Octup! (?)

[g3rmanium]

Hello,

first post here... Has someone gotten the Octup! from http://geocities.com/tpe123/folkurban/fuzz/snippets.html to work? I built in on veroboard. I corrected a wrong placement of the MPSA13 today (thought they were wired like other NPN transistors), but I still do not get any signal after the input cap. There's some 2 V at the base of the MPSA13, but no signal.

Here's a photo of the layout.



And here's the same photo with annotations:



Also, what octavers would you recommend that are easy to build? I'm toying with the Dan Armstrong Green Ringer.

Thanks.

[bioroids]

Can you measure the voltage at the pins of the first transistor?

Miguel

[g3rmanium]

Sure,

MPSA13 Collector -> GND: 6,6 V
MPSA13 Base -> GND: 1,4 V
MPSA13 Emitter -> GND: 0,8 V

The battery only has about 6,6 V left.

I set up two resistor decades at the input voltage divider.

[bioroids]

So, the battery has 6.6v instead of 9v? And you have also 6.6v at the collector of the transistor?
Seems like there's no current flowing there. How is the setting of the pot at the input?

Miguel

[g3rmanium]

I don't have that pot-1 MOhm-setup at the input. I use a voltage divider setup (currently 47 K from +9 V to the signal path and 47 K from the signal path to GND).

[bioroids]

The voltage divider thing it's ok I think.

I suppose the battery is too low to supply enough current for the circuit to work.
You said you have no signal after the input cap? That's strange, how are you testing it?

Miguel

[R.G.]

(thought they were wired like other NPN transistors)

There is no "like other NPN transistors" to the best of my knowledge. Every possible combination of three leads and three definitions (E, B, or C) have been used for NPN and PNP transistors.

You'll save yourself many headaches if you never assume that any transistor is wired like any other until you have checked the datasheet for the specific parts you're using.

[MartyMart]

http://www.alldatasheet.com/

This should help you with your transistor pinouts 

MM

[g3rmanium]

http://www.alldatasheet.com/

This should help you with your transistor pinouts 

Or http://datasheetarchive.com/

[g3rmanium]

You said you have no signal after the input cap? That's strange, how are you testing it?

I'm using the output lead.

[bioroids]

I don't have that pot-1 MOhm-setup at the input. I use a voltage divider setup (currently 47 K from +9 V to the signal path and 47 K from the signal path to GND).

You have an input impedance of 24K using the voltage divider without the 1M resistor. You shouldn't ommit it. You can still use the two 47K resistors to replace the pot, but connect their juntion to the signal path with the 1M resistor.
Also be sure to check the transistor pinout as everyone suggests, though looking at the datasheet and your pic it seems to be right.
Also check all your connections, in particular at the input stage for bad solders/broken paths/etc
Finally try to test it with a good battery (at least 8v I'd say)

Luck

Miguel

[g3rmanium]

Also be sure to check the transistor pinout as everyone suggests, though looking at the datasheet and your pic it seems to be right.

You mean the corrected version? The photos show the MPSA13 the wrong way around.

Also check all your connections, in particular at the input stage for bad solders/broken paths/etc
Finally try to test it with a good battery (at least 8v I'd say)

Thanks, I will do that.

[bioroids]

As it looks in the picture, with the flat side pointing to the right, it is placed ok (according to the datasheet I checked).

If you changed it, then you should check it again. The Fairchild MPSA13 datasheet says it is Emmiter-Base-Collector looking at the transistor flat side.

There should be a voltage difference between base and emmiter of about 1v. And there should be some voltage drop in the collector resistor. The voltage at the collector can't be the same as the battery potential, because that means the transistor is off (unproperlly biased).

Luck

Miguel

[petemoore]

  6.6v isn't enough to test an octave circuit, unless you know it's enough to test that octave circuit.
 

[g3rmanium]

As it looks in the picture, with the flat side pointing to the right, it is placed ok (according to the datasheet I checked).

You are right. Apparently, I can't read.

There should be a voltage difference between base and emmiter of about 1v. And there should be some voltage drop in the collector resistor. The voltage at the collector can't be the same as the battery potential, because that means the transistor is off (unproperlly biased).

Well, this evening, I changed the input cap. After this change, I could hear the input signal before the base of the MPSA13. I assume the old cap was damaged while soldering.

I soldered the output cable after the cap following the emitter of the MPSA13. With some tweaking of the resistor values, I was able to get a (low) sound out of it and even some fuzz. I used 680 K from +9 V to the input signal and 1 M from the input signal to GND. Voltages are

Emitter -> GND: 4.2 V
Base -> GND: 5.17 V
Collector -> GND: 4.85 V

[bioroids]

You are right. Apparently, I can't read.

Well, this evening, I changed the input cap. After this change, I could hear the input signal before the base of the MPSA13. I assume the old cap was damaged while soldering.

I soldered the output cable after the cap following the emitter of the MPSA13. With some tweaking of the resistor values, I was able to get a (low) sound out of it and even some fuzz. I used 680 K from +9 V to the input signal and 1 M from the input signal to GND. Voltages are

Emitter -> GND: 4.2 V
Base -> GND: 5.17 V
Collector -> GND: 4.85 V

Tha transistor is now saturated, lol. You want the collector to be higer voltage than the base. That's because you got too much current on its base. As the MPSA13 is a darlington, it only needs a bried amount of current to be set in the precise place. You should put the 1M resistor as per the schematic, and use a voltage divider there (yeah, I know you'll have to tweak a little the board). I dont know this particular circuit, but try to set the voltage at the collector at around 6.5v (with a 9v fresh battery), so the input stage doesnt get saturated or cutoff.

Luck

Miguel