Kalamazoo works except for drive pot

[Hemmel]

Hi guys !

I just finished my first build. Well, almost. I tried doing Tagboard's Kalamazoo (3rd layout) and the effect works finem except the drive pot doesn't work. If I turn it it changes nothing to the sound. And it sounds as if the drive is always fully on. I'm thinking it's being bypassed in some way, but I re-checked all solder joints and can't find anything wrong...

Also, it seems the tone pot works in reverse. CCW is a clear sound and gets darker as I turn it clockwise. Would simply reversing the connections work ? I think so but I'd like to be sure...

Some pictures are coming up, but I'd like to know what to look for about the drive pot problem.

Thanks guys !

[GGBB]

Check that the drive pot isn't faulty - measure the resistance across it with the pot set at both extremes, and check that lugs 2 & 3 are properly connected.

The tone pot is used as a variable resistor, so to reverse it all you need to do is connect lug 2 to lug 1 instead of lug 3.  Lugs 1 & 3 remain connected to the vero as they are.

[Mark Hammer]

I don't know if you have any pictures posted here, because my work filters them out.  But there is a schematic here: http://revolutiondeux.blogspot.ca/2012/01/lovepedal-kalamazoo.html

You can see that it is basically a Tube Screamer with a few changes here and there.  Like the Timmy, and so many others, the pair of clipping diodes is exchanged for a 2+2 combination, and the tone section (Glass) works exactly the same as in a TS, but with a slightly different range.

A standard Tube Screamer has a 1k resistor and .22uf to ground, immediately after the clippipng stage.  This forms a lowpass filter that starts rolling off around 720hz.  IN the Kalamzoo, we have a 1k resistor, in series with a resistor/pot network that can add to that resistance, and a .1uf cap to ground.  When the pot is set to zero resistance, all you have is 1k and .1uf, rolling off around 1590hz.  When the Tone pot is at max resistance, you will have 5k in parallel with 2k7, giving a parallel resistance of 1k75, added to 1k.  That gives you a lowpass filter with 2k75 and .1uf, having a rolloff starting around 580hz.  

Because the rolloff point goes down as you increase the resistance (i.e., 1k + 0, 1k + 500R, 1K + 1k, etc.), you want to wire the pot so that the pot resistance gets smaller as you go clockwise.

If you want more bass from the unit, put a 5k pot (wired as a variable resistor) in series with the 100nf cap to Vb.  This will reduce the maximum gain a bit, but by having more bass you will still get a lot of clipping.  This is used in the Timmy.

Make sure your Drive pot is wired properly.

[Hemmel]

Thanks Rob and Mark !

The pot isn't faulty, I checked it before and re-checked it just now. It checks out at 491K and behaves correctly on its own. Connections are good to the stripboard (according to the layout). Now the major change I made to the layout was to put a 120nF cap instead of the 150nF (which I don't have on stock yet). I'm pretty sure minor corrections such as 2K7 instead of 2K74 and 330R instead of 332R would change anything either.

As stated before, the effect works great otherwise. Volume, tone and glass all work. Thanks again Mark for the details on the tone pot. Didn't quite understand it all, but I'm learning.
It's funny you mentioned the Timmy, I also have a layout I want to box up.

I tried the "optional 820pF cap" and I didn't like the sound but if it's not present, you think I should jumper those pads ?

EDIT : Just tried jumping the pads and it completely cuts the drive, left with volume tone and glass working on a clean sound. ooooooh I get it, it shorts out the diodes.... just saw that... still looking...

[Mark Hammer]

It is quite posible that whoever reversed the circuit misinterpreted "820" as 820pf instead of 82pf (on a small ceramic, 821 would be 820pf and 820 would be 82pf),  That may be why you didn't like it.

[Hemmel]

Thanks Mark.
I tried a 100pF (don't have an 82 in stock) and it does sound a lot better. I'll keep it that way.

I'm still baffled as to why the drive pot still isn't "working". I measured between pin 1 of the 4558 and the junction between 10K (R1 on the following schematic) / pin 1 of the drive pot. Whilst turning the pot, the readouts were ok. I got up to 457K which I think is normal since the adjoining resistors have an impact on the readout. (Right?)

I'm not quite sure about how the pot is supposed to affect the drive. I think, looking at this schematic, that adding resistance (so turning clockwise) will force more signal through the diodes whereas removing resistance will allow clean signal to go through. I'm not sure I used correct words and/or phrasing here...
But if my supposition is correct, then I really don't understand what's going on because when the effect is on, it sounds as if the drive pot is at full clockwise all the time.

So I tried to measure the resistance between pins 1 and 2 of the IC (the schematic from revolutiondeux uses pins 6 and 7 for drive but tagboard's layout uses pins 1 and 2.) to see what the resistance was. I got 9.1 Mega-Ohms ! with drive pot at CCW and 9.05 M at CW. Is there resistance in Mega-Ohms within the 4558 I'm not aware of I checked the datasheet and didn't find any numbers in that range...

[Hemmel]

Not sure if it helps but I thought I'd show some voltage measures from that side of the IC.
DC in = 9.4V
Pin 1 = 4.2 V
Pin 2 = 4.2 V
Pin 3 = 3.82V
Pin 4 = 0V

These reading don't change at all when drive pot is turned.

[slacker]

So I tried to measure the resistance between pins 1 and 2 of the IC (the schematic from revolutiondeux uses pins 6 and 7 for drive but tagboard's layout uses pins 1 and 2.) to see what the resistance was. I got 9.1 Mega-Ohms ! with drive pot at CCW and 9.05 M at CW. Is there resistance in Mega-Ohms within the 4558 I'm not aware of

With no power to the circuit you should measure about 10k between pins 1 and 2 with the drive pot turned all the way anti clockwise and 500k between pins 1 and 2 with it turned all the way clockwise. It looks like your pot isn't actually connected to the circuit, or the 10k resistor from pin 2 to lug 1 of the pot is not 10k or is damaged or not connected properly.

Your voltages look fine.

[Hemmel]

I felt stupid for a few seconds.
The 10K resistor was actually lifted at IC pin 2, I just noticed it while checking if I didn't mistake a 10M for a 10k. So the only components connecting IC pins 1 and 2 were the diodes.
Re-connected the 10k, and now it works !!!!

All I need now is to change lug 2 of the tone pot and connect it to lug 1 instead of 3. I just hope it won't have too much of an impact on the glass pot which works great.

Thanks to all for your help and pictures are coming soon.

[Hemmel]

Update : I reversed the tone pot and now it sounds awesome. No notable impact on the glass pot.

Pictures of this first build coming up tonight.

Thanks again to all who helped !