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 » Mon Dec 31, 2012 6:25 am

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Last edited by Don T on Mon Feb 04, 2013 7:58 pm, edited 1 time in total.

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Re: OB-Xa - unresponsive front panel/keyboard

Post by Steve Jones » Mon Dec 31, 2012 7:32 am

Don T wrote:
DesolationBlvd wrote: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.
Ok, so the power supply seems to be ok. Yeah, see if you can find someone with a meter that has a frequency counter function just to make sure the clock crystal is ok.
If you aren't using an oscilloscope to measure the power supply rails then you cannot assume that they are OK.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Tue Jan 01, 2013 1:29 am

I don't have an oscilloscope or a frequency counter. Looks like this one's on hold while I try to look for one for cheap.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Tue Jan 01, 2013 2:50 am

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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Tue Jan 01, 2013 7:56 am

So I should replace the clock crystal? It's 4.9152 MHz, right?


Also, a dead clock on the control board doesn't kill the modulation box?
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Don T
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Tue Jan 01, 2013 9:26 pm

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Last edited by Don T on Mon Feb 04, 2013 7:58 pm, edited 1 time in total.

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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Tue Jan 01, 2013 9:33 pm

But didn't you say that an unmeasurable 3VAC means a dead clock crystal?

And by "kill the mod box", I mean rendering it unresponsive just like the keyboard and front panel.
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Don T
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Wed Jan 02, 2013 2:56 am

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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Sat Jan 05, 2013 12:48 am

I looked at the prices of the frequency counters - I can't justify that much on something I would use rarely. At least the desoldering iron will see lots of use in other projects, like guitar part replacement.


Actually, I have another question, for synthparts: Did the top board have to be switched with yours for your unit to respond?

If so, then that means the clock crystal on mine is pretty much the culprit.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by synthparts » Sat Jan 05, 2013 2:32 am

No I was using both your boards.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Sat Jan 05, 2013 4:09 am

So it worked in yours but not in mine...

But the voltages seem right, according to Don.
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Yamaha SY99/DX100
Waldorf Microwave 1
Simmons SDSV/SDS9
Elektron Analog RYTM

Don T
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Location: Marietta, GA

Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Sat Jan 05, 2013 10:42 pm

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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Sun Jan 06, 2013 3:58 am

On the top CPU board? I do not see any 74LS42 chips. The closest I see are chips that say "F-7442APC 8009" in spots like 106, 107, and 160.

I think I put the 74LS42 backwards (the writing was rightside-up when the top panel was flipped up), and got all the lights on at once.

I measured at pin 16 of all the 4051s (that's just across from 1, right?) and got some fluctuating reading that went from 0.6 to 0.8 V (DC, right?)


I also had a keybed issue - two keys were stiff and sluggish after replacing the bushings. One just needed more lube, but the other would not respond to any amount of lube. Luckily, I had way more than 61 bushings.

Now the keybed is reasonably level and feels very good, if a bit heavy. Isn't the weight of the Pratt-Read on OB-X/Xa/early 8s the reason why it is preferred over the Panasonic on most OB-8s?
Moog Matriarch
Oberheim OB-8/DSX/DMX
Sequential Prophet 2002
Ensoniq Mirage/SQ-80
Roland JP-8/SH-101/RS-09/Kiwi-30/D-550
Korg MS-20 Kit/ARP Odyssey FSQ
Yamaha SY99/DX100
Waldorf Microwave 1
Simmons SDSV/SDS9
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Fri Jan 18, 2013 2:07 am

Don T wrote:...find yourself a 74LS42 chip. Do NOT use a chip that has any other letters in the part number, and especially don't use one that just says 7442.

Good luck.
Just bought the 74LS42. Funnily enough, the 74LS42 in my top board said "74LS42N".


Sorry I have to bug you guys a little.
Moog Matriarch
Oberheim OB-8/DSX/DMX
Sequential Prophet 2002
Ensoniq Mirage/SQ-80
Roland JP-8/SH-101/RS-09/Kiwi-30/D-550
Korg MS-20 Kit/ARP Odyssey FSQ
Yamaha SY99/DX100
Waldorf Microwave 1
Simmons SDSV/SDS9
Elektron Analog RYTM

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Re: OB-Xa - unresponsive front panel/keyboard

Post by Steve Jones » Fri Jan 18, 2013 11:57 am

Did you check that the CPU NOT RESET line is sitting at 5V when the machine is on? (pin 26 of the Z80 CPU should rise to 5V with a slight delay when the synth is turned on and it should drop to zero when the power is turned off, again with a slight delay). If does is not then the computer will not run. If there is no 5Mhz clock on pin 6 of the CPU it will not run. If the 6.8uf capacitor connected to pin 26 of the CPU is a tantalum and it is shorted the computer will not run.

Really to service a stalled computer a multimeter can only get you so far. If you can find a cheap used scope then you can see if the DC supplies are actually OK and don't have a sawtooth ripple on them from a dead rectifier diode, then you can go around all of the pins pins the CPU and see if the processor is actually running. If any of the data or address pins are latched up or down the computer will not run.
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