Tube Screamer Debug...Help!!!!

[Zben3129]

Okay, so...

I can't get it to work.

I have a strange issue going on with my voltages. First of all, I am reading V+ on pin one, and V+ minus about .5v or so on pin 2. Second, I am reading about 5.3 at Vb. I measured the resistance across the top resistor in the Vb network (10k), and couldn't get a reading. Then, I unplugged the battery and got a reading of 5k ohms. Plugged the battery back in, no reading again.

So, in short:
  -V+ on pins where it shouldn't be
  -Vb is about a volt high
  -Top resistor in Vb voltage divider reads open when a battery is plugged in, and 5k when there is no battery.

The last bullet is the one that makes me think of a transistor problem, or opamp, as the fact that the power being on/off affects this value makes me think there is an active component issue.

Zach

[kurtlives]

The only reason I can think of that would cause your Vref ro be high is a wrong or failed resistor or failing filter caps.

[Zben3129]

Actually I fixed the Vb problem, but I don't know how...it just fixed? Maybe I wrote down the wrong voltages. Oh well, its better 


But I'm having a little bit of confusion here with the transistor pinout. I am getting the 9v, 3v, 2.5v readings on the transistors, but the emitter and base are backwards from what they should be. E at 3v, B at 2.5v. I know I have the pinout right, is the layout right, is GGG right? On the GGG voltage chart http://www.generalguitargadgets.com/pdf/ggg_its8_instruct.pdf is where it says base should be .5 above emitter, and on the tonepad layout http://tonepad.com/getFile.asp?id=81 I am reading (from left to right on the layout, Q1)  3v, 2.5v, 9v


Zach

[kurtlives]

Actually I fixed the Vb problem, but I don't know how...it just fixed? Maybe I wrote down the wrong voltages. Oh well, its better 


But I'm having a little bit of confusion here with the transistor pinout. I am getting the 9v, 3v, 2.5v readings on the transistors, but the emitter and base are backwards from what they should be. E at 3v, B at 2.5v. I know I have the pinout right, is the layout right, is GGG right? On the GGG voltage chart http://www.generalguitargadgets.com/pdf/ggg_its8_instruct.pdf is where it says base should be .5 above emitter, and on the tonepad layout http://tonepad.com/getFile.asp?id=81 I am reading (from left to right on the layout, Q1)  3v, 2.5v, 9v


Zach

That cant be right...the GGG voltage chart says otherwise and also the base should be approx one silicon voltage drop higher than the emmiter.

[Zben3129]

NEW INFO NEW INFO NEW INFO NEW INFO

Okay, step one, be an idiot. Complete!

I had a battery that was only 6.8v, and I forgot about this, so my voltages were wrong,

I have 9v at v+,
6v at Vb,
4.5v at the emitter, 3.7v at the base, and 9v at the collector for both transistors.


Audio probe shows signal is lost at the output of the opamp (pin 1). Normal signal at input, pin 3, and pin 2.


Zach

[Zben3129]

Did I burn up both of the transistors or something 

Zach

[PerroGrande]

The transistors are NPNs in a basic emitter-follower configuration.  The base *MUST* be a diode-drop above the emitter.

Based on the schematic, I would expect to see exactly what GGG published -- around 3v on the base and around 2.3 ~ 2.4 on the emitter.

Have you taken voltages from your op amp?

[R.G.]

OK, this is a good example of why I wrote "what to do when it doesn't work". I admit that the approach seems overly restrictive and regimented, but it's that way because there is a good, logical progression of what to do first, then next. It's not simply a matter of "go measure the voltages."

In this case, one of the first steps is to measure the battery voltage at the battery. That would have nailed the low battery problem first, before the confusion.

Zach - you're not following good debug process. Go back to "WTDWIDW" and follow the steps. It's likely that you'll find your problem all on your own.  Skip the steps, miss a problem and you (one) get lost.

[Zben3129]

Okay, something must have been wrong last night, either with me, or the DMM (it was me I'm sure, I fail equipment more than I fail me). I used a 9v source and still got bad voltages at Vb, pin 1, etc.


I did end up fixing it, but I don't know how 

I unfortunately got impatient and just reflowed a lot of the joints, replaced the transistors with sockets and added new transistors, and changed the opamp. Works great now. Unfortunately, I don't know how I fixed it.


I probably had one of those tiny solder bridges that are hard to see anyways, and I am working on switching to contact lenses right now so vision isn't so great.

Thanks

Zach