it’s been very busy year for me especially since I became father in March. It is one of the reasons who so few of the repairs made it to this blog. I had quite a impressive selection of instruments on my table.
Oberheim OB8 which initially looked like it’s all working and is in good shape but it ended up being very time consuming excercise to get it to 100% reliable condition. All keys needed new bushes and J wires, nearly all trim pots needed to be replaced and I’ve learned one lesson there. I replaced the filter tracking trimpots with temperature stable BOURNS multi turn trimpots. They are quite expensive at about EUR 6 each and the outcome was actually less stable filter tuning then before. Changing this for regular cheaper multiturns with higher temperature coefficient made things better. Lesson learned was that sometimes temperature drifts compensate each other partially and therefore better components do not always bring better results. Another lesson was a weird behaviour of the demux circuits. Filter wasn’t tracking well. There was always some offset. It all went away after factory reset.
Shortly after OB8 came another Oberheim this time OBXa. While it was expected to be in rougher shape then the OB8 it ended up being better off. Most of the upgrades were already there such as higher spec rectifier bridge in the PSU and multi turn trimmers at critical positions. It was actually pretty smooth ride, there was one faulty TL084 so I had to recalibrate the offsets but rest was just working great.
Prophet 5, very little needed to be done to this one, mostly cleaning job and calibration.
And in the row of Oberheims there came two Xpanders and two Matrix 6Rs and one Matrix 6.
Each had a different fault. One had a dead CEM3372 what I’ve learned is the suffix does matter. This Xpander had CEM3372B and when I tried to use CEM3372D there, this voice won’t pass the autotune procedure. Basically if I would replace all 6 with D all will be fine but different suffixes cannot always be mixed and matched as you like. It is safe to say it’s recommended to have them all of the same suffix.
Second Xpander had a problem with demux chip in charge of panorama so some of the voices couldn’t be reassigned.
There was a funny error with the Xpander rotary encoders. I suspected dirty or worn out encoders itself I suspected the debounce circuit isn’t right for the replacement encoder but all suddenly was fixed with replacement of Schmidt trigger IC. I later verified it would work even with original encoders.
Matrix 6 all 3 had a same problem which was just contact on the CEM chip socket and needed general cleaning.
Minimoog original D needed mainly thorough chemical cleaning of keyboard contacts to get tracking back in shape.
I thought after dealing with Farfisa Soundmaker as described in earlier post that next time I encounter that instrument it will be easier because I already had quite decent understanding of how the circuitry works for most part. What I did not expect was how big of a nightmare it can still be. I have spent over 150 hours of troubleshooting to restore this one. There was no single difficult fault but neverending row of problems throughout. I started on the main board for Mono /poly generation where there were the usual suspects 74C85 comparators faulty. There were two faulty 3080s in the VCA VCF mono section, about 6 bad small signal transistors, poly section had multiple shored tantalum caps in the sustain modules. Keybed had a funny fault where one of the contact springs was pulled and sticking out a bit. When the corresponsing key was pressed the sticky out wire shorted against chassis. Last but not least was a impossibility to create decent sounding flute. Some of the transistors in the DA converter on the main sound generation board had a very low beta and things improved dramatically when they were replaced with modern BC549/559 equivalents with plenty current gain. I had to make new circuit board for PSU because it got shattered during transport so if you need one give me a shout. Its really cool to have a proper FR4 material.
There was too many other instruments to count them all. I’ll add some mention of them if I can think of something with useful lessons learned.