OB-Xa - all EGs gone bad?

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DesolationBlvd
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Re: OB-Xa autotune issues - recap?

Post by DesolationBlvd » Thu Oct 18, 2012 1:07 pm

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.
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Re: OB-Xa autotune issues - recap?

Post by DesolationBlvd » Sun Dec 23, 2012 5:59 pm

Bump. Updated the first post.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by synthparts » Sun Dec 23, 2012 8:16 pm

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?
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Sun Dec 23, 2012 9:27 pm

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?
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Re: OB-Xa - unresponsive front panel/keyboard

Post by synthparts » Mon Dec 24, 2012 11:42 am

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.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Mon Dec 24, 2012 6:21 pm

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?
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Tue Dec 25, 2012 4:42 am

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Last edited by Don T on Mon Feb 04, 2013 7:56 pm, edited 1 time in total.

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Re: OB-Xa - unresponsive front panel/keyboard

Post by synthparts » Wed Dec 26, 2012 8:52 am

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...
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Wed Dec 26, 2012 8:48 pm

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).
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Thu Dec 27, 2012 11:44 am

.....
Last edited by Don T on Mon Feb 04, 2013 7:56 pm, edited 1 time in total.

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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Thu Dec 27, 2012 6:32 pm

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.
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Don T
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Sat Dec 29, 2012 8:46 pm

.....
Last edited by Don T on Mon Feb 04, 2013 7:57 pm, edited 1 time in total.

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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Sun Dec 30, 2012 8:23 pm

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.
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Don T
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Mon Dec 31, 2012 3:15 am

.....
Last edited by Don T on Mon Feb 04, 2013 7:57 pm, edited 1 time in total.

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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Mon Dec 31, 2012 5:39 am

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.
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