Comparous oscillation

[EfarmosmenosStokos]

Hello all,

I'm having a problem with a Comparous PCB I etched from tonepad. I assembled the board and powered it to check if it was working properly.
What I get is a very fuzzy and gated guitar that reacts to the sustain knob, and a steady wobbling sound like a metronome, even when there is nothing plugged in the input.
The wobble does fade with the level control and settles with the trimpot.

diagram: http://www.tonepad.com/getFile.asp?id=9

Any help would be much appreciated

[EfarmosmenosStokos]

Some more observations:

There was no sound or noise when I removed the first transistor Q1.
Q1 VCE was 6.67V with 10.97V power supply
Q4 and Q3 VCE around 9.23V
Q5 VCE 0.590V
Q2 VCE 0.002V

[PRR]

> Q2 VCE 0.002V

All _three_ voltages to ground on Q2, please.

And anticipating a *very* common problem.... where did you get your '3080? The market is flooded with fakes.

[EfarmosmenosStokos]

Thanks for the reply, I will report the voltages when I get back home tomorrow.
IIRC, I purchased the chip from BanzaiMusic.com in Germany. Is there another online supplier you could recommend for a chip?

[EfarmosmenosStokos]

I tried to measure the three voltages, but they were all fluctuating greatly. They were slowly oscillating around the following values:
VEG 0.0 - 0.03 V
VBG 0.0 - 0.06 V
VBG 11.50 - 11.70
I switched Q2 with Q1 around, but the same problem persisted.
Any help or ideas on what I may have done wrong?

I have redone all the soldering on the back, switched all the transistors around, but to no avail.
The 3080 is supposedly from a trusted online shop in Germany, so I don't know what to make of this. Next step would be for me to purchase a digital oscilloscope and follow the signal along the circuit.

Please help me

[PRR]

> Please help me

We can't see your probes. Please proof-read your report:

> VEG 0.0 - 0.03 V
> VBG 0.0 - 0.06 V
> VBG 11.50 - 11.70

"VBG" is zero? Or eleven?

Point "Vb" (junction of 56K and 27K) should be around 3 Volts. Is it?

The Q2 Base to Ground voltage should be a little less. Is it?

If the IC is socketed, remove it, and test Q2 Base to Ground voltage again.

> 3080 is supposedly from a trusted online shop

There have been too many reports of bad '3080s from otherwise trustworthy shops. There are tons of fake '3080s and maybe the shops do not have a way to test them. The only '3080s today that I would trust without reservation are from Small Bear. There may be other known-good sources.

But if it is not a bad chip, then 98% of the time it is a "wiring problem". Hair or whisker of copper on the PCB between two traces, bad solder, etc etc.

[EfarmosmenosStokos]

Sorry for the typo, I took another measurement today:
My powersupply is 11.40 Volts.
Vb is at 4.1 so that would be around 3V if my PS were at 9V.

My voltages were steady today, Perhaps I need a new multimeter.
VEG at 8.21V
VBG at 8.89V
VCG at 8.23V

I doublechecked, turned on and off, and even switched transistors.

So, no, my Q2 base to ground is higher(!) than Vb by more than 4.7V.
I removed the CA3080 and measured VBG at 3.7V, while Vb was at 4.1V.
Would that be an issue of fake 3080?

Thanks for your time, PRR

[PRR]

> removed the CA3080 and measured VBG at 3.7V, while Vb was at 4.1V.
> Would that be an issue of fake 3080?

Are both input pins on '3080 at the *same* voltage?

[EfarmosmenosStokos]

I measured pins 2 & 3 on the PCB as per the datasheet and both inputs had the same voltage, with or without the 3080 in its place.
~5.3V with, ~6.0V without the 3080 in place.

[PRR]

Get a genuine '3080. I suggest SmallBear.