My VFX SD 1989 stopped working after replacing the battery.
reference to main board N 4001011001 REV A
ASSY N 400900110 rev B
OTIS R2 as at the early SD-1 1990
This is the VFX SD assembly that already has an AUX-output but doesn’t have the DAC from PHILIPS TDA1541A -That have the DAC of the previous assy version and therefore I could not find the original mainboard circuit for this particular version, so I used the “Ensoniq VFX” and “Ensoniq SD-1 (32 voice)” Mainboard Schematicsat the same time…
After replacing the battery, The synth stopped showing the “low battery” message and for a while everything worked, but I was not satisfied with the level of the output signal. It was 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 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 made a mistake when checking the output circuits with a multimeter and the next time I turned it on I again got the “low battery” error, and then VFX stopped responding to the keyboard. The sound is gone, although I can control it from the 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 - silent.
and some time I can to produce random sound by keyboard playing…
When checking the analog testings (holding RESET + COMPARE), I get the following indicators data:
PICH = 059 (but there is no reaction to the wheel)
MOD = 127 (but there is no reaction to the wheel)
PED = 127 (did not check because there is no pedal)
VOL = 000 (but there is no response to fader VOL)
KNOB = 000 (but there is no response to the buttons)
PSL= 000
REF = 000 (but voltage 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…)
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. But 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 and troubleshooting?
Help me please! Unfortunately I don’t have an oscilloscope or digital probe, just a multimeter and audio-probe…
