The obvious problem is the very well-known calibration error. Fixing that is simply a matter of reading the Service Bulletin that's posted way earlier in this thread. Take off a few center keys, solder flaky pins, replace keys, done. Also, putting the ribbon cable in backward after servicing the keyboard and popping a couple fuses is semi-typical. Those two things cover the good majority of keyboard issues, but I'll expound upon some other issues in detail.
First thing is to make sure there is continuity over the 20-pin keyboard ribbon cable. Test each and every pin on both ends, and make sure the PCB headers on the keyboard processor and mainboard are free of oxidation/corrosion. If that checks out, make sure the keyboard cable is routed away from and around the GLU chip. It's the square chip (gate array) in the brown PLCC socket on the mainboard.
No joy? As mentioned in the last post, +VU is important for the keyboard processing unit. Put your red probe on pin 1 on LM2926 and your black probe on pin 3 to make sure it's 8V DC or higher. Move the red probe onto pin 2 to make sure +5VDC is being output.
Motorola 68HC11A is something to focus on a lot. It's basically a CPU/UART combo unit that controls the processing and communication with the mainboard. If the ribbon cable is good, any bad serial communication with the keyboard is probably going to be traced back to this chip. Don't take that to mean that all serial communication problems are the keyboard's fault, though. Trace the serial path all the way from the DUART on the mainboard to the display header into the keyboard ribbon cable and see if it disappears into the 68HC11. Also, if one or both footswitches are dead then this chip a good place to look.
Here are some key things to focus on with the MC68HC11A1P:
Pin 39 should see the RESET sent by the LM2926. It works the same way as the CPU on the mainboard -- blip low then stuck high when booted.
Pins 48 should be +5VDC, as generated by LM2926. Pin 23 should be grounded.
Pins 42/43 should occasionally show bursts of serial traffic. Pressing buttons on the display keypad should generate some blips on the serial communication lines.
Pin 6 should show the same activity that pin 4 on the keyboard ribbon cable should show which, in turn, should be the same as pin 15 of the DUART on the mainboard.
Pins 29/30 are of critical importance and are tied into the 8 MHz clock from the crystal. Unlike some other MCUs in the VFX, this chip has two clock pins, and so both pins need to be connected to the crystal and pulsing. If not, measure 74HC00 pin 2 for 8 MHz. If that is not present either, there's a problem with the crystal or its related circuit, so look at the crystal itself as well as C1, C2 and R1 which are mostly obscured by the black heatsink that LM2926 is cooled by.
Pin 30 on 68HC11 is XTAL which basically "rings the bell" of the crystal and puts it into oscillation. The crystal then acts as an external clock whose timing is fed back into 68HC11 on pin 29 (EXTAL).
R1 should have one end connected to EXTAL (pin 29), the other end to XTAL (pin 30). The crystal should have one leg connected to one side of R1 (thereby connecting to EXTAL) and the other leg to the other side of R1 (thereby connecting to XTAL). The crystal should also have one leg connected to one side of C1 (and thereby XTAL), and the other leg to one side of C2 (and thereby EXTAL). The other sides of C1 and C2 should be ground.
Assuming that all looks correct, the 8 MHz crystal is cheap and easy to find, so replacement is a trivial issue. One off-the-wall idea to try if the crystal seems dead is to take it out and put it back in reversed. The crystal has no polarity, but might be "shaken up" enough to live again.
As far as the footswitches go, check pins 7 and 9 on the keyboard ribbon cable for activity when a footswitch is in use. If so, check pins 7 and 8 on 68HC11. Both signals should ordinarily be high, but one will go low when the corresponding footswitch is hit. Also double-check that pin 22 is stuck high at +5V and pin 21 is stuck low (grounded) as these two pins are compared to pins 7 and 8 as reference points.
Another semi-unlikely but entirely possible problem is that the keyboard EPROM is shot. Put the EPROM in an EPROM reader and generate its checksum (4B72 on v2.33). If in doubt, the EPROMs for the VFX SD can be found toward the top of this thread.
Probably the most mysterious chip in the whole synth is the Motorola S38BC010PS01 (aka KPC chip). It appears to do most of the low-level interaction with the actual inductance keys themselves (key note value, aftertouch, velocity, etc.), but replacing a defective one will prove to be a chore because they don't seem to exist outside of Ensoniq keyboards.
In order to even operate at all, the KPC needs a few easily checked things, though:
Pin 24 needs to get +5VDC, and pin 12 needs to be grounded.
Pin 8 is the clock input, but verifying clock on 68HC11 should be done before addressing any clock issues on KPC.
If 6HC11 is clocked correctly but there's still no clock on KPC pin 8 then take a close look at 74HC00 because chances are that pin 3 will be floating or stuck.
There is something very important to note about the 68HC11. While 68HC11 chips are pretty easy to find, the particular revision used on the KPC board contains a built-in EEPROM. What this means is that it has been programmed with special code and some random, generic 68HC11 may not work as a replacement. Unless the 68HC11 already contains the Ensoniq code or the EEPROM inside the OEM EEPROM is read out and programmed into the donor 68HC11 then things may not work as designed.
The good news is that multiple Ensoniq products (SQ80, et al) use a similar or identical 68HC11, so they can be found without too much difficulty. The other good news is that the EEPROM in a generic 68HC11 of the same revision can be rewritten with the Ensoniq code (provided you have access to the original Ensoniq code to write in the first place). Long story made short, the 6HC11 in the VFX/VFX SD/SD-1 series is specialized but not nearly as impossible to replace as the S38BC010PS01.
Also, the ICs on the KPC board do not like to operate without being physically connected to the keybed. Unless you have access to the special keyboard simulation test board then it will very likely be necessary to have the full keyboard assembly connected to the mainboard as you troubleshoot a large portion of the VFX system.
KPC BOARD OPERATION
Motorola 68HC11A1P is the main brain in the keyboard processing board:
Port A selects which group of music keys are currently to be scanned and also reads in the state of the footswitches
Port B is addressing (exclusively)
Port C is address/data lines which gets data to/from the EPROM and KPC chip
Port D is serial transfer to/from the mainboard and the display/keypad PCBs
74LS373 is a latching IC that demultiplexes Port C of 68HC11 into separate data and address streams.
The EPROM (27C256) contains the instructions for the operation of 68HC11.
The KPC CHIP [U3] handles the reading of the actual music keyboard:
F0~F3 are output lines coming from the keyboard, to be decoded by the KPC chip and the result is forwarded to the 68HC11; the result of F0~F3 is dependant upon which keys are currently being driven by 68HC11 Port A (CS0~CS3).
68HC11 processes both the display/keypad button and music key data and then sends it to the mainboard as serial data.
As far as the actual inductance functionality goes, this thread explains the theory of operation:
http://electronics.stackexchange.com/qu ... d-function