I spent some time troubleshooting a Flight 2000 over the last 2 days. The sounds work fine but no speech was working.
There was also a problem with the all of displays strobing, so that problem was tackled first. When I was jiggling the display-related wires around, it was almost like the thing that refreshes the displays got "stuck" on a digit, or just a comma, then jiggle around some more and it could proceed but still strobing, very weird. I also tried taking out individual displays by unplugging them. When I unplugged Player 2, they all actually went blank! I stopped after player 2 and re-plugged all displays back in. After testing the rectifier test points which were normal (except for TP4 which is important later), I decided to swap MPU boards to an Altek and then the displays were all back to normal. However, no change for speech problem.
As a side note, it seems like the Altek MPU mounting location, one of the header pins connector is just in the perfect spot so intereferes with a large heat sink on the SB-300 sound board, that's annoying.
Verified dipswitch 17 set to on, at some point, all dipswitches were set to match the old MPU.
Swapped in a difefrent Lamp Driver board, no change to speech. Keep in mind, the VSU-100 speech board connects to lamp driver. Returned original lamp driver board.
Swapped in a working VSU-100 from Stern Lightening, no change to speech
Tested original F2k VSU-100 in Lightening, it works correctly (heard speech at beginning of game).
Swapped in a SB-300 sound board of unknown condition from a project Stern Cheetah. I had to do a mod where I run an extra wire on the back of the SB-300 to enable speech, ran this wire. Reference : http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Stern_SB-300_Sound_Board_Modification_for_VSU-100_Usage
Again got all sounds but no speech
Also the Cheetah sound board was swapped in using it's current ribbons from Cheetah
Now, keep in mind the VSU100 speech board connects to the SB-300 with multiple connectors and also to the lamp driver
So I suspected that those connectors might be the issue and thought about re-pinning.
However it was noticed that all GI was off in the game. (not sure why this wasn't noticed sooner!) Also the schmematic for VSU-100 says
J3-1 GEN ILL.. (side question: why isn't there a GI Return on the schematic?)
So lesson learned, if you have sound but no speech on a classic Stern, double check that GI is working.
Looking back on the problem, I probably should have tested TP3 earlier on VSU-100 in the troubleshooting process to get to a quicker solution. Without looking at the VSU-100 schematic, I simply tested the pins on EPROM chip on the board and got 5 or 6 volts and just assumed the board had proper power supplied to it. However that is obviously not the case.
Now I am left with fixing the orginal MPU board with the strobing displays. As a first step, I swapped U10 and U11 PIA chips to see if the problem could be bad PIA chip.