troubleshoot VCO: no SAW, only SQUARE

by **kokoon** » Tue Feb 28, 2006 3:12 pm

so i've built the psu and the vco and when probing the waveform switch i can only get square. isn't that a bit funny? as far as i understand square is a waveshaped saw so this shouldn't be happening... help?

by **kokoon** » Wed Mar 01, 2006 2:56 am

okay i found out i put C11 upside down. correcting that doesn't help - still the same. also i found out that i'm get that square even with both Q8 and Q27 out. (i have them socketed)

did i break something with putting that cap in backwards? i mean besides the cap itself - it doesn't look broken but i replaced it anyway - didn't help.

by **kokoon** » Wed Mar 01, 2006 3:12 am

anyone notices anything else wrong (apart from that C11)?

by **guest** » Wed Mar 01, 2006 8:23 am

it is common to be able to hear only one
of the waveforms at the switch
although i think its the saw that people
tend to be able to pick up better
one of them has a stronger signal

i am suprised that you heard a signal
with the transistors out though
are you sure it wasnt just 60hz pickup
what are you using to listen to the signals

the board looked ok though

by **kokoon** » Wed Mar 01, 2006 8:33 am

yeah that's what freaks me out - it's there even without the 2 transistors...
i've used some cheap headphones. i've read all i've found about such problems and people always heard saw - it's the square that's sometimes trouble to hear.

what i'm picking up is actually not a clear square - it's much more complex - it sounds like heavilly pulsewidth modulated square. also amplitude goes up and down quite a lot. it sometimes sounds quite clipped...

is it okay if i have transistors socketed that way (see photo)? is it possible i damaged something really bad with putting the cap backwards? i'm really scared now - i always thought this can never happen to me. i guess i shouldn't do this at such late hours

any tips on probing some leads to localize the problem?

another thing - my wallwart outputs 12V DC. i just read it's supposed to be AC... but from what i can tell the psu takes care of that (the diode bridge) - and my voltages are okay now...

please guys tell me my x0xb0x is going to be fine... :cry:

by **kokoon** » Wed Mar 01, 2006 1:48 pm

well... i've UNsocketed the 3 transistors and now i get both saw and square. but still the pot and the trimpots do nothing. if i apply some voltage to R89 the sound changes but not exactly the pitch... i just get different sound and variation in amplitude.

should i continue to the VCF? should i keep away from the sockets for transistors now?

anyone any idea what's going on here?

by **guest** » Wed Mar 01, 2006 9:02 pm

putting the capacitor in backwards wont hurt the circuit
as long as the capacitor wasnt damaged
you should be fine

im suprised that your wallwart is working for you
check your 5v and 6v supplies
a dc wallwart shouldnt work with the power supply

perhaps the powersupply isnt working quite right
so the sound is getting modulated when you
load down the circuit with the headphones

by **kokoon** » Thu Mar 02, 2006 1:41 am

really? so i DO need an AC/AC wallwart! i've never seen one of those and i was in a shop yesterday looking for it. are they like... super-rare?

i'll check 5V and 6V but afaik they're not affecting the circuit yet...?

thanks for the help! i'll go look for the wallwart

by **kokoon** » Thu Mar 02, 2006 8:14 am

okay i modded the wallwart (i went to 12 shops and found ZERO AC/AC wallwarts) - so i just pulled everything out of my AC/DC wallwart and connected the wires. now i can get a nice 4.5 - 13 VAC out of it (selectable)

however, plugging it into the x0xb0x didn't help one bit. everything is the same, all the voltages are the same... it looks like the whole thing could run off a 12VDC as well.

so - anyone - PLEASE help me! what can i do to check some parts of the VCO? i'm afraid to go on to the rest - what if it doesn't work in the end? it will be even harder to localize the problem...

thank you "guest" for your help very much!

by **guest** » Fri Mar 03, 2006 5:40 am

is q26 a 2sc1583 on your board
if you havent done so yet
triple check all the parts
and solder joints

can you apply a test voltage of 2v or 3v
to r89 and measure the voltages on
all three pins of the tuning pot

by **kokoon** » Sat Mar 04, 2006 9:29 am

q26 is okay. i'll do a third check on all the resistors later today.

voltages of the tuning pot: as they should be - 1: 0V, 2: 0-5.33V, 3: 5.33V
the voltages don't change if i apply 3V to R89...

by **kokoon** » Sun Mar 05, 2006 5:34 pm

okay i've quadruple checked everything. every part of the VCO. i'm positive all of them are the ones they should be.

i'm wondering if maybe everything is okay from the start... maybe it's normal the thing floats so much... i can't even say float cause it's a much more complex modulation. i've recorded it and looked at the waveforms... they just barely resemble saw and square... could all of this be normal? but what about non-working of the trimpots and the pot?

is it possible that my solder's rosin core conducts? any way i can test the ICs used in the VCO?

i'm planning on going on with the VCF. or should i investigate some more? this is becoming really depressing... i thought i was going to be finished by now and i'm only at VCO...

is there anyone else that can recommend me anything? how come noone else but "guest" has any idea? does this stage really work for everyone else?

by **guest** » Mon Mar 06, 2006 4:14 am

im not sure what isnt working with your circuit
it might be working fine
but it is odd that you dont hear a clear pitch shift
when you adjust the tune pot

it wont hurt anything to continue building
but if youre interested in
poking around the vco some more
check the following voltages

q26 pin1 to pin5
where pin1 is the pin closest to r103
and pin5 is the closest pin to r118

pin1 5.333v
pin2 5.333v
pin3 4.8v
pin4 8.18v dc with 5.6vpp of triangle wave
pin5 5.24v to 5.30v depending upon note selected and tune pot

ic11 pin1 to pin3
pin1 4.57v
pin2 5.333v
pin3 5.333v

q28
gate should be same as pin4 of q26
source should be 8.8v dc with 5vpp of triangle wave
drain should be 11.8v

q24
emitter should be at 5.4v

the rest of the transistor voltages are difficult to measure
as the meter will load down the circuit
you can also try cleaning the flux off of the board
its possible that some stray conductance
through the flux is messing it up

by **kokoon** » Mon Mar 06, 2006 6:10 am

the freakiest thing just happened:
after finishing the VCF and non-succesfully testing it, i went to check the voltages you wrote about.

at first pin2 of q26 was like 2 times it should be but after probing it 3 times it went down to 5.33 so all was good. all other votages too. more or less. then i tried hooking up the phones to the saw again and it was there! a clear saw!

then a loose wire on my desk made a shor-circuit on the x0xb0x psu and loud sparks went out of it.
i quickly disconnected everything and removed the loose wire. after turning it back on the PSU output voltages on J4 were exactly 2 times they should be. i disconnected everything, also the main board from the psu board. i waited a little and said to myself FROM NOW ON ALWAYS CLEAN THE DESK!!!

then i checked again the psu alone and it was outputting okay voltages again. i hooked up the main board and... EVERYTHING WORKS!

the tuning pot, the filter section, everything works...

what-the-banned?

by **guest** » Mon Mar 06, 2006 9:38 am

im glad to hear things are working
especially considering the exploding psu

perhaps there was a bad solder joint on pin2
and pressing the meter probe on to it fixed it
you might want to rehit that joint

troubleshoot VCO: no SAW, only SQUARE

troubleshoot VCO: no SAW, only SQUARE

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