Haven't been following this thread in detail (and I9;ve never seen an OBAa in my life) but I just watched the video.
I think it's strange at 0:04 that he's seeing 2MHz 4Vpp (!) signal on that scope if he's measuring the +5MEM supply...
There's not just small dips in the supply, but bursts of HF and relatively high voltage.
Also, if it's this scope:
- Measurable bandwidth: DC … 1MHz
- Visualization: 0 …500kHz (real-time), 1MHz (repetitive)
Should it even manage to show 2MHz signals? I wonder if the scope is acting up.
The 5V also just seems to disappear off the scale sometimes.
This was measured on pin 24 of U164? Do you get the same signal if measuring on the cathode of D9 or D10? It should be +5MEM as well.
I was wondering if some clock signal was being shorted to the +5MEM but pin 23 is A8 so I doubt it would carry a constant 2MHz signal. Could be something else.
That +5MEM and battery circuit in the schematic looks interesting.
LAC (K1) is carrying AC from the transformer and is rectified by D1 and I'm guessing it's buffered by Q1+C2.
I assume it's being used as a mains detection circuit.
There is a NAND pair with feedback (U117).
What happens if they go bad?
Maybe something is causing oscillations there?
Hope I'm not adding to the confusion. Just sharing some thoughts :)
On 22 March 2015 at 14:47, Bob Grieb bobgrieb@... [oberheim] <oberheim@yahoogroups.com> wrote:
In the video, over at the left edge the scope seems to be showing adip in the +5MEM supply. That supply is created by ORing the 5V digitalsupply and the battery, so there will be a diode drop in it. But that should beconstant, not a dip like we seem to be seeing. Maybe you shouldscope 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 orsomething? Then if it triggers, move the trigger point and the glitch to the centerof 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 youare at it. You can use the AC coupled setting of your scope to see 15V suppliesat 0.5V / div. In that case you are not measuring the voltage, just lookingfor noise and dips.Did you replace the power transistor in the supply with the exact type usedoriginally? 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]
[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.