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Re: OB-Xa autotune issues - recap?

Posted: Thu Oct 18, 2012 1:07 pm
by DesolationBlvd
One more question about the capacitor: I see these voltage ratings - what's the minimum that will work in the auto-tune circuit?

...Never mind. I found out the tropical fish code - 250v. Just ordered the replacement caps.

Re: OB-Xa autotune issues - recap?

Posted: Sun Dec 23, 2012 5:59 pm
by DesolationBlvd
Bump. Updated the first post.

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Sun Dec 23, 2012 8:16 pm
by synthparts
If you have the original 5V bridge rectifier still in here you might need to replace it. Have you measured all the voltages at each board?

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Sun Dec 23, 2012 9:27 pm
by DesolationBlvd
I tried to follow the instructions in the manual to find where to put the leads of the voltmeter - but pin 4 of connector C is blocked by a plastic plug thing.

And the rectifier is the little black cylindrical thing below the voltage trim pots, right?

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Mon Dec 24, 2012 11:42 am
by synthparts
The service manual says what all the voltages should be at which pins. Just put the black lead from your meter on any of the Grounds (usually I can wedge it securely in the connector hole) and use the red lead to measure the other pins.

Yes the black round things are the bridges. If you look at the bottom of the power supply board you'll probably see burn marks under the one for the 5V rail.

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Mon Dec 24, 2012 6:21 pm
by DesolationBlvd
Just measured the voltages using the pin tables, and the +15 and -5 were adjusted to be dead on.

But, the voltages that were supposed to be -15 and +5 turned out to be -15.2 and 4.88 respectively. Aren't those within the +/- 250mV tolerances?

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Tue Dec 25, 2012 4:42 am
by Don T
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Re: OB-Xa - unresponsive front panel/keyboard

Posted: Wed Dec 26, 2012 8:52 am
by synthparts
Don, did you see my post above or the last post in your other Xa thread? The reason you have burn marks under the 5V bridge rectifier is because the stock ones can't handle the current and should be upgraded...

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Wed Dec 26, 2012 8:48 pm
by DesolationBlvd
Don T wrote:There is a 5.5Volt AC line that feeds the memory protect circuit. If that is not correct, all kinds of crazy things can happen. The solution turned to to be repairing a burnt spot on the power supply circuit board, underneath the 5V rectifier. It's on the far left looking at the board while mounted in the synth.
My 5.5V AC is reading 5.6V. Isn't the tolerance +- 2V?

Also, I do not have any burnt spots on or under my power supply board.

The meter keeps switching from 5.6 AC to 4.something DC (it's a cheapie).

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Thu Dec 27, 2012 11:44 am
by Don T
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Re: OB-Xa - unresponsive front panel/keyboard

Posted: Thu Dec 27, 2012 6:32 pm
by DesolationBlvd
Don T wrote:If the meter samples slowly, that reading may not be out of line. [EDIT]: Ooh! Just thought of something: measure the DC voltage across the + and - leads of that bridge rectifier. It still may ripple a little bit, but it should be more stable. If it reads close to 7.7VDC, you're doing fine.

Anyhow, I'm confused on what is, and what isn't working now. What is still misbehaving?
I measured the bridge rectifiers, assuming that the - lead is the one that is neither + nor ~. From left to right: 11.4, 11.6, and like 52.

:shock:

As for the problems: The front panel does not respond, and the keyboard does not play. This popped up when I was fumbling around.

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Sat Dec 29, 2012 8:46 pm
by Don T
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Re: OB-Xa - unresponsive front panel/keyboard

Posted: Sun Dec 30, 2012 8:23 pm
by DesolationBlvd
Was going around in circles before I figured out that "U115", "7404", and the numbers referred to the different pins on the IC that says 7404. :oops:

At pin 1 of the 7404, the AC voltage just jumps about (it reads 1.6V DC for some reason), and I don't have a frequency counter.

Re: OB-Xa - unresponsive front panel/keyboard

Posted: Mon Dec 31, 2012 3:15 am
by Don T
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Re: OB-Xa - unresponsive front panel/keyboard

Posted: Mon Dec 31, 2012 5:39 am
by DesolationBlvd
Okay, I measured at the diode. Before the diode, it reads 5.56V AC. After the diode, it reads 5.15V AC.

The jumpy voltage that was DC was the one that is supposed to be 3V AC.