566 question..

kas · Joined: Aug 26, 2008 Posts: 8 Location: Greece

I have finished building the 566 VCO with the magicsmoke pcb.There is a strange thing happening though..The volume of the tri and sine out is very very low.After I looked at the schematic and checked the resistors with the polymeter I discovered that the resistor (R19) in the loop of the opamp at the tri out was 43k instead of 120k despite the correct color code. I desoldered the resistor and put a new resistot 120k only to find out after soldering that the new resistor was a giving a reading of 50K. I measured some other unsoldered 120k resistors from the same and they read out 120K. Does anyone has any idea why this thing is happening?

Loukas

Danno Gee Ray · Joined: Sep 25, 2005 Posts: 649 Location: Telford, PA

If you were reading these resistors after soldering them in, you were also reading through the rest of the circuit as well which was skewing your results. To measure a resistor you must first unsolder at least one leg of it and lift it out of the circuit before taking measurements to ensure you measure THAT reasistor only. Chances are the resistors were Ok and were being read in circuit and through the circuit as well.

kas · Joined: Aug 26, 2008 Posts: 8 Location: Greece

I thought of that , that's why i removed the tl072 from its socket when I was measuring .Looking at the schematics It seemed to me that the tl072 is the one and only way that closes a circuit around R19.Many other resistors that I measured were found to be ok.Maybe I was wrong about the actuall value of the r19 but still the output i get from this opamp is low and I cannot find any other obvious reason..Any ideas?

davebr · Joined: Jun 09, 2007 Posts: 39 Location: portland, or

The input side of R19 connects to R21 and to both ground and Vee through R22. The other side of R19 connects to R41 through R44 to ground. If R41 and R22 are mid-range, that's 93.2K in parallel with 120K, or 52.4K which correlates with what you measured. R19 also connects to R55 but those are higher resistances. Certainly a low triangle output will affect the sine as well. I'd check all the parts for correct values by their color code, not by measuring them. It can get confusing if you are using 1% resistors. The extra color band makes it hard to tell which is the first digit and which is the tolerance band. There's just a slightly larger gap between the multiplier and the tolerance band. I know I read them from the wrong end sometimes.

Good luck. - Dave

kas · Joined: Aug 26, 2008 Posts: 8 Location: Greece

Finally the problem was solved.I just replugged the ics and soldered better some legs and magically the problem was solved .Seems that I will not find where the problem was but it' doesn't matter.. Cool

Thanks for the replies...
Loukas

Danno Gee Ray · Joined: Sep 25, 2005 Posts: 649 Location: Telford, PA

Glad you got it working.

Tim Servo · Posted: Sat Dec 27, 2008 1:21 pm Post subject: 566 question..

Hey Loukas,

Glad you got your TH-101 working. Thanks also to Dave Brown and Danno G for helping out. I'm sure we've all been bit by badd connections and / or bad solder joints, but it looks like you've found the problem area. Have fun, and upload some samples if you make any cool noises.

Tim (recovering from post-Christmas shopping) Servo