Finished Build: No Cymbal, weak Handclap

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Finished Build: No Cymbal, weak Handclap

Postby the_sauce » Oct 15th, '19, 19:30

Hi everyone,

I just finished my Yocto build, PCB v1.0a. Everything sounds great except I don't get sound from the cymbal and the clap is very weak / quiet. Maracas are fine.

I get 127-135mV AC in the noise section and 529Hz on R44 / 798Hz on R45 in the cymbal section.

I've tried to reflow all solder joints in the handclap section. Haven't tried yet in the cymbal section.

Here is a recording of a kick-clap-snare sequence. Clap on second downbeat to compare loudness with other sounds: https://clyp.it/egigvslp

Here is just the clap, recorded louder: https://clyp.it/md3eb3qc

The master volume and instrument volumes are all the way up, no accent.

Any help troubleshooting this would be greatly appreciated. I'm not an electrical engineer and have no experience with hardware debugging but I'd like to eliminate all possible causes before replacing potentially faulty chips.

If necessary, I can provide closeup pictures of the board.

Thanks very much for your time
Last edited by the_sauce on Oct 18th, '19, 07:14, edited 1 time in total.
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Re: Finished Build: No Cymbal, weak Handclap

Postby the_sauce » Oct 17th, '19, 15:05

I remeasured the voltages for the Cymbal and Handclap ICs. I was fairly certain the voltages were correct while building the instrument sections but that doesn't look to be the case now.

Cymbal:

U5: 13.86v, -13.81v
U3: 14.5v, -14.6

Handclap:

U23: -14v
U22: 14.7v, -14v
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Re: Finished Build: No Cymbal, weak Handclap

Postby e-licktronic » Oct 22nd, '19, 07:25

Hi there,

the VCA and waveshaper of your clap seems not to work properly.
check with a scope the shape you have on U23 Pin13

Have a look on the TR-808 service manual page 6. Everything are explain on how works the clap.

Parts number of the Yocto v1.0 is corresponding TR-808
Best regards,
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Re: Finished Build: No Cymbal, weak Handclap

Postby the_sauce » Oct 25th, '19, 17:59

Hi Vincent,

Seems like AN6912 needs to be replaced? Here's a scope screenshot of U23, pin13. Voltages are off at the output despite -14v on pin 12 and the waveform is also missing the last three peaks after the initial peak, as shown in the service manual.

https://imgur.com/a/ehAhnTQ

Regarding the Cymbal, it is now fixed. A transistor pin was shorted to ground due to how close it sits next to one of the potentiometers. It was making contact but after bending it out of the way, the output is crystal clear now and sounds great!
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Re: Finished Build: No Cymbal, weak Handclap

Postby the_sauce » Oct 30th, '19, 19:34

I just replaced the AN6912 with an LM339N and I still have the exact same clap problem. I guess I can rule out AN6912 being faulty. Sucks since I had to trash it in order to replace it...
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Re: Finished Build: No Cymbal, weak Handclap

Postby helectromagneticus » Oct 31st, '19, 02:42

the_sauce wrote:Hi Vincent,

Seems like AN6912 needs to be replaced? Here's a scope screenshot of U23, pin13. Voltages are off at the output despite -14v on pin 12 and the waveform is also missing the last three peaks after the initial peak, as shown in the service manual.

https://imgur.com/a/ehAhnTQ

Regarding the Cymbal, it is now fixed. A transistor pin was shorted to ground due to how close it sits next to one of the potentiometers. It was making contact but after bending it out of the way, the output is crystal clear now and sounds great!

What do the other key handclap signals look like? Take a look at figure 13 in the original 808 service manual and compare your readings to what's given there. You will have to take multiple readings to get a complete picture. That circuit is fairly complex, so a lot can go wrong there.
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Re: Finished Build: No Cymbal, weak Handclap

Postby the_sauce » Nov 9th, '19, 14:08

Hi helectromagneticus,

A bit late since I had to return the scope and wait until I could borrow it again.

I managed to take readings of all but Pin 8 of BA6110 since the service manual shows an audio waveform for comparison. I didn't find a way to inspect that with the tools I have available. Happy to try if anyone has suggestions on how to do that.

https://imgur.com/a/0ke85PI

To be honest, I don't know how to interpret these readings in order to troubleshoot. The only things I can tell are some of the readings are off in terms of the signal shape and all readings are off in terms of voltage. None of them go from -15V to 0. You can see the relevant measurements in the bottom left of each screenshot.

Also, note IC23 is now an LM339N, not an AN6912.

Does anyone know what I should be looking at next?
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