ARP Pro-Soloist and Pro/DGX -1972/1977    Weight= 18.5 Lbs. Number mfg.= ? MRP=$1350

User Manual:
Reset Proceedures: N/A
Operating System code: N/A
Patches or knob settings: N/A
Circuit Overview: below
Scematics/Service Manual: fdiskc archive
Common Service Issues/Tips: below
Parts Sources: Keys knobs semiconductors misc
Uncommon chips/modules used: 4034 filter sub-module, ROM chips
General Info Links: Vintage Synth Explorer

Circuit Overview:
     Board E needs some clarification. I worked on my second Pro-soloist tonight and after having been puzzled by what I saw in the schematic, see what the problem was. Not that this circuit first generates a clock with gates from Z8 and Z2. This signal is divided in 4 stages which creates address code for the 74150 data select chip and simultaneously can update the memory cells of Z3 when a gate signal is detected, and this data feeds the dac board A along with the other two bits similarly buffered in the other two f/f memory cells of Z3 but which represent octave rather than note value in the codes, and are generated by two more dividers from Z5, a 7473. Z6 a 7403 open collector NAND takes that information to create a octave decoder.
     Here's where the funny looking thing happens. Those open collector outputs go straight to the 4 octave busses (one dedicated to the high C key only). And the switch wires for all note types are tied together. So for instance all the D's are tied to pin 6 of the 74150 which is selected if the address code is 0010. So if the high D gets hit, then there is now a connection between the third octave output of the 7403 and that pin. There is nothing else there. NO PULL NOTHING! Sadly my unit had a bad 74150 which even w/ a pull up wouldn't work. Once I replaced the chip then the pulses appeared. NOt square of course. THere's a time constant. When an open collector output on the 7403 goes high then a small current from the 74150's input floats the lines up! And since it's a fairly slow process (scan cycle is 100/sec I believe it said) this works just fine normally.
     Anyway there are a few other details on Board E but once those codes are there then the DAC of Board A is able to convert to a control voltage for the VCO with the Portamento and pitch control between of course. Then there is a divide down responding to the octave code for both a Frequency to voltage converter (which is fed back to the non-inverting input of the op amp driving the vco's CV) and for the actual generated signal to be conditioned on by a pulse and saw step shaper. And these signals are fed to Board C where they are filtered for the various sounds.
     The panel switches for patch selection create a static digital code on a group of wires that form an address buss for a set of ROM chips. These chips are programmed to give the proper high or low outputs on various lines that each alter the active components in the feedback loop of an op amp or the like. Thus changing for instance the filtering characteristics of a circuit. Tones generated can then be routed through the Moog filter copy 4034 module or not, also depending on the setting of a bit in a ROM chip for that particular patches address. Aftertouch can control all attributes but panel knobs only allow control of brilliance of the filter, AT amount, portamento amount, vibrato speed and volume. A reiterate switch is also present. The PRO/DGX model had improved switching, but lost the moog ripoff filter design. The rare 'Soloist' was the first product and I don't know much about it.

Repair Tips:
     The first one I've worked on, mine, there were several issues. The most significant was that some of the sounds wouldn't work at all. Looking in the service manual at the functions which the ROM chips were activating for each of those, (very nice manual!) the common link was obvious. They all use the moog filter copy 4034 module. I found arp tech was a very valuable resource even if it did take me a Sunday afternoon to build the filter and get it in there. The modules are potted and can't be repaired without melting the stuff. Not sure if it's doable on those. Download this and many other PDF's with diagrams of arp submodules on their site!

     The aftertouch strip went deader and deader on mine til parts weren't responding at all and other part with good pressure only. I decided to try my contact material I sell for repairing rubber key contacts. I cut strips that overlapped the tape spacers inside the strip after cleaning up the surface of the conductive substrate a bit. A few notes are more sensetive than others and I'll have to put some stiffening under those notes but generally it worked great AFTER I swapped the 20K pot (that slider is 1K if you look at the manual's notation on the TS amount control! Not sure why.) for a 5K and had I had an audio one here that would have been even better but it works fine enough for me as it is. If the whole things was destroyed it appears these strips from Interlink would work about as well maybe. Total length is about 520mm I think but the 500mm might work well or the 610mm could be cut down I think. The resistance goes even lower from the spec sheet data and I'm not sure but what the whole circuit would have to be redesigned to accomidate them. My material works fantastic so let me know if you want to order some or just send the strip and i'll repair it if it's not completely rotted out or something. Or you can experment with the 20.00 strips and let me know if it works for you! I'm skeptical and think that you might have to do some other things to get a proper range of responses.

Lots of discrete parts in these. I have a scrap Omni Mk 1 unit with no upper board. Always feel free to ask or check my Parts
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