OB-Xa - all EGs gone bad?

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Re: OB-Xa - unresponsive front panel/keyboard

Post by synthparts » Fri Jan 18, 2013 2:00 pm

Yes, good advice. You can sometimes get away with measuring the AC ripple on the power lines by setting your meter to AC but a scope is really what you should use...
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Fri Jan 18, 2013 8:48 pm

Okay, I finally have an oscilloscope on the way for relatively cheap.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by polyjuno » Fri Jan 18, 2013 9:04 pm

DesolationBlvd wrote:
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".
The N just means that it is in a DIP package (as opposed to the more modern and smaller SSOP packages for instance).

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Re: OB-Xa - unresponsive front panel/keyboard

Post by Steve Jones » Fri Jan 18, 2013 9:13 pm

Once you get your scope one thing to be aware of is that there will only be a single probe that has a lead coming from it with a clip. The clip goes to a ground point on the synth and then you probe things with the tip of the probe. The clip must be grounded to the synth before you will be able to take measurements with the probe. Don't treat it like a multimeter and use the earth clip and the probe to randomly measure things because the earth clip is grounded through the scope and will pull things to ground with it if you touch them with the clip.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Fri Jan 18, 2013 10:53 pm

So I would clip the earth clip to, say, the side of the metal lid?
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Steve Jones » Fri Jan 18, 2013 11:02 pm

Usually an exposed metal part of that machine is grounded. At this point I should also mention that it would be a good idea to do some safety checks on the machine - unplug the mains plug from the back of the machine and check that the mains earth wire from the IEC socket is securely connected to the machine's chassis and that the nut and bolt are tight. Use your meter to measure from the earth pin on the IEC socket on the machine to the chassis at various points and ensure that there is a good connection there with no significant resistance. Inspect around the mains input section of the machine inside and make sure that there is nothing loose, frayed or damaged and that the transformer mounting bolts are tight. If in any doubt get a technician to check for you. Make sure that all covers and screens around the mains area are in place before you work on the innards of the synth.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Fri Jan 25, 2013 12:03 am

I now have the oscilloscope. Setting it to 60hz, I found that the 5.5VAC does not look like a smooth sine, but (pardon my clumsy way of describing it) a series of traffic cones lined up:
_
/ \
/ \
_/ \_
_/ \___/ (and then the next "traffic cone")
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Fri Jan 25, 2013 4:37 am

.....
Last edited by Don T on Mon Feb 04, 2013 8:01 pm, edited 1 time in total.

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Re: OB-Xa - unresponsive front panel/keyboard

Post by Steve Jones » Fri Jan 25, 2013 8:25 am

Don T wrote:
DesolationBlvd wrote:I now have the oscilloscope. Setting it to 60hz, I found that the 5.5VAC does not look like a smooth sine, but (pardon my clumsy way of describing it) a series of traffic cones lined up:
_
/ \
/ \
_/ \_
_/ \___/ (and then the next "traffic cone")
Replace the bridge rectifier in the 5V supply. Use a higher rated bridge rectifier to do so, and definitely NOT the same part as the original. Make sure the board underneath the rectifier is not scorched. Let me know if it is, I've been meaning to post on this subject.
Those rectifiers fail regularly. They were very under rated. Hence my post saying that it is not a good idea to use a multimeter to test DC supplies in an OBXa, a scope is really necessary.
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Re: OB-Xa - unresponsive front panel/keyboard

Post by synthparts » Fri Jan 25, 2013 12:12 pm

Yes, it's a very common prob with OBs...I mentioned this back a couple pages in this topic (and in other OB topics)... :D
synthparts wrote:If you have the original 5V bridge rectifier still in here you might need to replace it.
synthparts wrote:Ah glad you figured it out. I had kind of assumed you knew of the common 5V rectifier probs on the OBs. Those W02Ms from the factory were underspec'd. This is especially true on OB-8s where pretty much every one I see has had it replaced with a beefier version. This is taking it to a bit of an extreme -

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Re: OB-Xa - unresponsive front panel/keyboard

Post by DesolationBlvd » Fri Jan 25, 2013 4:56 pm

Should I replace all of the rectifiers, or just the one on the 5V line?

I read somewhere else that the crappy rectifier that kills Oberheim keyboards is 1.5A, and a 3A one is recommended. What about the other ratings like the voltages?
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Fri Jan 25, 2013 6:30 pm

.....
Last edited by Don T on Mon Feb 04, 2013 8:01 pm, edited 1 time in total.

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Re: OB-Xa - unresponsive front panel/keyboard

Post by Don T » Fri Jan 25, 2013 6:48 pm

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

Post by DesolationBlvd » Fri Jan 25, 2013 10:33 pm

Condition of the power supply board:

No burn marks on/under the -5V. Something suspicious on the right side - scorching, or just old flux and dodgy soldering at the factory?

http://i1193.photobucket.com/albums/aa3 ... 12d7af.jpg

The Radio Shack only had a 100V 1.4A and a 400V 8A bridge rectifier (I assume a 50V one will not be enough), and I picked up the big one. Will it be overkill like the one in the shot of the OB-8 in this thread?
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Re: OB-Xa - unresponsive front panel/keyboard

Post by Steve Jones » Fri Jan 25, 2013 10:41 pm

If one of the diodes in the bridge fails there is often enough voltage that the 5V rail shows up on a multimeter as just below 5V. However if you look at it on a scope you will see that it is a sawtooth wave with a 5V DC offset. This sawtooth plays h**l with digital circuitry, the computer runs but is very erratic. I have seen this on Oberheims, Yamaha GS-1's and several other computer based poly synths over the years. I saw one tech spend days on a GS-1 trying to find a computer fault that caused key scanning and switch problems, all the while there was a vicious sawtooth wave on the computer's DC supply due to one failed diode in a bridge. He had confirmed the DC supplies with a multimeter and never saw that the bridge was faulty. When he eventually went back and tested the power supply properly and replaced the bad bridge the machine worked perfectly. Hence I stand by my advice - If you are working on a CPU controlled vintage synth the very first thing to do in every repair is to use an oscilloscope to examine the DC supplies. If you go into a machine like an OBXa with nothing but a multimeter you are very likely to spend time chasing phantoms.

Also a scope is necessary for looking to see if all of the data and address lines are working and not latched up or down on a dead machine, which is pretty much the next thing to look for once you confirm that there is clock and that the CPU chip enable line is set correctly.
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