Welcome to Mindburner's DAC71-CSB-I & Prophet 5 Site


 

My Prophet


I write and record my own electronic music and own several vintage synthesizers. I have owned my Prophet for several years now. I think it's one of only a few in Ireland. It was in a sorry state when I got it and it has been a learning curve trying to fix it and sort out the numerous problems. I bought it with a dead voice for about £250. After many weeks spent with the technical service manual, I managed to track down a faulty op-amp IC. I have since replaced nearly all of the older logic chips to improve tuning etc.

Please be aware that information is provided for help only and not binding. As I get quite a few emails now, I can't guarantee I will be able to respond every time but hopefully this page will help others to get to grips with troubleshooting this fabulous synth. I'm not an expert on these synths, so for detailed troubleshooting, check out the Prophet 5/analogue synth forums. Contact me for any corrections or improvements also. If in doubt about a repair, always take the synth to a reputable synth technician.

Background


Everybody's used this analogue American beauty, from Duran Duran to the Orb. Japan's 'Ghosts' shows a fine example of the FM modulation possible.

In 1978, a few guys in a garage in San Jose, California who called themselves Sequential Circuits created the Prophet 5, a synthesizer with five voices of polyphony and enough memory to save 40 sounds. This may seem like nothing today, but back then it was pretty impressive. But what really set the Prophet apart from it's contemporaries was it's sound, a sound that is now legendary.
Sweet is how most people describe it. It used two oscillators to create it's sound, and a powerful filter to shape it. The key to the 5's performance, though, was it's Poly-Mod signal routings, which allowed the second oscillator and the filter to modulate the first oscillator. Such a little thing made a huge difference. The Prophet 5 was also gorgeous, with it's wooden housing and thick black knobs.

Synth players fell in love with the Prophet when it hit the streets, pushing Sequential into one of the largest American synth manufacturers by the late 80s. Soft Cell, Devo, Gary Numan (who had five!), OMD, Frontline Assembly, INXS, Kraftwerk, PIL, and the Talking Heads, among others, all rocked the Prophet.

If you're looking for a Prophet 5 nowadays, expect to pay around £2500++ for a non midi'd one in good working order. Also, keep in mind that there were several different versions (or revs) of the Prophet released in it's lifetime. The early revisions are very expensive.

The Prophet 5 contains five individual voices. For it's principal sound sources each voice contains two VCO's (voltage controlled oscillators), OSC A and OSC B, and a white noise source which can be mixed into a resonant low-pass VCF (voltage controlled filter). The filter modifies the voice timbre under control of it's four-stage envelope generator. The filter may also be resonated and serve as a sound source. Following each filter, a VCA (voltage controlled amplifier), also controlled by a four-stage envelope generator, shapes the voice amplitude. Supplementing the basic voices are polyphonic modulation (POLY-MOD) signal routings within each voice that allow OSC B and the filter envelope generator to function as modulation sources applied to OSC A frequency or pulse width, or the filter frequency. Finally, there is a single LFO (low-frequency oscillator) and a pink noise source which can be mixed to modulate all five voices, as adjusted by the MOD wheel.

Random


Allegedly Chet Wood, a programmer  at Sequential Circuits ASCII-encoded mantras (Om Mani Padme Hum) into the Prophet 5 operating system.
The original code was written so that it would include a mantra,so each time the Prophet CPU cycles, that Buddhist programmer becomes more enlightened!

Here's a screen grab I did of the OS EPROM in 2017


The idea is that the code cycles over time like Tibetan prayer wheel. Hence the silk screened Buddha, Shiva, mandalas etc on the circuit boards.

Revision Numbers


The Prophet-5 sustained six revisions (or revs). Rev 1 was the original design. Rev 2 was a refinement of the original design and largely transparent. Rev 3, however, was a vastly different synthesizer than Revs 1 and 2. Introduced to Rev 3 were new voltage controlled IC's (CEM), an improved ADC, DAC, and a different control voltage distribution scheme. More sophisticated editing and tuning routines were designed, and to improve serviceability, voice trimmers were reduced from 80 to 45. Some believe that the Rev 3 synthesizers are slightly inferior (sonically) to their predecessors by revealing an absence in the lower frequencies. While this may be true, the majority of the Rev 3 synthesizers are far more operationally stable than their Rev 1 and Rev 2 counterparts.

Instrument serial numbers are coded by model number, followed by a full stop and then the rev level. Mine is a Rev 3.3.

1000.1 -- Rev 1: Serial Numbers 1-182
1000.2 -- Rev 2: Serial Numbers 184-1299
1000.3.0 -- Rev 3: Serial Numbers above 1300 (My Serial no: 4191)
1000.3.1 -- Rev 3.1 adds some changes to the RAM
1000.3.2 -- Rev 3.2 (USART, Analogue)
1000.3.3 -- Rev 3.3 with 120 Programs
1001 -- Prophet-5 Synthesizer with Remote Keyboard
1005 -- Prophet-5 Synthesizer with Polyphonic Sequencer"

Below are a few random pictures of DAC's etc.

Micro Networks DAC
Metal type DAC
Burr Brown metal platform DAC type
ceramic DAC type
Factory fitted Rev 3.3 memory mod
4000 series on PCB3
guy who checked the synth in factory
old UAF772 op amps - now replaced with new tl082's
cem3340
voltage reg
circuit board design
my new sample & hold caps
Tants
decoupler capicators - spawn of satan:)
new PSU smoothing caps, I used 10,000 uf
panel
USART
new 5mhz XO clock crystal
new decoupling caps fitted (in blue)
Hit Counters
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