TR-909 scratchy claps?

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synthnoob
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TR-909 scratchy claps?

Post by synthnoob » Wed Feb 10, 2010 6:48 am

Whats up everybody. So I bought the 909 knowing that the claps were not working properly ( i got a sick deal on it so its all good). Here's the deal. When you turn up the level on the claps you here a lot of static and very faintly in the background you can hear the claps. As you turn the level up the static gets louder as does the faint background claps. Any ideas? I'm hoping I just need to clean the pots. So what do you guys think? Here is a pic of my baby
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Altitude
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Re: TR-909 scratchy claps?

Post by Altitude » Wed Feb 10, 2010 1:19 pm

Does the snare and toms sound fine? They all share the same noise source. There is an OTA in the clap circuit that could go bad also

synthnoob
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Re: TR-909 scratchy claps?

Post by synthnoob » Wed Feb 10, 2010 7:22 pm

Thanks Altitude. Yeah the snare and toms sound ok. I think i need to bring it in and get serviced. the 909 sounds good but I feel like it doesnt sound like it should. Some of the drum sounds sound a little staticky, nothing too bad as you can barely here it. Its just not I guess crisp and clear. I guess the sounds are a bit muddied.
But as far as the snare and toms they sound fine compare to the claps. What is an OTA by the way

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Solderman
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Re: TR-909 scratchy claps?

Post by Solderman » Wed Feb 10, 2010 8:09 pm

Operational Transconductance Amplifier ie. Voltage Controlled Current.
Looking at the block diagram, I see the handclap circuit has two VCA's before going to the OTA(which you control with the level knob). This part is labeled IC28, and is probably a BA662A chip.

The handclap is just two sounds: Bandpass Filtered Noise is the source for 2 paths, so the first path sends the noise through sawtooth amplitude-modulation, while the second path has the noise fade away slowly(with an AR envelope). The noise source comes from the snare. Both of these go to the OTA for loudness, and this is probably what has failed if you can still hear a perfectly generated handclap faintly coming through.
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Re: TR-909 scratchy claps?

Post by schmidtc » Wed Feb 10, 2010 8:17 pm

If you have access to a scope, check the signal in and out to be sure it's the cause. BA662's can be a little tricky to find. People in the ladyada forum should be able to help you find a replacement, the xoxbox world has been hunting these down for a while now.

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Re: TR-909 scratchy claps?

Post by Solderman » Wed Feb 10, 2010 8:35 pm

Sorry, IC28 is not a BA662a, IC30 is. Your tech should be able to figure out what IC28 is and replace it.
I am no longer in pursuit of vintage synths. The generally absurd inflation from demand versus practical use and maintenance costs is no longer viable. The internet has suffocated and vanquished yet another wonderful hobby. Too bad.
--Solderman no more.

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Re: TR-909 scratchy claps?

Post by schmidtc » Wed Feb 10, 2010 8:56 pm

If you're hearing an increase in volume when you turn the clap level pot then IC 28 is working. The level pot just changes the gain of this op amp by controlling the resistance in an inverted feedback loop. Sounds like something is messed up before this stage in the signal path.

synthnoob
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Re: TR-909 scratchy claps?

Post by synthnoob » Thu Feb 11, 2010 2:27 pm

schmidtc wrote:If you're hearing an increase in volume when you turn the clap level pot then IC 28 is working. The level pot just changes the gain of this op amp by controlling the resistance in an inverted feedback loop. Sounds like something is messed up before this stage in the signal path.
Well, with the level of the clap all the way down you hear nothing. As you roll the level knob up the static volume increases with he clap faintly heard in the background. the more you turn the level up the louder the static is and the louder the claps get. The claps are always in the background compared to the static.
I really need to sit down and read up on some electronics stuff. I guess I should take it to a tech. It shouldnt be that hard of a fix right?

Thanks for all the help by the way

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Re: TR-909 scratchy claps?

Post by Solderman » Fri Feb 12, 2010 5:16 pm

You've already got two different prognosis from two people, one or both of whom could be wrong, who haven't measured any voltages on your 909. My advice is if you aren't already experienced in electronics troubleshooting, and do not own an oscilloscope, then take it to a tech.
I am no longer in pursuit of vintage synths. The generally absurd inflation from demand versus practical use and maintenance costs is no longer viable. The internet has suffocated and vanquished yet another wonderful hobby. Too bad.
--Solderman no more.

ripe
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Re: TR-909 scratchy claps?

Post by ripe » Sun Feb 14, 2010 6:52 pm

I would also check for a bad 2SC2603 transistor in the clap circuit, I don't have the 909 schematics handy but this is a common part to fail with similar symptoms to what you describe.

cheers
ripe

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