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TR808 BD modulation...
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author TR808 BD modulation...
Hi, i have a friend who bought an 808, he tried it quickly and got a little scam.
He realized that later, that kick isn't ok, it did not get along with other sounds playing at the same time ... Dead Banana

So initially, the kick had a problem as if the decay modulated, when it was at most, he replaced the pot decay and it has not changed anything. Then he left 808 for me to watch.

So i started by replacing components on the decay part, no difference ... Then persuaded that it came from the analog part, i replaced all the capacitors. No difference ... Then all the transistors. Then all the resistances.
The bug is less obvious, but if we listen well, we still hear a little, i recorded:

the bug is very light, you must listen to hear it...

To a certain extent, we hear more beug, decay thoroughly, full tone and full accent, but still it seems to me always present, it's just that some settings of the BD optimize its harmful presence .
Out of curiosity i compared with my yocto clone, the TR808 has a more pronounced attack, detail that i noticed.
I did some tests, the scope of the trig kick, has some little variations. But i do not think that small trig voltage irregularities could pollute the sound of kick.
I tried a few things, without really knowing, i disconnected trig from kick and snare and reversed it. If i sequence the kick with the trig of the snare, the kick is still not good, no difference. Does anyone have an idea to propose to me?
Are the PSU voltages clean? I mean no ripple or noise... may be some voltage variations causes the modulation.
Yes the power supply is OK, this is the first thing i checked. By cons, only BD jokes, all other instruments work perfectly.
I checked pin 17 of the power supply, which is the BU point. When the TR808 runs, the voltage oscillates from 4.2VDC to 4.5VDC. I ask if this voltage variation would not be too high. hmmm.....
I think the variation is a bit high, but that voltage is for the memory part of the circuit, I think, so don't think is going to affect the analog part... but if you have this variation in a part of the PSU may be you have a problem in it...
Yes i think this allows a reserve of charge for the memory, when the batteries are out of order. I compared with my TR606, BU point oscillates too, but much less, that's why i have a doubt.
On the other hand + 15 / -15 / + 5 are perfectly stable.
My old 808 had a kind of distorted BD caused by a bad trace on the pcb. Maybe that's the source?!?!? seriously, i just don't get it
I thought about that for now, i checked the via visually with a magnifying glass, i'll check with the tester. But this is not distortion the problem, I think it looks like variations of attack, it means that when the decay potentiometer is more than half.
I was wrong it is not the pitch of the bass drum that is affected, but the accent.
I also noticed that the bass drum is very bory when when the rotary switch "INSTRUMENT SELECT" is pointed to Accent. But it's a bit boring, pointed BD. And bug even less rotary switch pointed to the other instruments.

I made other recording, where it's very bugz, we hear that kick drum are stronger with randomly, there are variations while the pattern is "total accent":

Besides, i photographed the recording, we see that there are higher peaks and it's random. (sorry i could not capture pics the computer's, it's an old XP dedicated to audio)

Nothing to do with my problem.
I found a funny detail on the service manual, the accent in of all instrument are named trig. . For example, on this image capture, i indicated in green the accent input and in red the trig input.

I continue my search for the beug. Last night i went to a friend who has a better scope than mine. So i sequenced 1/2/5/13 steps for BD, SD and accent. We see that the trig of the kick and snare, seem to be OK:

By cons my ears were not wrong, we see that at R165, the input of common accent, there is variation, the signal should be the same and it is not:

By cons what is strange only the BD is affected, the other instruments have no variations, while the TR808 accent is common to all instruments. I must concentrate my research on the ACC module. idea
so, that shorter hieght pulse is only when BD is accented? otherwise it is tall like the rest?
Yes if i program only accented notes, there are short and high pulses, whereas there should be only high, i think. And i hear the abnormal behavior.

If i turn the potentiometer accent down, there are only short pulses and we do not hear the abnormal behavior.
i would suggest remving R152 and seeing if the pulses work as they should (you wont hear the BD, though). there is probably a short or bad component right where the TRIG signal enters the BD circuit.
No, the accent should be present all time.
The transistors at the input of all drum circuits are a AND gate, so to trigger the sound it should be present both trigger and accent, the non accented notes have an accent of 5V, and the accented notes have an accent of something beteween 5V and 15V (depends on the setting of the accent knob), but always have to be a signal in the accent input.
Thank you for your answers, i have not desoldered R152, i will try soon. But i did a quick test, i sequenced the accent alone and probe it, without any other inst. With the ACC knob set to the lowest, we can see that everything is normal:

There is 4.6V in a regular way, I put the "measure" option with my scope.

But when Accent potentiometer thoroughly, nothing goes well. I have 4.6V to 13.2V randomly. MY ASS IS BLEEDING While i should get 15V. d'oh!

is this with only the BD active, and all BD hits accented? it would be good to set the BD to hit once a second, or something slow like that, and see how often the accent trigger appears on your scope. is it just once a second, and in time with the BD sounding? it might be that the small pulses you are seeing are due to the line being reused elsewhere in the circuit when voices are not being triggered.
The last measurements that i did (pictures with rigol scope), it was without sounds, only the accent was sequenced and we can see that there are pulses variations.
yes, i think it would be good to do with with sounds. the digital line from the microcontroller that controls the accent, also is used to read and write to the memory, so there will be pulses on that line which have nothing to do with accents. so whats important is the level of that line only when a note is being triggered. to do this, use a second probe into your scope, and take a look at the BD control signal (the one going to R152), and set a second probe to the accent signal. trigger on the BD signal, so youre only looking at the accent when the BD goes high.
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