Author Topic: Rump Roast help  (Read 2319 times)

night-B

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Rump Roast help
« on: January 31, 2011, 03:44:33 PM »
Hi,
I just finished my rump roast. Wiring's OK, checked the solders with the DMM...
I've done the bias to 4.5v on each drain. Bypass is OK. I wired it as an overdrive.
Here's the problem : with gain to 1 the signal is close to the bypass.
On 2 it gets a subtle crunchy overdrive, and on 3 it starts whistling like a larsen.
If I continue to turn the gain knob the larsen note changes.
I don't know where to start with this debugging, any idea welcome!
Tell me if you need voltages, or pictures and I'll make some.
Thanks in advance  ;)

night-B

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Re: Rump Roast help
« Reply #1 on: February 01, 2011, 12:44:20 PM »
I have spent two hours debugging on this squealing beast but still haven't got any idea of the problem...

oldhousescott

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Re: Rump Roast help
« Reply #2 on: February 01, 2011, 01:16:58 PM »
Pictures would be good at this point. Which JFETs did you use? Make sure you haven't inadvertently routed the output of the board back to the input. Check your bypass switch wiring. Check that C11 is inserted with the correct orientation.

gtr2

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Re: Rump Roast help
« Reply #3 on: February 01, 2011, 01:51:45 PM »
an easy to build signal probe can help
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night-B

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Re: Rump Roast help
« Reply #4 on: February 01, 2011, 02:04:37 PM »
I used J201. No out return to input. Checked the polarized caps, the + is in the good place.
Here are some pics . I don't have an oscilloscope and an audio probe actualy, but I'll remedy soon.
Thanks for the help!

[attachment deleted by admin]

gtr2

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Re: Rump Roast help
« Reply #5 on: February 01, 2011, 02:43:46 PM »
http://diy-fever.com/misc/audio-probe/

I wired mine to a reg mono jack so I didn't have to cut a guitar cable though.  Make sure to use the cap to block dc current... ;)
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night-B

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Re: Rump Roast help
« Reply #6 on: February 04, 2011, 11:22:50 AM »
I checked my components before soldering, but today I double checked the resistor values with my DMM after soldering and I found a weird thing :
R1 and R5 are giving a 0R resistance to the circuit (but they had the great value before soldering) .
I checked on the resistors wire and on the solder under the PCB.
I can't see a solder bridge...  ??? ??? ???
Is it from where my squealing problem starts? (I think R1 is the input buffer right?)
Is there a solution or I must search a problem somewhere else?

oldhousescott

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Re: Rump Roast help
« Reply #7 on: February 04, 2011, 12:23:05 PM »
You can't check components in-circuit. They must be measured out-of-circuit. If you really want to check again, unsolder one side and pull that side free of the PCB, then you can check that component (resistor in this case).

night-B

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Re: Rump Roast help
« Reply #8 on: February 04, 2011, 01:38:01 PM »
Woops! Noob question...
Since that i have no idea of what's going wrong in there. I gather parts to build an audio probe  :'(

night-B

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Re: Rump Roast help
« Reply #9 on: February 06, 2011, 05:38:21 AM »
I found another mistake (think I must drink more coffee...), banzai has sent me the wrong pots!
They are all linear instead of log ones. I don't have some in stock, is it worth buying them with shipping rates?
I think it can solve my overgained pedal problem right?

oldhousescott

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Re: Rump Roast help
« Reply #10 on: February 06, 2011, 06:51:44 PM »
As long as the pot values are the same, the pot taper (linear/log) shouldn't cause the problems you're experiencing. Once you find and fix the problem, you may want to use the correct pot tapers so the action is spread evenly across the rotation.

madbean

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Re: Rump Roast help
« Reply #11 on: February 07, 2011, 01:18:07 PM »
Some general thoughts:

The RR is a monster gainiac, so it could be that the transistors you are using simply have to much gain themselves. You could try replacing them with 2n5457 or subbing in different J201s (socket the positions on the board).

Does the oscillation change pitch when you reduce the volume knob on your guitar, as well? If so, increase R2 to something more like 220k. Although you should not require a decoupling cap at the input of the circuit, you could try testing one temporarily to see if that solves the problem. Simply put a 22 or 47n cap in place of the 68k (or 220k) might do it. Or, perhaps raising the input impedance by using 2M2 in place of the 1M pulldown resistor.

Increase R3 to 4k7 to further reduce gain.

Try dialing down the bias on T1...4.5v is a general rule, not absolute.

One of these things (or two) will likely solve the problem,

Also, you could try reducing the vol pot to 10k, too!
I own madbeanpedals (duh). I am part owner of Function F(X).

night-B

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Re: Rump Roast help
« Reply #12 on: February 07, 2011, 03:15:14 PM »
I switched R2 and lowered the bias to 4V and now it's an amazing overdrive pedal!  ;D
Didn't thought the J201 could be different in the suppliers (mine are from banzai, I'm close to Germany)
Thanks Brian for such a great and original project, and the debugging help. 
Need to work on the enclosure finish and I'll post it in the build reports  :)

madbean

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Re: Rump Roast help
« Reply #13 on: February 07, 2011, 04:02:31 PM »
Awesome! I'm glad you like it.

This is definitely one of those projects where you can dig in and modify to your hearts content. It could also serve as a good learning tool for those who would like to experiment with different component types, values, etc. You could even through in some clipping diodes if you want. I kinda hoped it to be a bit "open source".
I own madbeanpedals (duh). I am part owner of Function F(X).

night-B

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Re: Rump Roast help
« Reply #14 on: February 07, 2011, 04:51:30 PM »
Can you give me advices for adding a clipping diode section?
I'm still a noob and haven't experimented yet on a pedal 'cause I don't fully understand on the schemic wich components are the high pass filter , the buffer, etc...
I read you guys trying this transistor and this type of diode but, there are so much types of them that I don't know wich ones to buy. Is there a retailer who provides an assortment of the coolest ones to try out?
I want to learn more and more and stop being a noob, maybe i'll design my own pedals someday ? ::)