no sound.. VCO DC approx correct, but not AC

by **djarago** » Sat Nov 05, 2005 12:51 am

So my x0xb0x doesn't work.. for as many times as I checked parts when building I can't imagine I have anything mixed up (and I had the right number of each part so i don't think I put anything in wrong). Also after it didn't work I double checked every polar component to make sure I had them in right. I suspect I have a bad solder joint somewhere, but can't find it by visual inspection.

Here's what I know when I connect headphones to the headphones out I hear noise when I turn it on for a short time, like most audio equipment. I also hear static while im turning the volume knob between about 2 o'clock and 5 o'clock positions. If I connect something to mix in I hear it on the headphones. The volume control doesn't affect the mix in. There is no other way for me to hear anything coming out of the unit.

I thought it would be best to start troubleshooting from the beginning. The PSU is outputting 5.33v as specified. I'm sure it is correct. The VCO when I probe at the waveform switch gives me approximately 1.5VAC on pin 3 and 0.75VAC on pin 1. I get 6.4VDC and 9.4VDC on those same pins respectively.

Any ideas as to where I should go next would be greatly appreciated. I'm just kind of randomly looking for bad solder joints...

by **djarago** » Sat Nov 05, 2005 1:05 am

I just noticed R84 appears burnt at one end. Any ideas as to what may have caused this? I have other 200k 1% resistors I could replace it with, but I'd rather fix whatever damaged it. Could this have been done by me shorting it out when I was trying to probe during construction?

by **Guest** » Sat Nov 05, 2005 4:20 am

r84 probably got burnt during soldering
there are no voltages on the board which are high enough
to burn that resistor

it seems that your vco is working
id try to listen to it through a good set of headphones
or an amplifier to make sure it sounds right

the next place to check is the vcf
put your probe on the resonance pot
i think this is described in the manual

if there is signal there
check for signal on the volume pot
to ensure that the vca is working

by **Jonnay** » Sat Nov 05, 2005 1:38 pm

I found testing through headphones to be a real PITA. THe signal was really low, so you really had to listen. On top of that, I found tha tif my hand moved at all, I would be inundated with static.

So in short, testing with headphones means wiggling it around a little, waiting, and then wiggling it again, waiting.

by **zero01101** » Sat Nov 05, 2005 1:59 pm

agreed... headphone testing is mui diffuculto. im pretty sure thats why i thought i had screwed my b0x...

by **djarago** » Sat Nov 05, 2005 2:02 pm

I forgot to mention. I did probe the resonance pot (the far right and far left pins just to make sure) and I was getting nothing... well more like 0.07VAC and 0VDC if I remember correctly.

I'm going to try to listen to the VCO with the help of an amplifier. Is the best place to do this at the waveform switch?

If that sounds ok I'll look at the VCF extremely closely.

by **Guest** » Sat Nov 05, 2005 6:10 pm

your measurements of the vcf could be correct
with the resonance turned all the way up
of the cutoff turned all the way down
you only get about 70mV of signal

with cutoff full up and resonance full down
you should get about 800mV of signal

the main reason why testing with headphones works poorly
is due to the flux covering the contacts
if you clip your multimeter probe to the headphone jack
you can press firmly into the contact and pierce through
the layer of flux and get a better signal

using an audio amplifier gives better signals though
as it does not load down the circuit

by **djarago** » Sat Nov 05, 2005 7:30 pm

I'll have to remeasure, but I'm pretty sure I had the filter cutoff fully CW and resonance fully CCW and that the measurement was 7mV, not 70mV. I may have been delirius as it was something like 3am and I had been working on it for about 9 hours straight at that point.

by **djarago** » Sat Nov 05, 2005 9:50 pm

The resistor I thought looked like it might be burnt turns out to be fine.. I removed it (so I could replace it), but I tested it with my DMM and it came out to 201k or so.

I've gone through each part individually starting from the beginning trying to check every single aspect of this thing. Each part I make sure the part is correct and inspect the solder joints. I'm nearly done with the VCF without finding any problems yet. I did find something odd though, it seems like the one leg of C13 was loose (inside the cap itself not the solder joint). I bent the capacitor upright (as it was bent sideways a bit) now it doesn't seem loose anymore.

by **djarago** » Sun Nov 06, 2005 4:02 am

Update:

I connected my soundcard (emu 1212m) to a cable and probed the osc selector switch and recorded it. I definatley see a saw and a square wave, but the amplitude is very low (and the frequency is about 29hz).. like approx 1/8 max for the saw and 1/16th max for the square.

When I do the same on the right (and I tried left) most pin of the res pot I also get an extremely small signal. This signal is about 1% of max and is so small that even when I enlarge it looks pseudo sinusoidal with an extra peak every cycle.

It appears to me that both the VCO and VCF have a signal passing through them, but it's so small I must have something in the VCO messed up. Any ideas what this could be? I extremely carefully checked every part in the PS, VCO and VCF many times. Each part individually. I do not believe I have made a mistake with the parts, but I suspect one (or more) are bad.

Oh yea I'm not sure how much affect the load on the circuit from my soundcard was. I measured the resistance of it to be about 9.8kohm. Much less load than headphones, but still not as good as using an oscope

by **Guest** » Sun Nov 06, 2005 4:45 am

is the digital section all done
can you program and run a pattern
if so listen to the headphone out whilst playing a pattern
turn the volume full up
mess with the filter knobs and see what happens

is it dead silence or are there pops and noise with each step
it seems you have some signal at the vcf
so if the vca was working correctly you would have
some signal coming out of the headphones
i think the vca may be the culprit

by **djarago** » Sun Nov 06, 2005 9:34 am

It has been completed for 2 days now. The diigtal section appears to work although there are some oddities. When in keyboard mode if I play any of the # keys no lights come on, but if im in (i forget which mode) they will come on. I had been using random and keyboard mode to attempt to test.

I hear dead silence, not pops or anything (i do hear pops when I first turn it on).

So my extremely low signal values are probably just from my soundcard loading the circuit?

by **zero01101** » Sun Nov 06, 2005 10:29 am

i know nothing of the electronics game, but if you ARE getting output but its just ludicrously low, id scream AMP!!! from the mountaintops.

by **djarago** » Sun Nov 06, 2005 12:58 pm

Well I'm trying to measure the raw output of the VCO and VCF, before it hits the VCA if that's what you're referring to. I was connecting to my soundcard so I could record and see the signal (and then amplify to hear).

I would think the VCO output would be fairly high to keep the signal strong before going into the VCF and so on.. perhaps this is the case and the attenuation due to the 9.8kohm load I'm putting onto the circuit is the problem.

by **Guest** » Sun Nov 06, 2005 4:05 pm

check to make sure the gate is going to 12v with each step

no sound.. VCO DC approx correct, but not AC

no sound.. VCO DC approx correct, but not AC

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