FatPants 2013 clipping / no boost - FIXED!

[mattc]

Hi all -

I am trying to build a new FatPants, with a 2013 board I got from Bean a couple of weeks back.   When bypassed the signal goes through fine.   When it's engaged, the signal level drops, and becomes very quiet.   When I bash on a E5 power chord (for example) to get a lot of output going, the signal clips very strongly.   I am a n00b, but it seems to me that low output and clipping would indicate that the circuit isn't getting enough juice, but what do I know?

Any ideas on where to look to trouble shoot this?  I can take pictures of the board, and/or uload a sound sample if that would help.

(1) Project Name
FatPants 2013

(2) General description of the problem
Low signal, clipping (more above)

(3) Steps that you have taken to try to resolve the issue
Checked all the solder points, made sure all connections were good

(4) List any substitutions you used for parts/values
I used a few 1//2w resistors instead of 1/4w, and a 63v cap instead of a 25v.   All the values are the correct values, so I don't think this is the issue?

Thanks in advance!
Matt

[madbean]

Sounds mis-biased. Double check your values for R4, R3 and R8. Have you verified you are getting 18v out of the charge pump? Have you tried messing with the trimpot as described in the build doc?

[mattc]

I've tried changing the trim pot, and that didn't have any noticeable impact.

I haven't checked to see if I'm getting 18v from the charge pump, since I don't know how to do that.

I'll check R3, R4 and R8 now.

[mattc]

R3 is reading .6M when it should be 2.2M.  I pulled the resistor out, and measured it and I get 2.194M. 

Any idea why I'm getting .6M when it's on the board, and 2.2M when it's not? Would this be the problem?

The other two are fine.

[GermanCdn]

Your resistors in circuit are going to read differently then out of circuit, as they are running in series/parallel with other components, so the 2.19M out of circuit would indicate that your resistor is the right value.

I'm not at my build desk, but have you adjusted the bias properly by measuring the DC voltage at the TP and turning the trimpot until you get the required readout (from memory it's 5V, but don't quote me on that, go to the build doc).

Easiest way to check if you're getting 18V out of the charge pump would be to take a reading between the 18V out auxillary and the ground - if you've got 18V there, you're getting 18V out of the charge pump.  My apologies if anything I've said here contradicts the build doc, I'm running off memory alone, and it's been a while since my last fatpants build.

[madbean]

GermanCdn pretty much covered it, except the bias point is fine anywhere between 10-12v on Q2. Verify you have 18v out of the charge pump (the build doc already tells you how to measure the voltage for the Test Point, and all you need to do is measure it at the 18v output pad). A remote possibility is fake JFET transistors...it could be the case depending on where you got them. But, that would not be my first guess.

Have you already wired this up? If so, lets see some pics of your wiring, etc.

[mattc]

I will take pics when I get home from work tonight.   The JFETs came from Mammoth, so I'd be surprised if they're fake.

I'll also check the voltage from the charge pump.

Also - thanks for all the help!

[mattc]

So, life got in the way, and was just now able to get back to this.

I tested the 9v, and I'm getting 9.11v.  Then I tested the 18v, and I'm getting 17.75v.  That all seems good to me.

Then I tested the "TP" test point, and I am not getting good readings.   With the trimmer all the way up I'm getting  83.5mv.   The build document says I should be in the 12-14v range, so I'm thinking that 83.5 millivolts, is a ways off.  I'm at a loss as to what to check next.

I've attached pictures of everything, in the hopes that one of you can see something I did incorrectly.

Thanks again to all of you helping me figure this out!

Cheers,
Matt

[midwayfair]

R11 looks like it's 33K in your pictures. It should be 3.3K. If you don't have a 3.3K, any small resistor around that value will be fine -- it's just a minimum resistance for the trim pot.

[mattc]

R11 was indeed 33k.   I swapped that for a lower value, and everything is working now. 

Thank you all for your help!