CA3080 in Tonepad Small Stone.

[served]

Hi.

I recently built a Small Stone Phaser with Tonepad material. Well I probably broke it as its not working anymore. It was working fine for couple of months.

http://tonepad.com/project.asp?id=11

If I turn the Switch ON and max out Rate pot, then something starts to woble. Otherwise clean signal is coming through and Rate knob has a minor efect on the sound but its not phaser, its similar to volume of some filter.

I measured voltages on CA3080. No signal to input. Voltages Measured with - wire connected to chassis(gnd)


IC1
1. 0
2. 3
3. 3
4. 0
5. 0,67
6. 4
7. 7,35

IC2
1. 0
2. 3
3. 3
4. 0
5. 0,66
6. 5,7
7. 7,35
8. 0

IC3 (pin 6 is odd)
1. 0
2. 3
3. 3
4. 0
5. 0,66
6. 0,68
7. 7,3
8. 0

IC4
1. 0
2. 3
3. 3
4. 0
5. 0,66
6. 5,7
7, 7,4
8. 0

So as they all work in very similar setup then I feel like they should be the same?

Any ideas what is causing it to malfunction?

[slacker]

As the voltage on pins 5 and 6 is nearly the same, you could have a short between pins 5 and 6 of IC3. Check that first, if that's not the problem and you used sockets swap IC3 and one of the other ones and measure the voltages, this will tell you if the problem is with the IC or that particular position on the board.

[served]

Can you suggest how to skip that CA3080 step?
Is it possible?

[lonewolf]

usually when something works and then stops for no reason..that sounds like a possible cold solder joint..you can find them by moving components..or visually look for bad solder points..check the wires too

[served]

Its not a cold spot.

I think IC3 is bad.
Or something around IC3 is bad for example transistors. I will get some new ones.

[served]

Hi.

So still no luck.

I switched IC3 and IC4. Now exIC3 instead of IC4 is giving good results. So IC3 was okay from the start.


Could some one explain to me how it works?

What does the IC1 do with the signal?

As I understand you need 4 times IC to get 180 degrees of phase shift?

I am abit in trouble to figgure this out, as I don't understand the principal.

I changed Q6 Q7 and Q8, but no luck.

What could hold the 6 th pin of IC3? Is it bad or should it be like that?

[PRR]

> What does the IC1 do with the signal?

Same as the other three stages.

> 4 times IC to get 180 degrees of phase shift?

Ignore that part. You must have good DC voltages before you can pass audio properly. You don't have good DC voltages. Fix that.

> What could hold the 6 th pin of IC3?

Either a fault in IC3 _and_ the parts around it, _especially_ the transistors (which you did not report voltages for), _OR_ because all stages get DC level from the preceding stage, a fault in the stage _before_ it (again: without transistor voltages we are missing clues).

The full phase-shift circuit is messy and unclearly drawn for DC analysis. The 3080 has a weak output, loaded with a cap for phase, then buffered with transistors. This makes an "op-amp". The DC levels are set by the 3V Vref reference, a DC level from the preceding stage, and the two 27K resistors connected in Inverting Opamp style.

An Inverter is a lever. In this case the center is tied to 3V, both arms of the lever are equal.

Ignoring details of 3080+transistors, if Vref is 3V and the input is at 3V DC, then the output will be 3V.



Focusing on the internals: if the inverting feedback forces the output to be 3V, and the transistors have typical 0.5V-0.6V offsets, then the 3080 pin 6 must swing to 4.1V, so the two transistor-drops leaves the output at 3V.

You must check transistor voltages. In the 'sick' stage, but also the stage before it.

And very-much agree with lonewolf: if it worked for a little while, quit, was not abused.... that's the _classic_ sign of a bad joint. Not likely to be parts. Replacing parts is not a good way to find bad joints. Get bright light, magnifier, examine. If solder is gloppy, get the solder off, tin the pad, tin the lead, then apply barely enough solder to join. Angelina Jolie, not Roseanne Barr. Also examine PCB traces for cracks; and of course solder bridges or copper-whiskers.

[served]

Strange is that the VCC is floating.
Transistor voltages

Q5
C 7,4
B 5,6
E 5,4

Q6
C 7,4
B 5,4
E 4,8

Q7
C 7,4
B 0,68
E 1,2

Q8
C 7,4
B 1,2
E 1,2

[PRR]

> the VCC is floating

Do not know what that means.

[served]

If I put my multimeter on and measure voltage from ground to transistor Collector, then this voltage is not stable. Its  variing and the speed of it is changing by turning the rate knob.
I don't think its normal.

[served]

I will now attemp to start it with only one CA3080.
Lets see what happens.

I am afraid that its the LFO that has failed but as I have not had much time to work with it, I will do it today.

[served]

Hi.

So for future and for others.

I managed to fix the pedal. One CA3080 was broken.

To check if the circuit works you can check each chip 6-th leg for signal.
I personally audioprobed it (use a cap to do it).
So from 6th CA3080 leg you can hear acutal signal from phaser. First IC will provide you a 90 deg shift (and modulation) and so on.
So listen to the 6th leg to find out how the signal is moving.


And you can use only one CA3080 for 90 deg shift (if its needed)