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 particular 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 particular 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 it's not the OTA though I've also seen the mylar capacitors I believe they are, fail. Very unusual. They appear ok but your sound will die die die because of one of those capacitors that are between the OTA output and buffer!
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 knobs and most other parts I should have in stock including divider chips, OTA's, etc.. 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 bp Bass (FM depth 7 FCm 8) vib. depth- 1.2 vib. speed- 2.3 Octave- Lo 16' 9.5 8' 5 4' 4 2' 2 Noise 0 Cutoff- 6.2 Resonance- 5 EG depth- 5 Filter Mode- BP Key Follow- Off Attack- 0 Decay- 5 Sustain- 4 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 :-)