Tubescreamer schematic question..

[JP19]

Hi. I am just looking at the excellent Electrosmash tubescreamer analysis and I have a question. I'm sure it's a simple electronics question but just trying to understand what's going on. Hopefully someone can enlighten me...  Schematic is attached.

I am trying to understand capacitor C8. The points before and after it are both pulled up to 4.5v, so what is it doing?

Thanks!

[POTL]

it blocks DC current and forms a high pass filter together with R12

[JP19]

Ok thanks, I guess I was wondering really why DC current would flow if both points are at 4.5v?

[GGBB]

Although it does form a high-pass filter with R12, that's not the main purpose of C8 (knee frequency is below audible). The main purpose is as a coupling cap to keep the bias on Q3 stable. Without it, other parts of the circuit can change that bias. For example the level pot would be a path to ground from Q2 gate through Q2 when the circuit is engaged.

[antonis]

To be more precise, corner frequency is set by C8 and R12//[(R13//RC) X h_FE], neglecting intrinsic emitter resistor & RB..

[PRR]

> points before and after it are both pulled up to 4.5v

Base of Q3 is not at 4.5V. It is surely somewhat less because a base sucks.

[bamslam69]

Hey guys,
Just trying to rescuscitate a mate's Tubescreamer Deluxe that got drowned after a garage flooding incident.
I've got it all going, but for some reason I can't get the LED circuit operating. I've replaced the zener diode, and all the bjt's but I'm stuck.
I'm starting to think I need to focus on the base side of Q7. Anyone got an idea of what kind of voltage or current it needs to get going? My circuit theory isn't that great. Any tips are welcome (apart from suggesting to throw it out and starting again... I want this thing out of my house asap)

Cheers

[antonis]

Switch ON-OFF analysis is a bit complicated so I'd suggest you to make some simple tests, like:

Short Q7 Collector to D6 Anode..
If LED lights up and considering Q6/Q5 switch works properly, your issue lies  either on Q7 itself or Q6 Collector/Q7 Base connection..
The later can be verified by shorting Q6 Collector to Q7 Base..

To simplify how it works:

When Q6 is OFF, Q7 Base is biased via R27 (56k) so its Emitter stands on a voltage level set by Q7+Q5 currents (less than 300μA total) times R29 (100R) plus D6 forward voltage drop..
Let's say it's about 1.1V (mainly due to MA150 high forward voltage drop)..
LED current is calculated by: (9V- LED voltage drop - 4.7V(*) - 1.1V) / 36k, considering Q7 V_CEsat negligible..
(*) IMHO, Zener voltage drop should be negligible due to very small breakdown current (Izk) through it..

When Q6 is ON, its Collector (which now bias Q7 Base) stands at a voltage level equal to its Emitter one (heavily staurated) which level is equal to Q7 Emitter one.. So, there is not practical V_BE for Q7 to be set ON, hence no current through LED..

P.S.
I presume you've already checked for LED good health..

[antonis]

Dunno if it makes things more clear or further confusing...



P.S.
Blue arrow is the sum of Red and Green ones..
(for Q7 fully ON, Green should be much larger than Red/min h_FE..)
Q5 and Q6 Collector - Emitter currents are considered equal..
(no Base current, ignore Grey arrows..)

[bamslam69]

Thanks heaps for the detailed explaination, Antonis. A fair bit went over my head but the arrows help with explanation of the circuit. I really appreciate it.

Yes the LED works, but I've also shorted out a few in my testing the other night. haha.
I keep coming back to it and double checking it's still functional.
Nothing is working via your inital test, and looking at the legs of the other bjt's, I think I've overlooked checking htey're functional.

Thanks once again and I'll report back when I find the issue. 90% of the time it's something simple.

[Rob Strand]

Hey guys,
Just trying to rescuscitate a mate's Tubescreamer Deluxe that got drowned after a garage flooding incident.
I've got it all going, but for some reason I can't get the LED circuit operating. I've replaced the zener diode, and all the bjt's but I'm stuck.
I'm starting to think I need to focus on the base side of Q7. Anyone got an idea of what kind of voltage or current it needs to get going? My circuit theory isn't that great. Any tips are welcome (apart from suggesting to throw it out and starting again... I want this thing out of my house asap)

Cheers

Up front you probably should not need to replace any components.  Just clean the PCB.

Does the foot switch work at all?  ie.  does the effect toggle between bypass and effect?   That means the whole flip-flop circuit is working and the problem is isolated to the LED part only.

That schematic has errors.  There should be a 1M resistor between the base of Q7 and the collector of Q6.   The LED resistor is 3k6 not 36k.

If you are just using the schematic for a general map it doesn't matter but if you are replacing components it could be an issue.

D7 is a zener diode, if you replace that with a normal diode the LED won't work.

What about the LED wiring.  Maybe you just wired the LED around the wrong way.

[bamslam69]

Thanks for the deets on the schematic errors Rob.
Most of the resistors are still good, while most of the caps, LEDs, diodes and IC absolutely shat themselves in the flood & battery shorting incident. Photos on my original thread below, I just wanted to jump on this thread to keep things relevant.
https://www.diystompboxes.com/smfforum/index.php?topic=132230.msg1287440#msg1287440

Yeah the zener diode has been replaced like for like, and the LED is facing the right direction. Double checking everything.

The effect is working. The switch is working. Just trying to jump this last hurdle.
I shall report back with the error (it's most probably me.)

I Love the help that members on this forum provide.

[Rob Strand]

The effect is working. The switch is working. Just trying to jump this last hurdle.
I shall report back with the error (it's most probably me.)

If you replaced the transistor driving the LED  you might have the wrong pin out.  Worth checking.  You already mentioned you replaced some transsistor, so it might not be that.

[antonis]

The LED resistor is 3k6 not 36k.

That makes much more sense..

Both for LED working current and Zener breakdown current..

[Mark Hammer]

RE: Biasing

Whenever I look at a schematic of a circuit intended to run off a 9V battery that provides a +4.5V reference to serve as "ground", I'll look through the overall signal path, to see where the biasing is inserted,and where it gets defeated and has to be restored.

So, it may be inserted at stage A, but stage A has a cap on its output, so it has to be re-inserted/re-established for stage B, and so on.  In many such instances, one does not want the bias voltage of a preceding stage to get amplified/raised by a subsequent stage, so the goal is to "re-set" to avoid that.

[antonis]

Well said Mark but your answer corresponds to a 4-year earlier query..