HELP [Lovetone big cheese clone]

[FuzzAldrin]

Hi guys, I built a clone of the lovetone big cheese using the nicely fabbed "OLC Great Cheddar" PCB.

I have triple checked components and orientation.

For the MOST part, it works. I don't have an original or another to compare it to. It just doesn't sound like I think it should. Its not as fuzzy as the clips Ive seen online, and when its in the 1st position, the tone can be adjusted. I thought that position removes the tonestack so only volume and fuzz would work....

Here we go...

Bypass works great.
LED comes on.

Power supply = Regulated WW, measured at 9.16v center neg

Q1: 2N3904
C = 1.23v
B = 0.614v
E = 37.3mV


Q2: 2N3904
C = 7.79v
B = 1.23v
E = 0.640v

Q3: 2N3904
C = 0.2mV
B = 0.2mV
E = 0.2mV

IC1: TL071
P1 = 146.8mV
P2 = 4.58v
P3 = 4.44v
P4 = 0.2mV
P5 = 146mV
P6 = 4.58v
P7 = 9.16v
P8 = starts @ 168mV then keeps dropping

IC1: TL071   (voltages are basically the same as IC1)
P1 = 146.3mV
P2 = 4.58v
P3 = 4.43v
P4 = 0.2mV
P5 = 146mV
P6 = 4.56v
P7 = 9.16v
P8 = FLUCTUATES pretty rapidly.

[PRR]

> Q2: 2N3904
> C = 7.79v
> B = 1.23v
> E = 0.640v

C=8V, very near battery voltage, smells odd to me.

A copycat schematic shows 2.8V there.

http://www.generalguitargadgets.com/diagrams/geo_great_cheddar_v6.pdf

Voltage across R8 10K and voltage across FUZZ pot 1K should be in the ratio of their resistances. They are not 10:1 different, more like 2:1 different.

Figure out why.

5K pot at FUZZ? 2K even 1K at R8?

[FuzzAldrin]

Oh wow, thank so much for the link to a similar schematic with voltages. This will definitely help.
Yea, something isn't right in that "area" of my build. This confirms it and should make troubleshooting easier.
I'll report back....

Thanks PRR!

[FuzzAldrin]

 

My Fuzz pot is NOT a linear 1K as it should be.
Some idiot (me) put in a 10k Linear pot.   

So, Im still new to all this. I haven't swapped the pot out yet for the proper value. I'm looking at the schematic trying to see how that would  fix my issue with Q2 measuring out of spec like that. Any info?

[PRR]

> NOT a linear 1K ... a 10k Linear pot. ...I haven't swapped the pot out yet for the proper value.

Why not? Should be easy and quick to swap. If you don't have a 1K pot, put in a 1K resistor--- you won't have fuzz-depth action but at least you will see the right DC voltages (or not, revealing there's another problem too...).

> I'm looking at the schematic trying to see how that would  fix my issue with Q2

I said: "Voltage across R8 10K and voltage across FUZZ pot 1K should be in the ratio of their resistances."

Actually the emitter voltage is fairly fixed for a large range of wrong values, around 0.6V. If Re is 10K and Rc is 10K, then Rc also drops 0.6V, down from 9V, giving the ~~8V you observe. If Re is 1K and Rc is 10K, Rc drops like 6V, down from 9V, giving about 3V at Q2 collector. Which exactly equals (near enuff) the 2.8V on the linked plan.

And it needs to have enough current and mid-voltage before it will pass audio well.

> Some idiot (me) put in a

Reminds me of my helper. (I work alone.)  

[FuzzAldrin]

> NOT a linear 1K ... a 10k Linear pot. ...I haven't swapped the pot out yet for the proper value.

Why not? Should be easy and quick to swap. If you don't have a 1K pot, put in a 1K resistor--- you won't have fuzz-depth action but at least you will see the right DC voltages (or not, revealing there's another problem too...).

> I'm looking at the schematic trying to see how that would  fix my issue with Q2

I said: "Voltage across R8 10K and voltage across FUZZ pot 1K should be in the ratio of their resistances."

Actually the emitter voltage is fairly fixed for a large range of wrong values, around 0.6V. If Re is 10K and Rc is 10K, then Rc also drops 0.6V, down from 9V, giving the ~~8V you observe. If Re is 1K and Rc is 10K, Rc drops like 6V, down from 9V, giving about 3V at Q2 collector. Which exactly equals (near enuff) the 2.8V on the linked plan.

Thanks Paul, your explanation makes perfect sense now. I will put in a new correct pot shortly. I moved and had to box up my iron and parts basically.