DEBUG: Tone Bender MK II/1.5

[Germanium_Boy]

Well, I have an issue with a TB that is being a headache...

Here is the schematic I followed (I used OC75 transistor)



I changed three things: I went for a 1k resistor instead of the 470ohm resistor, for a bit more of output volume; 20k trimpot instead 8k2 resistor, and 100k trimpot in place of the 47k resistor.

And one more thing: a DPDT switch to change between MK1.5 and MKII TBs. The schematic of the MK1.5 can be found here:



And well... what is the trouble? The MK1.5 mode souns fantastic, I really love it. It's the first time that I use OC75 trannies, and they make the difference. Smooth and classic tone.

But what about the MKII position? Well, a very low volume ouput, a bit bassy sound, and not too much fuzz at all. So I checked the voltages. And all it's ok, except collector voltage in Q1.

And it makes sense, because Q1 is bypassed in MK1.5 position.

What it's happening, and I don't understand, is around 10k resistor at Q1 collector: at the node between that resistor and -9V rail a I get around 9.45v. But at the node between the same resistor and Q1c I  get... less than 1.0v... Maybe a bridge to ground? Faulty tranny? Oh, what a mess...

At last, here is the layout I followed. Sorry, I only found it in PDF:

http://turretboard.knucklehead.dk/wp-content/uploads/2011/09/tbmk2profinal.pdf

Thanks a lot... Let's make some fuzz with this pedal!

[duck_arse]

have you tried swapping the 10k on the base of Q1 for the 100k some circuits show? and how have you connected your mk1.5//mkII switch, diagram please?

[Germanium_Boy]

I didn't try it, but it's more suitable for OC75 the 10k resistor. 100k is the best option for OC81 trannies.

About the switch, I'll try to explain with this diagram, and having the PDF layout as reference.

What I did was lifting up the leg of 5uf capacitor (at left, the blue capacitor that is at the input of the schematic) at the node with Q1 base. I call the tie of 5uF 'A'. In the place of the layout where this leg should be soldered, I soldered a wire, 'B'. In the other hand, I did the same with the 100nF capacitor that is feeding the base of Q2 base. The lifted tie is 'C' and the wire soldered at the node with Q2 base is 'D'.

So the DPDT switch would be connected as:

   
-B-  -C-
-A-  -D-
--x-  -x-

And being X and X soldered together. Yeah, it's a mess explaining it, but I'm not very good at using drawing software!

[Germanium_Boy]

Maybe this quick layout could make it easier to understand...

http://imgur.com/vdBUUCn

[Quackzed]

  i get how you said you had it wired, both caps are lifted on their right sides, in 'up' (ill call it) of the dpdt the caps both connect to their normal spots, in 'down of the dpdt the first cap gets connected to the place where the second cap gets connected in the up position, bypassing the first stage... but the drawing shows both 1st and second cap connected together on the 3rd row, i'm hoping thats not how you actually have it wired. if its wired like you said and not how you drew it, i'd guess its the q1 biasing , on q1 try a 100k instead of the 10k from q1 base to ground  as suggested... that should get you closer, then  you can adjust that resistor up or down a bit to get it juuust right for your needs... sometimes gains of transistors are higher or lower than normal and they need a little help to be biased and happy.
also you can put a resistor between the transistor emitter and ground instead of connecting its emitter directly to ground, to lower the gain of q1 a bit, try a 1k first (probably too much gain cut) but to see if it helps, maybee a 330 or 220 is better to try out...

[LucifersTrip]

just a couple notes/questions...

you really shouldn't need a 1K instead of the 470. MKII is loud enough if Q3C voltage is ~ 8V, plus you will be altering the actual tone.
also, what's the value of the cap on Q3C when you switch (MKII is 5uf and M1.5 is 25uf)?

...and yes, try 100K on Q1B (or use trim to get best value, then sub permanent R) instead as others suggested. even though 100K was used for 81 version, it's still depends more on the gain/leakage of your specific Q, rather than the model number. Q1C voltage should be close to 9V anyway...

lastly, you haven't shown any gains, leakages, or voltages.

[Electric Warrior]

...and yes, try 100K on Q1B (or use trim to get best value, then sub permanent R) instead as others suggested. even though 100K was used for 81 version, it's still depends more on the gain/leakage of your specific Q, rather than the model number. Q1C voltage should be close to 9V anyway...

It's so far off, I bet something else is wrong.

[Germanium_Boy]

I'm afraid Electric-Warrior is right... I suspect some issue, apart from the possibility of the 100k resistor fixing a bit the things...

Here are the voltages, with the 100k bias pot (Q2 collector) at about the half of its sweep. Where it's sound 'better':

Q1
B 0.07   
C 0.70                     
E 0.00                                    

Q2
B 0.12
C 0.32
E 0.00   

Q3
B 0.96
C 8.59
E 0.96

And about gains and leakage... it's a transistor set purchased at SmallBear, and here is the data:

Q1. Gain 69, L=359
Q2. Gain 79, L=278
Q3. Gain 91, L=370

Anyway, as you can imagine, the gain expresed is the real one, substracting the leakage

And as I said, the more extrange for me is the difference between Q1-collector 10k resistor node with -9V and his connection with Q1 (from -9.45V to less than 1.0 V).

[LucifersTrip]

Q1
B 0.07    
C 0.70                     
E 0.00                                    

Q1. Gain 69, L=359
Q2. Gain 79, L=278
Q3. Gain 91, L=370

...that's why posting voltages/gains/leaks is the first step...now you KNOW it's not the 10K. There's most likely a wiring/solder error/bridge/crack or bad/wrong component.  Check to make sure Q1 and all components connected to Q1 are making the right connections.  

Are those gains/leakages measured by you or written on a bag? Did you re-test these (especially Q1) after soldering. To quickly test Q1 itself, just swap Q1 and Q2 and see if measurements are still bad in the 1st stage.

...and of course, B and E cannot be equal.

Q3
B 0.96
C 8.59
E 0.96

What are the voltages with the MK1.5? Did Q3 change, since it shouldn't work well with above voltages?