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Re: OB-Xa tunes same four oscillators sharp

Posted: Sun Oct 07, 2012 3:02 am
by DesolationBlvd
I found a photo online: http://retrosound.de/obxa-inside3.JPG

Look at the upper left - missing connector? Or perhaps it was never used.


Going to run down to Radio Shack for the chip puller tomorrow so I can swap chips around.

Re: OB-Xa tunes same four oscillators sharp

Posted: Sun Oct 07, 2012 5:14 am
by synthparts
No that connector is not used so that is normal...

Re: OB-Xa tunes same four oscillators sharp

Posted: Sun Oct 07, 2012 11:27 pm
by DesolationBlvd
Two things:

1. Ouch! I shredded my hand on the bottom of the upper control board.

2. I assumed FT1A, FT7B, etc. were the fine tuning for oscillators 1 and 2 on each voice.

I swapped the 4051s at 46 and 50: no effect.

I swapped the TL084s at 70 and 71 - while on - one of them blew up. I attribute this stupidity to the blood loss.

Going to need a few TL084s and TL082s.

Re: OB-Xa tunes same four oscillators sharp

Posted: Mon Oct 08, 2012 12:54 am
by Zamise
@Don T - I don't have a manual for my OB-SX :( Are they available anywhere for cheap?

@DesolationBlvd - Sorry to hear, I don't think I can be of any more help in regard to your synth, but did you know your avatar is pedobear?

Re: OB-Xa tunes same four oscillators sharp

Posted: Mon Oct 08, 2012 1:49 am
by DesolationBlvd
Zamise wrote:@DesolationBlvd - Sorry to hear, I don't think I can be of any more help in regard to your synth, but did you know your avatar is pedobear?
Look closer at his arms and legs...

Also, I just used up my remaining eBay Bucks on TL084CNs and TL082CPs.

Re: OB-Xa tunes same four oscillators sharp

Posted: Mon Oct 08, 2012 6:34 am
by Don T
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Re: OB-Xa tunes same four oscillators sharp

Posted: Mon Oct 08, 2012 3:26 pm
by Zamise
Thanks Don!

DesolationBlvd - I noticed the Oberheim logo, but why is pedobear with it :? Do hope you get your OB fixed, but your avatar is contradictory like a skittles commercial.

Re: OB-Xa tunes same four oscillators sharp

Posted: Fri Oct 12, 2012 12:38 am
by DesolationBlvd
Zamise wrote:DesolationBlvd - I noticed the Oberheim logo, but why is pedobear with it :? Do hope you get your OB fixed, but your avatar is contradictory like a skittles commercial.
The Oberheim Logo and Pedobear are in the same pose, and I just found it amusing. :D


Got the TL084s and TL082s. Voice 5 is now right, but 6-8 are just way off in consistent but arbitrary ways. I must have cooked the (capacitors, right?) near the TL084s that seem to do with auto-tune when I was stupid and switched the chips while on.

It looks like this is going to be my first recap. Any advice in selecting capacitors (or whatever I blew up) for an autotune circuit? I should replace them all anyway to be consistent, right?

Re: OB-Xa tunes same four oscillators sharp

Posted: Fri Oct 12, 2012 7:08 am
by Don T
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Re: OB-Xa tunes same four oscillators sharp

Posted: Fri Oct 12, 2012 10:26 pm
by DesolationBlvd
Don T wrote:Well, "cooking" a capacitor while the power is on is usually VERY obvious (In other words, they usually explode), it's also not likely.

That said, if your motherboards have any film caps that are painted a series of horizontal stripes of various colors, CHANGE THEM. They are sometimes called 'tropical fish' caps. They are notorious for all kinds of issues when they age, especially if the shell cracks near the leads and allows moisture/humidity in. They WILL cause the kinds of problems you've bee having when they start to fail.

Sorry, I forgot that the OB-Xa I worked on recently had some of those things in it, or I would've mentioned it earlier.
By "cooking", I meant that when the TL084 started to smoke as I put it back in when on (bad!), the caps were really close, and maybe that damaged them further.

So what kind of caps do I replace the "tropical fishes" with?

I only have a desoldering vacuum-plunger-thingy (bought so I could replace the SH-101's slider), not a fancy desoldering station. Is the desoldering station a must, or is it just to make things quicker?

Re: OB-Xa autotune issues - recap?

Posted: Sat Oct 13, 2012 10:05 am
by sam
good technique and equipment are essential to do these thing properly

Re: OB-Xa tunes same four oscillators sharp

Posted: Mon Oct 15, 2012 4:02 am
by Don T
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Re: OB-Xa tunes same four oscillators sharp

Posted: Tue Oct 16, 2012 1:50 am
by DesolationBlvd
Don T wrote:About the desolder station: Given my experience with solder-suckers and double-sided boards, especially THICK double-sided boards like the ones in the OB-Xa, I'd do my best to find a friend with a desolder station. The problem isn't so much the method as the fact that by the time you get the solder hot enough to lift it with a solder-sucker or desoldering braid, the solder pad and possibly the trace attached to it may lift from the board. The desolder station runs so hot that by the time the heat builds up enough to heat the fiberglass too much, the solder has long been melted. If you have to make a choice between a solder-sucker and desoldering braid, go with the braid. Sometimes the solder-suckers will suck the pad up along with the solder if the fiberglass gets too hot.
Good thing I didn't jump in with the desoldering, then.

c**p, one more thing I have to buy (nobody I know has one - nobody I know is into electronics). Does it matter which one I buy, or can I get away with a relatively cheap one?

I can only see it used once or twice more in the future (possible MKS-30 80017A replacement / when KIWI-MKS-30 comes out), assuming no more major vintage analog purchases.

Re: OB-Xa autotune issues - recap?

Posted: Tue Oct 16, 2012 2:46 am
by synthparts
I've been using the Hakko 808 desoldering gun for several years and highly recommend it. I like it better than much more expensive desoldering stations.

Re: OB-Xa autotune issues - recap?

Posted: Thu Oct 18, 2012 7:42 am
by Don T
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