Keyboard Calibration Error
Most common fault on the VFX series synths where the dreaded Keyboard Calibration Error.
While it took years for Ensoniq to come up with a permanent fix for this issue some of the old more or less useless remedies still exist on internet.
The fix where described in Service Bulletin #11 March 20-1991 and later refined in bulletin #12 where also important firmware releases are listed.
The fix consist of soldering 12 short wirestrips across the two coil boards on the keybed. There are no need to remove the keybed for this fix. Only remove 4 of the keys for access to the soldering points from topside of PCB.
Checking the software version:
Hold down Presets button, then press the Master button.
WARNING: Use correct KPC firmware revision for the keybed at hand. For instance using version 2.27 with grey foam pads the after touch will stop function.
- v1.50 white foam pads glued on the undersides of each key
- v2.03 grey foam pads glued on the undersides of each key
- v2.27 foamless - no reference coil
- v2.33 foamless - with reference coil
Static noise, Cracles and Pops
This phenomenon is most noticeable in sounds with an extended decay or reverb tails. At times, it may resemble the sound of hand claps in a concert hall. While reading about others experiencing this issue, I couldn’t find a solution on the internet.
Since the issue also involves the digital effects processor, it is unlikely that the fault lies in the analog section. And since the DAC (AD1856) and the rest of the analog section use +/-12V, I first focused on the 5V power supply. The capacitors C8 and C11 on the PSU were replaced. These are responsible for stability in the 7805 and LM2926 voltage regulators.
No luck there, — measured several other caps and voltages on the main board without finding any issues. Then I remembered that there was a service point I hadn't completed yet. On the elevated PCB referred to as "the tower PCB" in the service documentation, there is an adjustment procedure that can be performed according to Service Bulletin #7. While the bulletin doesn't mention noise or pops & cracle as symtom this adjustment where the culpit on this VFX running with +0.4 overvoltage.
Before adjustment: Disconnect all audio outputs and turn the volume slider all the way down to prevent load cracle sounds when contact glitches occurs while turning the trimmer.
Note that in the service bulletin #7 the following adjustment where performed without the Ensoniq ESP chip installed. While I have successfully carried out the adjustment with the chip in place without damaging it, it is probably appropriate to caution that Ensoniq originally intended this adjustment to be done differently.
- On the large Ensoniq ESP chip 5510 0001 0x, there is a colored* handwritten sticker. Make a mental note of whats written.
- Measure the voltage across the 22µF tantalium capacitor** soldered onto the Ensoniq ESP chip.
- Use the trimpotentiometer to adjust the voltage to match what is written on the sticker.
Lower voltage seems to be better than higher if you have trouble adjusting it precisely. There are two types of tower bords and trimpots. On the newest rev C PCB the trimmer is made of plastic. The metal one shown in the image is mounted so that turning it clockwise will lower the voltage.
What we are adjusting is the traditional implementation of the variable voltage regulator LM317 (Q1), which in turn supplies power to the ESP chip. LM317 itself draw current from 78L05 (Q2). Both regulators are located on the tower pcb.
*) The color of this sticker indicates chipset revisions in the synth. See bulletin #7 april '92.
**) If this capacitor is missing, you likely have a very early VFX model and should check multiple service bulletins.
Miscellaneous
The monolithic AD1856 16-bit pulse code modulation (PCM) audio DAC is multiplexed to the analog stereo outputs.
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