Cosmopolitan Help

[pryde]

Hello,

I just finished a cosmo circuit and built it exactly to spec. First, I checked all my values, solder joints, etc and everything checks out so here is the issue:

I get sound and all the pots work but there is very little gain or fuzz produced. Its like the tone is pretty clean and "muffled" sounding overall? Also, the fuzz pot is noisy when turning and only a very little difference in fuzz when full CCW to CW.

I checked BC109 and AC127 pinouts and they are correct. I switched around to be sure and got crappy-fizz so I am pretty sure they were oriented right?

Any ideas what to do next? All advice greatly appreciated.

[cjkbug]

some clear photos of both sides of the board would help. although you said the pinouts where fine It sounds like one is in wrong to me.  some voltage readings on the transistors might help as well. who sold you the ac127? are you sure it's good?

[oldhousescott]

The fuzz pot (and the gain pot to a lesser degree) will crackle when turned since there is DC across the pot.

Can you post the voltages for each leg of each transistor, as cj suggested?

[ckim715]

I had the same problem; ended up being that I needed a new BC109 and a new BS170. I was getting voltage into the BC109, but nothing coming out into the ge npn.

[pryde]

OK. Back home now

Here are voltages I am getting

BC109: C= .6                 Different BC 109 same values
           B= .87
           E= 0

AC127 C= .87                 Different AC127 C= 3.3
          B= .92                                         B= .60
          E= .79                                         E= .50

BS170: S= 2.16              Different BS170 Same values
           G= 2.99
           D= 6.99


So there is a change voltage with CBE of a different AC127 but it still does not sound much different.

[pryde]

Pic of the board:

Basically no real difference swaping out all trannies. Fuzz pot is noisy/scratchy and doesnt produce much fuzz, all other pots seam to work ok?

[ckim715]

the fuzz knob is actually the gain knob of the BS170, and by its nature, it will be scratchy. Think ZVex SHO or the Fatpants Fat knob.

The "Gain" knob is actually what controls the overall gain of the NPN.

As I said, try changing out the BS170 and the BC109. My BC109 was bad, and the BS170 has a tendency to blow pretty easily.

[oldhousescott]

BC109 values look reasonable. The base of the AC127 should be at the same value as the collector of the BC109 (since they're directly connected), so the first reading doesn't make sense. The second looks better. The BS170 gate should be at half the supply voltage based on the divider formed by R1 (+R2) and R4. Your gate reading is probably correct; your meter is likely loading the circuit bringing down the reading.

Your readings (with the different AC127) are comparable to the those listed on a drawing at FSB, so I'm not sure where to point the finger in this instance. You could try building a listening amp or probe to find out where the signal goes awry.

[pryde]

thanks everyone so far.

to ckim: I did put in all new trannies and got the same results; however, the second AC127 did improve things (i.e voltages re better).

To oldhousescott:
I do have an audio probe but am not sure what to listen for since the circuit actually does work, except for the weak gain/fuzz and muffled sound? Depending on where I probe I get everything from no sound to high-pitch squeals. Anything in particular I should be listening for?

Again, the pedal just sounds like a really weak/low gain overdrive at max fuzz/gain settings?

[cjkbug]

can you post a photo of the trace side of the board. double check that the pots are the correct.with five controls it's easy to get a couple mixed up.

[cjkbug]

I noticed that you got way too large of a value for c2...c2 should be 200 picofarads not 220 nano farads. thay would have a big impact on the tone. swap it out and see how it works.

[pryde]

Jeez good eye! C2 is wrong  .

OK, my local radioshack has only 100pf caps. Can I hook 2 together somehow to get the needed 200pf?

Thanks alot

[timbo_93631]

Caps are the opposite of resistors, if you put them in parallel the value increases, in series the value decreases.  If you put two 100pF caps together in parallel you will get 200pF, a 100pF + 100pF in series = 50pF.  The opposite of how resistors work where two 100k r's in parallel = 50k, and two 100k r's in series = 200k.

[Sigesmundninja]

Yes you can, just put them i parallel. Did this in my last lunar module. 2 x 1n instead of 2n. The leads of both capacitors may not fit in the holes but just solder one in and then wrap the leads of second one around the first one and you should be good to go!

[pryde]

Awesome.

I will get this done and report back.