Circuit Overview:
Delta uses selected transistors for a single
VCO that feeds S50241 TOS chip to produce top octave square waves. This signal
then feeds the three divider chips which are triggered by the
capacitor/resistor/diode networks that provide poly release envelopes. And upon
triggering all notes being sounded are mixed to the footage outputs (16, 8, 4
and 2'). The outputs of the three chips are mixed along a ribbon cable mounted
flat to the board and a pull up resistor at the input of an op amp for each
footage. These are then used as the base signals for both string and synth
sections. The string section filters and introduced the effect via MN3004BBD
chips on the ensemble board. Then has it's own A/R while the synth section is
routed through the ota based 4 pole filter. (resonance control lower right of
cutoff slider under rubber cap. These machines come stock with a very mild
maximum resonance. ccw 1/8 turn :-) )
Service Tips:
I have seen the OTA's fail. Cleaning issues
common. People seem to like to put these keyboards through a lot. (Finishing one
attacked by a mouse 6/06) After cleaning up this machine I replaced about 40
diodes that had rotted off to half off and several transistors, capacitors and
most of the op amps down there. (These AN6551's can be replaced with any
symmetrical 9 pin sip unit like the M5218 or 4558 versions but you can also just
skip pin 1 and install pin one in pin2 then wire over the power from pin one OR
since they are symmetrical just put them in backward with pin 8 on pin one where
required.
WARNING! Do not use heat gun on these boards or flux remover.
Whatever they put on the bottom isn't a good mix with those things. :-)
TROUBLESHOOTING ROUTINE:
If no power light verify transformer continuity
when switch is on by holding ohmmeter across AC cord legs with it..unplugged of
course :-)
If no continuity trace through to find problem. Primary should
have more than a few ohms across it.
If continuity ok there, proceede to
check voltages. Ensemble board in the middle has a three pin Interconnect with
ground in middle and +15V on red and -15V on other wire. If voltages ok probably
bad led. If not verify regulator legs and look for short circuits in the
impedence measurments between V- and V+ and grounds (measure all possibilities
there)
If something off board is shorting begin pulling power interconnects
to see if you can find the board it's on. I use a milliohmmeter sometimes to
sniff out the precise proximity of shorts.
If machine powers on but has no
sound scope the legs of the TOS chip which is a DIP package chip with a metal
top near the back right of the main board. Most of the legs should have a high
audio frequency signal on them. If nothing verify that the input pin has signal
from the vco.
If no VCO signal I've seen problems with failure in poly
capacitors though not common I'm sure. Check the transistors and connections.
Get diagram.
If VCO then TOS is bad obviously.
If TOS is ok and
absolutely no sound is produced check wiring to jacks and so on. Breaks in
sliders? Since from this point on at least some notes should be sounding on some
footages there must be something terribly strange if you have a problem where
none of this helps. Call me.
If notes are weak it's likely the capacitors
(1uF 50V) that surround the big chips.
If footage is weak it's likely the
associated interstage capacitor next to the sip op amp in the right rear of main
board going open.
If multiple notes sound on release it's likely a bad
diode...in the row nearest the big divider IC's. They can leak and give false
triggering during release envelope.
If decay is too short likely the
associated 1uF capacitor is leaky.
If single note is missing check key
contact or scope input for that note to divider chip. You should see a large
transient when key is hit. If that's ok then it is likely the divider chip
itself.
If an entire note is missing (all "D" for instance) and the TOS is
ok to the divider input on that note then it is also a bad divider chip for
certain.
If notes don't trigger wiggle interconnects as they die from
corrosion easily.
If string section is all ok but no synth output I've seen
the OTA chips die. Scope a logical path through there.
If string section is
dead check Ensemble output on near pin of left interconnect. If it's good then
problem must be on front board somewhere. Check slider for continuity and pull
board down to trace signals.
If no signal out of ensemble check inputs to
pin 3 on MN3004 bbd's. If ok then check their outs at 13, 14. If dead bad BBD.
If not scope path through transistor etc. If signal doesn't get to BBD in then
check IC1.
Parts:
I have a scrap
Lambda which has some common parts but no silder caps; I used a mouser mixer
slider cap for the cutoff on one of mine which works great. Most other parts I
should have in stock including divider chips, OTA's, etc.. <a
href="../../address.htm>Contact me.
Harpsichord1 vib. depth- 1.0 vib. speed- 3.8 Octave- hi 16' 6 8' 4 to 6 4' 3 2' 4 to 3 Noise 0 Cutoff- 10 Resonance- 2.5 EG depth- +.5 Filter Mode- Band Key Follow- Off Attack- 0 Decay- 4 Sustain- 2.2 Release- 4 VCA EG- On trig. mode- 1 Basic Brass Synth (phat lows, rev. env.) vib. depth- 1 vib. speed- 2 Octave- Lo 16' 8 8' 4.2 4' 3 2' 1 Noise 0 Cutoff- 2.7 Resonance- 6 EG depth- -3 Filter Mode- LP Key Follow- Off/On Attack- 6.2 Decay- 3.7 Sustain- 7.6 Release- 0.5 VCA EG- Off trig. mode- 1 Friendly Synth: (Harmonicaish) vib. depth- 1.0 vib. speed- 2 Octave- hi 16' 5.3 8' 4.8 4' 2 2' 1 Noise 0 Cutoff- 7.5 Resonance- 7 EG depth- 1 Filter Mode- BP Key Follow- Off Attack- 3 Decay- 4 Sustain- 3 Release- 4 VCA EG- On trig. mode- 1 Thwak Bass: vib. depth- 1 vib. speed- 3 Octave- Lo 16' 6 8' 1 4' 2.5 2' 2 Noise .8 Cutoff- 2 Resonance- 6 EG depth- -3 Filter Mode- BP Key Follow- Off Attack- 2 Decay- 4 Sustain- 5.5 Release- .5 (fine adjust for note rate/attack relationship) VCA EG- Off trig. mode- 1 Buhgadihl (or close to it! with string mix..nice.) vib. depth- 1.2 vib. speed- 3 Octave- Lo 16' 8 8' 1 4' 2 2' 1 Noise 0 Cutoff- 2.5 Resonance- 6.3 EG depth- -3.8 Filter Mode- BP Key Follow- Off Attack- 2.8 Decay- 4.2 Sustain- 5 Release- .3 VCA EG- Off trig. mode- 1 Rhodesish (on a couple notes second octave down :-) ) vib. depth- 1.2 vib. speed- 1 Octave- Hi 16' 7.5 8' 2 4' 5 2' 1.8 Noise 0 Cutoff- 2.5 Resonance- 6.3 EG depth- -1.8 Filter Mode- BP Key Follow- Off Attack- 6.5 Decay- 6 Sustain- 5 Release- .2 VCA EG- Off trig. mode- 1
Mods The first thing to do is get the resonance control
tweaked to your liking. For some reason they left the setting very conservative
from the factory. The filters can resonate wildly if adjusted. Remove the rubber
hole plugs below the cutoff slider and use a jewler's screwdriver..probably 2mm
wide or like that...and tweak and listen on those controls and get them set the
way you like them. Since these machines lack computer involvement a lot of real
modifications are possible. They are inexpensive machiens to play with. If
nobody buys my spare...I'm going to write up some more stuff eventually here :-)