Hi Rich,
There are many CMOS chips in this synth. On my schem, U164 is a 6116 SRAM chip.
I don't know exactly how the MIDI kit is set up, or what parts it wants on the MB, so it's tough
to answer your question. But assuming that it has its own SRAM and EPROM, then probably
it wouldn't be using U164. Removing that chip would reduce the load on the data and address
buses. There may be a timing problem that is fixed by leaving it in place. Every so often
the wrong data is latched somewhere when the right (wrong) conditions happen. If you want to
try to learn more, assuming that RAM chip is not used, you can bend out data and address lines
individually and maybe find out which ones are helping to solve the problem. But it may be more
than one. Also, you can only bend the pins so many times before they break. Your scope probe
can also be used to add capacitance to a signal. You can remove that chip, and then try probing the
pins of the socket one by one. If it's just one signal that is making the difference, you might find
it that way as well.
If you do leave that SRAM chip installed, check with your scope to make sure it is not
being selected. Trigger the scope on the high to low edge of chip select and you should not see
it triggering.
Bob
From:
"richtrix@... [oberheim]" <oberheim@yahoogroups.com>
To:
oberheim@yahoogroups.com
Sent:
Thursday, March 19, 2015 6:25 PM
Subject:
Re: [oberheim] Finding Clock frequency of my OBXa
Thanks again Bob,
I am not going to worry about the memory problem right now as I've gone back to the midi kit for control. I got my oscilloscope in and was able to compare waveforms. I got a +5V offset 500 KHz wave on pin 6 for both stock and the Midi Kit. They looked pretty close to identical. The strange thing is that I sort of fixed the Blipping problem. While switching between stock and the MIDI kit I got sloppy wanting to play with my new oscilloscope toy and forgot to remove the stock ICs besides the Z80 which is required for the installation of the Midi kit. Well when I turned it on it sounded fine... No blips or nothing just like when it was stock. I scratched my head wondering why the hell it was working fine now when I realized I had left the ICs in. Happy that it didn't catch fire or something and seemed to be working fine despite this I began removing one chip at a time to see if I could isolate something there. I ended up finding the one chip which if removed would begin the blipping. U164 the CMOS chip, if left in place, eliminated the blips. If removed they came back.
Anyone here have any idea as to why this might be
I have tested all crucial functionality and everything seems functional with the midi kit. Sysex dump and read, MIDI in and out all function well. I have not tested everything but those important things are fine.
I really don't know what it means for the system to leave in U164 along with the Midi Kit. What I have experienced is that after an hour or two of being on the lowest octave keys began to act weird. Delayed notes on keypress or on holding down a key producing a repeating note on every second or so. This was cleared up with a cycle of the power but I imagine somehow the CMOS is starting to conflict with the Midi kit. Any theories why the CMOS would smooth the blips out though
if I could keep that function but ditch any higher function that might conflict (by bending out pins maybe) That'd be perfect.
Sincerely,
Rich Clarke