oberheim obx-a no sound

[jack c.]

I turned on the obx-a this morning and i start to play some notes and notice some octaves are louder than others,and the sound was going on and off.i heard I few loud cracks coming out of the amp.now no  sound.could anyone help?

 

[haima]

I guess the first thing to check is the power supply...

is there any lights on the machine at all?

fuses blown?

do you have a multimeter? or do you know someone who has one? if not you're going to need one, if you want to fix anything yourself... cheap ones are like $10 or $15.

 

[radiance]

You'll NEED the schematics.

 

[Nele]

check this thread: http://www.groupdiy.com/index.php?topic=46465

 

[jack c.]

thanks for replies:
I guess the first thing to check is the power supply... is there any lights on the machine at all?
        it has a new power supply
fuses blown?
        will check
do you have a multimeter?
        will check into this
You'll NEED the schematics
        have this
i need a cem3340 Analog Synth VCO Chip which i see on ebay for $75
this is what me tech found so far
i started going through the service manual, found a bad voice chip, but it still had nothing to do with the problem of notes not triggering. Either way you will need at least 1 CEM3340

 

[radiance]

Either way you will need at least 1 CEM3340

Are these socketed...yes? Try swapping them between voices.

 

[jack c.]

update from me tech
I have the service manual and I am going through each section/part one at a time and checking voltages. There are test EPROMs you can get for accurate diagnostics on it, but they're not easy to get a hold of. Without them at the current rate, spending a few hours a week on it, it is going to take a month to figure it out.

The issue with yours is that there is some fault in one of the logic boards. It appears (although I'm not entirely sure) that the voice boards are okay. The CPU/control section also seems to be okay, since the LEDs trigger from hitting the various buttons. I've narrowed it down to the control board section that interacts with the voice boards. The power supply is fine (that was the first thing I tested out yesterday with the meter). It could be anything from a faulty logic chip, to a bad EPROM, to bad memory. Either way, the auto tune now no longer even cycles through, and I get an error as if all 8 voices are bad/no communication with them.

I'm thinking instead of sitting around and poking at all of these parts that I may just order brand new logic chips and opamps for it. I can't imagine it being more than $60 to replace all of them, and these logic chips from the early 80's are nearing the end of their lifespan anyway.

 

[s2udio]

I am sure all dry joint and suspect electrolytics / switches and supply rails would have been checked first.
Just starting simple first rather than diving in the deep end !
It just seems every so called tech nowadays seems to blame the chips before doing
The homework..........
Just to mention . :-\

 

[Nele]

be sure to check the interconnect-cables. bad contacts can cause really unpredictable behaviour in these beasts.

 

[jack c.]

from tech
Are these socketed...yes? Try swapping them between voices.
            Did that, that's how I found out that one of the CEM3340's was bad.