ok, then assumingly something in the keyboard matrix is wrong and simulates an active key.
Try to monitor the MIDI-Out, when you switch on the synth having no key pressed.
I still think this is a ttl logic issue.
There is (nearly) no ttl logic involved in the sequencer. The sequencer itself runs completely in the CPU. There are only a 40H245 and a 74LS138 which form the column and row for the keyboard matrix. The 40H245 is used for the panel also, so if it was broken, then something on the panel shouldn’t work too. One may replace them, but my suspicion is more some short (dirt? spilled coke?) on the keyboards pcb.
Thanks for your inputs, much appreciated. I will take a look and also remove the keyboard and give it a clean.
Regarding stuck notes, there is no issue with all other features, e.g there is no sound on start up
Circling back on this dead post for resolution. After years on this sequencer issue I finally addressed it. I wanted to be able to trigger the sequencer via my 707 so I took this think apart..again. I started in the panel board and probed all the traces. They looked good. Took apart the rate slider and adjusted the wipers, no change. I then decided to clean the rubber membrane under the keyboard and the contacts. This brought my sequencer back to life. Hopefully someone else with the symptoms won’t have to wait 6 years to get it working again.
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