Author Topic: First Post: Somthing's not right  (Read 5705 times)

GroovyMan

First Post: Somthing's not right
« on: December 05, 2009, 06:16:08 PM »
Hi all,

This is my first post on this forum and my first build. I've built the beginner's project circuit using vero layout from here:

http://www.diystompboxes.com/smfforum/index.php?topic=74762.0

When I test it I seem to get a cut rather than a boost. I've been testing it with a signal generator and a scope.

The boost control seems to vary the gain from about 0.5 to 0. Pulling out the transistor has no effect on the output.

I've varified that I have +9v on the board and have checked that I have DC voltages at the collector, base and emitter. I can see the input signal at the base using the scope, so it's getting through the input cap, but the signal at the output is just weedy.

I'm using a 2N2222A, any idea what the voltages should be for this tranny? Any other clues what might be going on?

Thanks,

Ben
Next project: Ross Compressor
Completed: NPN Boost, Vox V847 True Bypass and DC socke mods.

pazuzu

Re: First Post: Somthing's not right
« Reply #1 on: December 05, 2009, 06:36:37 PM »
i built mine with a 2222 as well, they work. is the pot running to ground? you should be going in on one wiper, and grounded off the opposite wiper with the cap running off that to the center.

also:

http://www.fairchildsemi.com/ds/PN/PN2222A.pdf are you sure your pinouts are oriented properly?
« Last Edit: December 05, 2009, 06:42:00 PM by pazuzu »

GroovyMan

Re: First Post: Somthing's not right
« Reply #2 on: December 06, 2009, 08:13:42 AM »
Thanks for your reply. I pretty sure I have the pot connected right. And I tripple and tripple checked again the orientation of the transistor and it seems right.

The voltages I have are around the Tr are:

Vb 2.73
Ve 2.17
Vc 3.91
y
The voltage at the where the 22uF cap, 10k, 100k and 47k meetig is 2.73v.

I'm tyring to verify if the voltages I have should make the transistor do something. However I can't work out what the resistance should be at the base. From the reserach I've done the V at the base is usually set by a voltage divider. From there it shold be easy to work out if the operating conditions are correct.

However in the NPN boost schematic the configuration is different and I can't figure it out.

Thanks,
Ben
Next project: Ross Compressor
Completed: NPN Boost, Vox V847 True Bypass and DC socke mods.

pazuzu

Re: First Post: Somthing's not right
« Reply #3 on: December 06, 2009, 09:18:17 AM »
well, you are already debugging in a much more sophisticated manner than me. when something doesn't work for me i just systematically rip out and replace parts until it works. and if that fails i smash it and do a different project.

i suppose that's what you get when you have been a welder most of your life.  :icon_lol:

GroovyMan

Re: First Post: Somthing's not right
« Reply #4 on: December 07, 2009, 03:13:57 AM »
Well, I've a done a bit more digging around and it seems that the configuration we have is a "bootstrapped input". I'm not exactly sure what that means yet, but it seems that transistors have low input impedance and by adding a little bit of positive feedback (boost strapping) you artifically raise the input impedance if the circuit. So in our case I think this is done by the 22uF cap connected at the emitter. Why we want a high impedance I'm not sure; something to do with noise pickup perhaps?

I guess the DC resitance conditions are unaffected by the feedback loop as there is no way for it to get across the 22uF cap. But I'm still not sure how to work out the DC voltage at the base of the tranny.

Anyway, as for debugging my circuit, the plan is to build a breadboard version, get it working and then try to work out what's different to my vero-layout. At the moment my money (perhaps naively) is on a broken tranny.
Next project: Ross Compressor
Completed: NPN Boost, Vox V847 True Bypass and DC socke mods.

aron

Re: First Post: Somthing's not right
« Reply #5 on: December 07, 2009, 03:18:22 PM »
What is happening is that there's a mistake somewhere on the board. Either a component is misplaced or a trace is not cut, or a bad solder joint or any number of other things.

The voltages you should see (approximately) are in this thread.
http://www.diystompboxes.com/smfforum/index.php?topic=63165.0

I've built the vero board several times.

aron

Re: First Post: Somthing's not right
« Reply #6 on: December 07, 2009, 03:20:55 PM »
What is the voltage of the battery? Your collector voltage looks low compared to everything else.

GroovyMan

Re: First Post: Somthing's not right
« Reply #7 on: December 08, 2009, 03:01:51 PM »
Thanks for all your help, I had a couple of shorts on the back of the board. Funny how you stare at the board for hours and hours and not notice the shorts, but go back the next day and there they are, plain as day! It's working perfectly now, wahoo!

The attenuation was caused by a short between the emitter and the other side of the 22uF cap where it meets the 10k, 47k and 100k resistors. I then had another short that was putting some DC on the input side of the input cap. The classic symptom of this is scratch noises when you turn the volume control on your guitar.

I'm now trying to put in a box. I have already wrecked one box because I only planned how the jacks, pot and switch would be layed out from the outside. I didn't considerhow they might interfer with each other on the inside of the box!! big doh!!

Ben
Next project: Ross Compressor
Completed: NPN Boost, Vox V847 True Bypass and DC socke mods.

aron

Re: First Post: Somthing's not right
« Reply #8 on: December 08, 2009, 11:44:35 PM »
Well congratulations Ben, I'm assuming it's working. GREAT JOB!

GroovyMan

Re: First Post: Somthing's not right
« Reply #9 on: December 09, 2009, 02:18:01 PM »
Yup, thanks Aron, it's working a treat!
Ben
Next project: Ross Compressor
Completed: NPN Boost, Vox V847 True Bypass and DC socke mods.