Moogah's Fabrication Journal.

Discuss x0x construction and related issues

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moogah
 
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Joined: Sun Jun 19, 2005 8:51 pm

Moogah's Fabrication Journal.

Post by moogah »

So, my x0x arrived yesterday and I promptly began to put a 8 hour day in at the workbench. Before I slept I managed to get the PSU, VCO and VCF assembled. The PSU and VCO both appear to work as they should, however, while scoping the output of the VCF It seems that I can't open the filter all the way, both the square and saw still look quite a bit filtered with the cutoff turned all the way up. turning up the resonance causes quite a bit of attenuation as well and the signal is completely cutoff at about halfway down whereas the fab manual seems to indicate I sould se a signal even with the cutoff all the way down. I haven't been able to trace this yet, but I'm fairly shure the caps and transistors are all placed correctly. I haven't stated checking the resistors yet, hopefuly one of you will have a pointer about this before I go checking them. This morning I began on the envelope, which appears to work fine so I moved onto the VCA. I can't seem to get it to pass the test from the manual. Neither scope nor headphones will pick up a signal at the rightmost pin of VR8 (which I havn't connected yet). Does anybody have a pointer or two for me?

djarago
 
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Joined: Sun Jul 10, 2005 1:45 am

Post by djarago »

i could be off on what you're describing, but the square wave on the tb303 does not look like a 'perfect' square wave. this is part of what gives it its unique sound. I suspect what you describe as the square wave looking filtered is actually what it should look like.. however I *just* ordered my x0x so take my info as you will.

steev
 
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Joined: Thu Oct 20, 2005 7:35 pm

Post by steev »

Check out the software Oscilloscope thread (assuming you are using a hardware oscillo) and take a screenshot of the waveform :)

moogah
 
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Joined: Sun Jun 19, 2005 8:51 pm

Post by moogah »

I've spent the remainder of yesterday and the beginning of today troubleshooting these issues. Last night I checked each resistor, capacitor and transistor in the VCA and VCF individually against th BOM for each section. Everything checked out except I found 2 resistors I missed in the VCA section. Still, however, I can't get any sign of life out of the 6110. Both the outputs and inputs (as I understand them) are mute, simply showing 5V. The signal from the vcf gets to the backside of C21 but is shifted up about 5VDC. On the other side of the resistor there, it dissapears simply reading 5V. Now, at this point my tracing of the problem comes to a halt as I can't reconsile what my board looks like against the pic of the schematic online. I'm hoping to get a comment from one of the masters about that. Now, about the VCF. Something is certianly out of whack with it, but as a whole it works, it even responds to the envelope trigger which I can watch with my scope. However, increasing the amount on envelope causes the cutoff to drop while there is no trigger present and it seems the decay value has no effect, but it's possible it's difficult to see the difference as a 303 doesn't have a long decay time. Can someone verify which pin on the res pot I should be probing? Currently I only get any kind of signal from the rightmost pin on the component side, on the leftmost pin I see no signal untill I turn up the resonance. Now, on to the questions:

Why are there two leads coming from the rez pot into the VCA and why does one of them show nothing on the scope (C20)? If I understand it correctly the connections leading from C21 and C20 lead back to the feedback controll for the rez, but why are the output and input to the filter tied to the input of the VCA. I've only made a OTA SVF before, and I don't quite understand how this one is working. What is the second half of the stereo pot doing? Making a wild guess I assume this provides volume controll to the resonance. I'm looking at the VCF like a ladder filter, is this correct such that R70 and R98 serve as feedback resistors?
While probing from the output of the VCO forward I've found that the AC signal at where R62 and C17 meet is greatly attenuated, I move my scope from the 5 setting to the .5 setting before I can see the signal again. At the top of R62 things look proper.

Jonnay
 
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Post by Jonnay »

Hmm, I am not sure if this is going to answer your question, but maybe this will give you a few hints.

The second half of the stereo pot (with D24, R46 and C13 hanging off the other side) is to give the resonance boost for the accented notes. So your guess about it providing volume control is pretty close.

The VCF is apparently a Diode ladder filter, but with transistors acting like diodes instead. To be honest, I don't know much more then that.

moogah
 
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Joined: Sun Jun 19, 2005 8:51 pm

Post by moogah »

Well, I'm still stumped. I just thoroughly de-fluxed the board as I hadn't yet done that and was hopeful that it would solve my problems. However, taking the measurements again, nothing has changed. The signal is still being filtered qite a bit (there really isn't much above the 2nd harmonic and showing up) with the cutoff all the way up.. Can someone please give me a yes/no about measuring the output of the VCF on pin 6 (rightmost) of the dual pot? If it is supposed to be pin 1 then I still have a strange problem as nothing shows up untill I crank the rezonance, at which point I can see a proper signal for the waveform that is selected, (although it is still too filtered) and sweep the frequency (noting the changes as harmonics are reached). the VCF aside I would greatly appreciate it if someone could take some measurements at C20, C21 and R120, R121 as the lack of signal into the VCA is whats preventing me from moving forward.

adafruit
 
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Post by adafruit »

moogah wrote: Still, however, I can't get any sign of life out of the 6110. Both the outputs and inputs (as I understand them) are mute, simply showing 5V. The signal from the vcf gets to the backside of C21 but is shifted up about 5VDC. On the other side of the resistor there, it dissapears simply reading 5V. Now, at this point my tracing of the problem comes to a halt as I can't reconsile what my board looks like against the pic of the schematic online.
the schematic picture is BANNED up, but....
note that the BA6110 is the top one and the '662 is the bottom one
for the '6110
the input pins are #1 & #2
control signal (env) is #4
#3 & #5 are gnd
#6 is unbuff out
#7 is buffer in
#8 is buffered out
#9 is pwr
we have a datasheet here somewhere...ill try to dig them up

Guest
 

Post by Guest »

if you have a scope check out the pins on the 6110
the voltages should be as follows

pin / dc bias / ac signal
1 / 5.3v / 10mv
2 / 5.3v / 0mv
3 / 0.0v / 0mv
4 / 0.0v / 1.2v
5 / 0.0v / 0v
6 / 5.3v / 750mv
7 / 5.3v / 750mv
8 / 5.3v / 750mv
9 / 12v / 0v

pin4 is the signal from the gate
so it should go to 1.2v when the gate is high
and 0v when the gate is low

the signal at pin1 is very hard to see except on a clean scope

Guest
 

Post by Guest »

in regards to your filter

the best place to measure its output is on the rightmost pin
of the resonance pot
this is the pin that gets fed from c14

the output here should not look like an exact copy
of the output of the vco
the output of the vco is capacitively coupled into the vcf
so only higher frequencies can pass through
basically the signal should look rounded off
not as sharp

with the resonance full down and cutoff full up
you should get about 2 to 3 volts of signal on this pin

Guest
 

Post by Guest »

in regards to the vca measurements
these should be conducted with the following settings

volume full up
resonance half up
cut off full down
env mod full down
decay full up
accent full down

to make the signals a bit easier to see
you can turn the cutoff full up
pins 6 through 8 will then be at 2v of signal
and pin1 will be at 30mv of signal
the rest shouldnt change

moogah
 
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Joined: Sun Jun 19, 2005 8:51 pm

Post by moogah »

"signal should look rounded off not as sharp"
I suspected as much, however, should turning up the rezonance cause a good bit of attenuation?
Also, the fab manual indicates that even with the cutoff all the way down some signal should be getting through whereas even around 100hz I lose signal about halfway down on the cutoff

! So both the 6110 and 662 are on the schematic!? That clears up things a bit! Still have the same problems tho, here are my measurements on the 6110
1: 5.3v
2: 5.3v
3
4: 0v/12v (should this really be 1.2v?)
5
6: 5v
7: 5v
8: 0v (!?)
9: 12v

These remain the same after making the adjustments above. I havn't yet placed the volume pot, perhaps this is a problem?

Guest
 

Post by Guest »

do not plug in your synth until you check the following
make sure q1 through q4 are in correctly and of the correct type
im guessing you have the pnp and npn transistors swapped
if they are there is a problem with those transistors
it should not be 12v at pin4
your 6110 may be damaged already

also pin8 being at 0v is very strange
double check ic15b is in the correct orientation


the resonance seriously attenuates the signal
dont worry about the vcf until you can listen to it
it will probably sound fine

the volume pot doesnt need to be in for any of this

Guest
 

Post by Guest »

sorry
line4 should read

if they are in correctly there is a problem with those transistors

moogah
 
Posts: 186
Joined: Sun Jun 19, 2005 8:51 pm

Post by moogah »

Ok, the tranny's are in correctly and soldered properly. I'm repalcing all 4 now. *crosses fingers for the well-being of the 6110*

UPDATE:

I just finished replacing all 4 of 'em. Still getting 12v at pin 4
Last edited by moogah on Thu Oct 27, 2005 12:38 pm, edited 1 time in total.

Guest
 

Post by Guest »

while you have q1 out you should try
connecting a 220k resistor from pin4 of
the 6110 to 12v
this will allow you to verify that it is still working

there is a possibility that the fault lies in
some other part of the envelope section
check q31 r131 r123

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