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This is the VFX SD assembly that already has an AUX-output but doesn’t have the DAC from PHILIPS TDA1541A (Is wich the DAC of the previous assy version of VFX) and therefore I could not find the original mainboard circuite for this particular ver. I used the "older Ensoniq VFX” and “Ensoniq SD-1 (32 voice)” mainboard schematics at the same time…
- main board N 4001011001 REV A
- ASSY N 400900110 rev B
- OTIS R2 as at the early SD-1 1990
After replacing the battery, The synth stopped showing the “low battery” message and for a while everything worked, but it doesn't satisfiyed me the output level.The sound was be quiet and I checked with an audio probe the passage of signals on four polar transistors located at the end chain of the main and aux outputs (according to the old VFX circuit this is Q10; Q11; Q12; Q13, and according to the SD-1/32 this is Q11; Q12; Q13; Q14 which are controlled by another field-effect transistor Q10 - upon receipt of the RES (reset) signal). I tried to check the serviceability of these transistors - for the absence of a breakdown that would suppress the signal from TL072. But apparently I have made a mistake when checking the output circuites. Then I turned on, I got the “low battery” error again, and the VFX is being unresponding to the keyboard. The sound is gone, although I can control it from the sirface of displayboard, change programs and so on, as well as receive/send MIDI messages, but the synthesizer is silent.
UPDate: MIDI send keyboard only (note on/off and program change). The Pich/MOD weel - doesn't nothing send, and some times I can to produce random sound by keyboard playing…
When I checking the analog tests (holding RESET + COMPARE), I get the following indicators data:
PICH = 059 stable value (there is no reaction to the wheel)
MOD = 127 (there is no reaction to the wheel)
PED = 127 (didn't check because there is no pedal)
VOL = 000 (there is no response to fader VOL)
KNOB = 000 (there is no response to the buttons)
PSL= 000 (nothing reaction from "preset select" buttoms manipulation)
REF = 000 (but voltage is in operating range 4.9V)
BAT= 252 (the voltage of the new battery = 3.6V - but being float the message “battery low” every time to switch on…)
Then I checked and compared the values of Volt DC at inputs pins of multiplexter 4051 between trouble VFX-SD and good SD-1 mainboards, wich I have for compare:
pin#1 - PEDAL: -2,2V
pin#2 - BAT: - 0,85V
pin#3 - OUT: +1,4V ??? (it's strange, couse at Ensoniq CD-1 mainboard have bitween +2,8 ~ +2,9V)
pin#4 - REF: -1,9V (Good, as on SD-1 MB: -1,92V)
pin#5 - VOL: ~ 2,0 ~ -0,75V (depending of fader position "Volume" 000 to 127)
pin#12 - VAL: -2,0 ~ -0,7V
pin#13 - Pitch: -0,8 ~ 1,2 ~ 1,7V (at extreme and MIDLE positions of "Pitch" weel)
pin#14 - Patch Select: -0,7 ~ +1,1 ~ +1,5 +2,0V (at depending of combination of buttom )
pin#15 - MOD: -0,8 ~ 1,95V (at extreem pos.of MOD weel)
pin#16 - V"+": +4,95V (This is good level 5V, but I did saw of ealy Ensoniq circuite scheme is mentioned up to +7V)
pin#7 - V"-": -5,0 V DC; and a little AC pulse 0,15+0,001V (at SD-1 mainboard is absent!)
pin#8 - = GND 0V
I double-checked the functionality of the display board by reinstalling it into a working synthesizer SD-1 - and it turned out to be in full working order (the VOL. and DATA faders also work properly when checking the analog test on SD-1 - everything perfectly demonstrates their functionality and the accepted values from 0 up to 127)
I was able to track down this behavior and found out the same discussion indicating a problem with the 4051 multiplexer or its TL072 op-amp.
I changed the U30 (4051) and U34 (TL072) after multiplexter to new op amps - nothing to changed! and I tryed the change ITIS IC bitween VFX-SD and SD-1 - nothing to change too!
Due to the lack of an original diagram, I cannot compare and verify the recommendations given here .
Can anyone give any other useful recommendations for troubleshooting?
Help me please!
Unfortunately I don’t have an oscilloscope or digital probe, just a multimeter and audio-probe…