In the video, over at the left edge the scope seems to be showing a
dip in the +5MEM supply. That supply is created by ORing the 5V digital
supply and the battery, so there will be a diode drop in it. But that should be
constant, not a dip like we seem to be seeing. Maybe you should
scope both of your 5V supplies more carefully. Can you put the scope on 1V/div,
or maybe even 0.5V/div, and set it up to trigger on a negative edge at 4.75V or
something
Then if it triggers, move the trigger point and the glitch to the center
of the scope so you can see it clearly. If the power supply is drooping too much,
that cause cause lots of strange things to happen. There are two 5V supplies.
I would check them both. You might want to check the other supplies while you
are at it. You can use the AC coupled setting of your scope to see 15V supplies
at 0.5V / div. In that case you are not measuring the voltage, just looking
for noise and dips.
Did you replace the power transistor in the supply with the exact type used
originally
Any other P/S parts replaced
Bob
From:
"richtrix@... [oberheim]" <oberheim@yahoogroups.com>
To:
oberheim@yahoogroups.com
Sent:
Sunday, March 22, 2015 8:51 AM
Subject:
Re: [oberheim] Finding Clock frequency of my OBXa [2 Attachments]
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Attachment(s)
from richtrix@... [oberheim] included below]
Hi again Bob,
The blips happen at all phases of keypress. They are seemingly random and occur so long as 5-8 are making any kind of sound (attack, sustain, decay).
I measured the 5V pin with my little digital scope and it was seemingly pretty clean as seen in the attached image. Then I tightened the time frame and that revealed some artifacts (see attached video)
I'm not hugely trusting this scope when it comes to correct ranging and proper voltage measurement but the video is probably what is happening in reality. The probes were hard mounted and not shifting.
So something may be wrong with my +5 V supply. This in not my first PSU problem if so. I had to replace one of the power transistors when I got the OBXa.
I will try bending out those different pins on U164 Monday.