the problem is in the Read-Out or Trip Circuits. If operation is normal, the problem is in the Write-In circuits, to be covered later.
Which is it: Read-out or Trip?
If the problem is in Read-Out or Trip, we must continue to eliminate one or the other. First, verify that the Read-Out timing is correct. Turn the machine off and disconnect the mechanism plug from the control center or selection receiver. Move the carriage to one end of the magazine, remove the records at that end and also remove the mechanism carriage cover. Set your ohmmeter for continuity and connect it between L1100 and ground. This is the inductor that is mounted to the Detent Timing switch next to the motor. Connect the Ohmmeter lead to the side of the inductor with the yellow wire and any decent ground should do. Slowly rotate either the turntable or the motor coupling a small amount and note which way the carriage moves. Now check the Contact Block Actuator slide or reversing switch (depending on jukebox model) to make sure that it is in the correct position for the direction of carriage travel you have chosen. Note that some models (most notably, LPC1, LPC480, PFEAU1, and all of the Home Stereo Consoles (APx, HSCx, and 600x)) only perform Read-Out while traveling in one direction. For those models, make sure you are moving the carriage in the Read-Out direction. Slowly move the carriage until you get continuity, then stop. Now if you sight down between the record separators, you should see the Transfer Arm Head perfectly lined up with (centered on) the magazine record separator. If so, the timing is correct for this direction of travel. If not, fix it. There are several inter-dependent mechanism adjustments which affect this timing. They include: Magazine - Horizontal Position, Transfer Arm 1, Tormat Memory Unit Position, Read-Out Contact Block 1, 3, and Detent Switch. Any one not correct will make this timing incorrect. Perform this check at each end of the magazine and in each direction for those machines doing Read-Out in both directions.