Living VCO Troubleshooting

bennethos · Joined: Apr 18, 2009 Posts: 31 Location: Belgium

Does someone have an overview of the voltages for the tl072 pin layout without an ic ? or a reference sheet which voltages should be get where ?

for the TL072 :

VCO 2 :

6&5 -0.34v with ic , wihtout : +9.2v
2&3 : with ic : 0.32v , (shouldn't this be 0 ?)
8&0 : +14.63 without ic
4&0 : -14.63 without ic

VCO3 :

6&5 with TL072 in socket : 0.2v , without ic 8.98v
2&3 0V
8&0 without ic : 15.6v ( this looks like it's getting too much juice ?, this happens to be the only vco that's almost completely working, except for the bump when the sawtooth ramp starts. see pictures above)
4&0 without ic -14.07v

help would be appreciated, been debugging 4hours, it's fun I'm learning more about reading schematics & electronics :p .

Danno Gee Ray · Joined: Sep 25, 2005 Posts: 1351 Location: Telford, PA USA

Here is the data sheet for the TL072.

tl072a.pdf

Description:

Download (listen)

Filename:

tl072a.pdf

Filesize:

1.08 MB

Downloaded:

598 Time(s)

bennethos · Joined: Apr 18, 2009 Posts: 31 Location: Belgium

I think I didn't made myself clear, I want to compare my measurements I posted vs measurements of someone's vco that works.

I know remote troubleshooting is hell, but what *can* I post so you can help me out ?

Uncle Krunkus · Posted: Wed Feb 10, 2010 1:40 am Post subject:

I think some reference to which circuit you're talking about might help. Wink

bennethos · Joined: Apr 18, 2009 Posts: 31 Location: Belgium

Hmm I thought I was in the living vco section when I posted. well that's the circuit anyways Smile

jhaible · Posted: Wed Feb 10, 2010 11:04 pm Post subject:

If the supply voltages are about right (+15V at pin 8 and -15V at pin 4), plug these chips *in*. Voltages without these ICs don't make much sense.
If you burn a TL072, you burnt 50 cents.

Strange that you have *different* supply voltages on different pins. Something fishy here - there should be no voltage drop to speak of, across the ferrite beads.

JH.

bennethos · Joined: Apr 18, 2009 Posts: 31 Location: Belgium

Hi Juergen,

Thank you for your time.

I have +15v & -15v on vco1 & vco3 . VCO2 gives +14.8v & -14.9.

If I put in the TL072's my sawtooth waveform has a dip when it starts (vco1 & vco3, vco2 is not working at all but I'm focussing on 1 & 3 atm) (see pictures I posted from oscilloscope).

VCO 1 has a second problem, when I up my frequency with the 10 turn after 3/4 the waveform rises flattens like a trapezium and drops. I think there's something wrong with the comparator not sending the reset impulse in time to the npn transistor or the input voltage of my tl072 on pin 6 causing problems with the GAIN calculation. It's all a bit new to me but I'm learning fast and it's exciting Smile

Could you tell me the voltage I should get on pin 6 & 7 of the TL072 please ? (vco 1 has -0.12v on pin 6, vco 3 0.21v with the 10 turn dial @ highest freq setting).

Other question, to make troubleshooting more easier (fixing my saw) I wanted to isolate the circuit by desoldering R27 1K resistor & removing the CA3086, is that OK or has it impact on creation of my saw ?

I will try to measure over the ferrite beads to see if I have a drop... One thing I noticed though, I followed Dave Brown's BOM and put in 10uF electrolytics near the Ferrite instead of 22uF...

thx
Benjamin

bennethos · Joined: Apr 18, 2009 Posts: 31 Location: Belgium

I solved my problem yesterday evening, as Juergen said "it's a silly problem most of the time" and he was right.

I actually inserted 1 wrong capacitor, the one between the base of the transistor and input of the comparator.

My saw waveform had a dip when it started and when my 10 turn was fully open (no resistance) I had a half trapezium :p .

on vco 2 pin 2 of the TL072 made no contact to anything, made a little bridge and that solved it as well Smile

can't wait to test those *beasts* Smile