Circuit Overview:
Analog subtractive similar to OBXa in some ways but condensed yet more of course as technology matured. Michael Bacich notes: The OB-8 is still one of the finest synthesizers ever made. They pretty much got everything right. It was designed (mostly) by Marcus Ryle and Michel Diodic, who were very young engineers at the time. They later went on to design the Alesis ADAT and many other groundbreaking products,then later started the Line6 company.
Service Tips:
First of all before you start tuning oscillators make sure the VCA offsets are proper such that the signal while in the test1 mode oscillator test is roughly balanced around zero volts. Otherwise the cpu may not see anything and no LED's will turn on. This puzzled me for a bit. I wonder why they didn't make a note of that in the tuning instruction?
Whatever the case first get the +15V and -5V trimmers set as closely as possible. A few mv I've done if the pots are clean. THen set the dac as per the instructions and the benders. Then it says if you need to to adjust the eg's. But then verify each vca offset and get all 16 vco's trimmed. Then at least the filter adjustments as noted in the service manual.
Also note the revisions. My newer board uses a 3080 instead of the CEM3360 for the vca and a TL082 instead of 081 (pin 2 for input and 1 for output instead of pin 6 on the 081). It's output can be seen on the 4.7K resistors between the 36K units along the right front of the board. A handy place to do tests.
I saw the strangest problem with my A8 version unit. Intermittently computer seems to slow down or something. I tried replacing what measured as BAD 7404 chips of whatever family they use. I put in same numbers that measured good on my chip checked..but as I did..various things started to get WORSE until I replaced the clock and it would not boot! I scraped the bad chips out of the trash and put them back in..bammo..boots up and works sort of ok again.! THERE IT SAT FOR A DECADE. 8/7/16 I started trying to fix the two I had setting here and found that this one had some memory corruption due to the PUP circuit mod needing to be installed probably. I edit out the glitches including TRANSPOSE settings for the upper and lower boards that had totally goofy intervals going. COnfusing at first since I didn't see that in my poorly printed manual. haha. Oh well what are the odds upper and lower would randomly be BOTH off by a minor third but 2 octaves shifted from each other? My 'luck' isn't the best. That totally confused me for a while. But it's working swell now it appears. I may never understand why my faithful chip checker told me all those chips were bad.
VCO chips fail sometimes also..More recently I found that good measuring 4051 chip was bad causing cv's to go out of bounds on one voice board. After replacing it worked...until I tried to trace the problem with the controls not responding and then apparently the DAC desided to bite it or the op amp buffer. Hopefully the latter. Here is a link to a page talking about the DAC and lots of other useful stuff alphaentek I just replaced a DAC that blew when power supply issues happened. ALWAYS CHECK to see that the first (left most) bridge rectifier has been replaced with a larger one. The 1.5A rated ones aren't so good there.
Parts:
Aside from some of the Curtis chips and maybe CPU I stock everything all the time electronically. The CEM3340 has been reproduced (Contact susanonchip@covad.net for details. Reasonable price but I hear for now at least pins are a bit weaker on them). Knobs may be difficult to find exact replacement. The switches are available from analogdiehard@att.net I hear.
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