OB-Xa - all EGs gone bad?

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

Post by Don T » Fri Jan 25, 2013 11:23 pm

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Last edited by Don T on Mon Feb 04, 2013 8:02 pm, edited 1 time in total.

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Re: OB-Xa - PSB issues

Post by DesolationBlvd » Sat Feb 09, 2013 7:48 pm

Finally got the new bridge rectifier installed after cleaning up the damage of removing the old rectifier, in order to rule out the common -5V bridge rectifier problem.

Still getting the "traffic cones" on the oscilloscope at the 5.5VAC and a non-responsive front panel. I would guess the next step is a recap?

Side note: I broke a pin on the keyboard ribbon cable, and had to order a replacement 16-pin ribbon connector.


Ideally, I would just buy a replacement power supply board, preferably with upgraded bridge rectifiers, to see if that solves the problem. Are early-rev and late-rev OB-Xa PSBs compatible?
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Re: OB-Xa - power supply issues?

Post by DesolationBlvd » Sat Apr 13, 2013 11:58 pm

New PSB installed. Still not working.

Here's a picture of the oscilloscope output on the 5.5 V AC:

Image
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Re: OB-Xa - power supply issues?

Post by Steve Jones » Sun Apr 14, 2013 12:21 am

Can you post a pic of your schematic and mark the point where you are seeing that waveform?
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Re: OB-Xa - power supply issues?

Post by synthroom » Sun Apr 14, 2013 2:18 am

DesolationBlvd wrote:New PSB installed. Still not working.

Here's a picture of the oscilloscope output on the 5.5 V AC:

Image
What are the scale settings? How high are those peaks, in volts?
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Re: OB-Xa - power supply issues?

Post by DesolationBlvd » Sun Apr 14, 2013 3:10 am

Steve Jones wrote:Can you post a pic of your schematic and mark the point where you are seeing that waveform?
Can't really follow, but it was on the upper control board, after the connector to the power supply board, the joint right before the first resistor.


Can't really remember the scale settings on the oscilloscope.
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Re: OB-Xa - power supply issues?

Post by Steve Jones » Sun Apr 14, 2013 7:22 am

Do you have a copy of the schematic? If you give me a connector number and pin where you are taking the reading (or a photo of the top board with the place marked) I will open one of my OBXa's and see what is there. Also are you making sure that you are earthing the oscilloscope probe correctly? The lead with the clip on the end that is attached to the probe should be connected to a ground point on the chassis when you are taking readings with the scope, that waveform looks like the scope probe might not be earthed correctly...
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Re: OB-Xa - power supply issues?

Post by DesolationBlvd » Sun Apr 14, 2013 12:55 pm

Steve Jones wrote:Do you have a copy of the schematic? If you give me a connector number and pin where you are taking the reading (or a photo of the top board with the place marked) I will open one of my OBXa's and see what is there.
Connector K, pin 1 - what should be the 5.5 V AC according to the table in the service manual.
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Re: OB-Xa - power supply issues?

Post by Steve Jones » Sat Apr 20, 2013 1:03 pm

That trace on the 5V AC line is what you would expect to be there.
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Re: OB-Xa - power supply issues?

Post by DesolationBlvd » Thu Apr 25, 2013 1:52 am

More observations:

- no matter how I tweaked the +15V, the -15V always read as -15.2V
- the 10K resistor to the right of the battery (the one that should be used to measure the CMOS current drain according to the service manual) - I got two measurements, one of over 100mV, but all subsequent measurements were zero.
- the Frequency and Pulse Width button lights are much dimmer than the rest

Which way should the switch on the motherboard be pointing - towards the back or front?
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Re: OB-Xa - power supply issues?

Post by synthparts » Thu Apr 25, 2013 3:08 am

The mobo switch is just the memory protect on/off so it shouldn't matter. So have you measured all the PS voltages on all the boards? I've run into wiring issues between boards several times so you might do a continuity test on all the connectors...
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Re: OB-Xa - power supply issues?

Post by Steve Jones » Thu Apr 25, 2013 4:03 am

How are you measuring the battery current?
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Re: OB-Xa - power supply issues?

Post by DesolationBlvd » Fri Apr 26, 2013 1:03 am

Steve Jones wrote:How are you measuring the battery current?
I was following the service manual's instructions - measuring across the 10K resistor while the unit has been off for over five minutes.
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Re: OB-Xa - power supply issues?

Post by DesolationBlvd » Wed May 01, 2013 2:00 am

Another observation: On the pot board, the +5.6V and -5V on IC 5 are instead fluctuating and unreadable. Reading it on the oscilloscope reveals a square/pulse wave. The connector that should be +5.6V between the pot boards is also displaying this square/pulse on the scope.

Can a problem on the pot boards freeze up the CPU or otherwise cause unresponsiveness?

The voltages on the control boards are good. Pin 16 of IC 145 on upper and 45 on lower are +5.55 (within the 0.2V tolerance of +5.6) and pin 7 of IC 149 on upper and 13 on lower are -10.34 (within the 1V tolerance of -10).

I had a feeling the pot board was fishy from the start...

Would it be
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Re: Unresponsive OB-Xa - pot board issues?

Post by synthparts » Wed May 01, 2013 1:59 pm

Ah yes that would make sense since both control boards and the power supply I sent you worked fine in my Xa...

If you disconnect the pot boards are the voltages OK?
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