From Brian Willoughby Sent Mon, Apr 8th 2019, 01:11
From Brian Willoughby Sent Mon, Apr 8th 2019, 01:11
Hi Peter, There are also the hand-calibrated resistors for the Rev 2 (and likely = earlier) Prophet 5 models. See sections 2-12 and 2-21 of the Technical = Manual for descriptions of the DAC. Section 5, Service Notes, 5-7 = specifically warns service technicians not to change or remove = resistors. =E2=80=9CYou may find extra resistors in the DAC ladder.=E2=80=9D= Sounds like you=E2=80=99re looking at tempco components, but I wanted to = point out that there are other =E2=80=9Ckludges=E2=80=9D in the Prophet = 5 due to the state-of-the-art of electronics at that time. Either better = solutions did not exist at all, or they were too expensive. Sequential = Circuits used every trick in the book to make the Prophet 5 perform as = well as it does for the price at the time. You=E2=80=99re probably = looking at a Rev 3 Prophet 5, so you probably won=E2=80=99t see the = ladder resistors, but this seemed like a good time to point them out. There are 41 channels of S&H from the Rev 1 & 2 DAC, but the DAC itself = only has 7-bit accuracy due to the resistor ladder. Coarse Frequency = only uses 6 bits of the DAC, while other CV values for Fine Frequency = and Tune are mixed in after being scaled so that the full 7-bit range of = the DAC covers a very small voltage. The total frequency resolution is = probably only 12 or 13 bits once all of the VCO pitch CVs are summed in = analog. The Prophet 5, Rev 3, changed from discrete, hand-selected resistors to = the DAC-71 integrated circuit, even though they only used 14 bits of the = total 16-bit resolution. I imagine that the firmware has to change = significantly due to the DAC technology changes between the Rev 2 and = Rev 3, not to mention the drastic switch from SSM to CEM chips. = Fortunately, the DAC-71 doesn=E2=80=99t (or shouldn't) require = hand-selected resistors or hand-soldered =E2=80=9Ckludge=E2=80=9D = resistors because the chip itself was calibrated at the factory. Brian Willoughby On Apr 7, 2019, at 3:00 PM, Peter Forrest <xxxxxxxx@xxxxx.xx.xx> wrote: Yes - found a pic online - thanks!! = http://studiorepair.com/gallery/Sequential_Circuits/Prophet-5/slides/SCI_P= rophet-5_SSM2030_VCO__STUDIOREPAIR_09050701_1002051858.html -----Original Message----- From: gm [mailto:xxxxxxxxxxxx@xxxxx.xxx]=20 Sent: 07 April 2019 23:00 To: xxxxxxxx@xxxxxxxxx.xxx Subject: Re: [AH] Prophet kludged chip Sounds like maybe SSM2030 VCO chips, with 1K tempco plus 121 ohm=20 resistors epoxied to body. Pictures? ~GMM On 4/7/2019 2:50 PM, Peter Forrest wrote: >=20 > I had a nice delivery of stuff from Genesis today to auction. >=20 > One of the items is a case of chips mainly relating to Sequential. >=20 > There are some 'kludged' chips, 16-pin DIL with components=20 > (resistors?) epoxied to the top, connectinr pin 2 to pin 13. >=20 > I remember hearing about it but I just can't remember the story behind=20= > that. >=20 > Anyone help me out? >=20 > Peter >=20