Mayqueen biasing issue

[Guybrush]

Hi all

I've just put together the Runoffgrove Mayqueen and run into a problem.

http://runoffgroove.com/mayqueen.html

Q3 needs to be set to 5.52v but using the 10k trim pot, it only goes down as low as 6.03v.  Would switching to a 100k trim sort this out?

Using the pedal with Q3 set to 6.03v I'm getting a gated splattery fuzz.  Will this be because of the incorrect voltage or have I got other problems?

Thanks guys.

[midwayfair]

Hi all

I've just put together the Runoffgrove Mayqueen and run into a problem.

Here's the link http://www.youtube.com/watch?v=BGWYGR3xy2Y

Q3 needs to be set to 5.52v but using the 10k trim pot, it only goes down as low as 6.03v.  Would switching to a 100k trim sort this out?

Using the pedal with Q3 set to 6.03v I'm getting a gated splattery fuzz.  Will this be because of the incorrect voltage or have I got other problems?

Thanks guys.

Try putting a resistor in series with the trimpot first. You can cut the connection to the drain, then solder the resistor to leg 2 and the drain (should fit sideways). Might save you some trouble. Start with a 10K since you know that's not big enough. Use sockets if you think you might need to go bigger than 10K.

Also, post some pictures of the build and let us know which FETs you're using. It's possible there's something else wrong. 10K should have been big enough.

p.s. wrong link. http://runoffgroove.com/mayqueen.html

[Guybrush]

p.s. wrong link. http://runoffgroove.com/mayqueen.html

Link fixed!   Thanks Jon. 


Ok, here's a pic of the board:



and a pic of the transistors I used:

Try putting a resistor in series with the trimpot first. You can cut the connection to the drain, then solder the resistor to leg 2 and the drain (should fit sideways). Might save you some trouble. Start with a 10K since you know that's not big enough. Use sockets if you think you might need to go bigger than 10K.

I appologise but I'm not understanding what you mean with this.  Could I remove the drain leg from the socket, solder on a 100K resistor and put the resitors other leg into the now empty socket hole?

Grrr...  It's such a simple circuit.  How can I get it so wrong!? 

[midwayfair]

Sorry, I forgot it was an etch (you told me earlier) and I thought it was their perfboard layout. Go with the larger trim pot. Sorry about that.

FETS are fussy, no doubt about it.

Also, can you give your voltages on the other pins of the FET? And have you tried making small adjustments and testing at basically every area of the trimmer?

If I were in the UK I'd send you a 25K trimpot, but you're probably better off getting it from Tayda. You can replace the trimmer with a regular pot if you suspect it really is the issue.

[Guybrush]

Sorry, I should have mentioned it was an etch.

The voltages for Q3 are as follows:

D  Between 9.28v and 6.03v at extreme of the trimmer to the other
S  0.734v
G  - 30mv  (but it never seems to settle and is a minus reading)

There's obviously something amiss.

Thanks

[midwayfair]

Let's try something else: Let's see if there's something wrong with the circuit or if it's really just FETs not biasing. Take one of your 3904s. With the flat side to the right, twist the leads so that the middle and bottom leads are swapped. Best to use pliers so that you can get clean bends and don't break them. Plug it in where the FET is.

Does it work or does it sound the same?

Also, check continuity on all the parts with your multimeter. Follow the schematic while doing so. I know it seems tedious, but if it's a bad soldering connection that you just can't see or a crack in the etch, this is the easiest way to find it.

[Guybrush]

Thanks Jon.

I'll try it after work this evening.

To summarize: Take out Q3, replace (but facing the other direction than what the J201 was facing) with a 2N3904 that has the E & B switched.  Sound good?

I checked continuity on the etch before I populated the board but there's every chance I screwed up when soldering so I'll check again.

[Guybrush]

So...

I had a fiddle, removed and changed a few componets with dodgy looking soldering and it fired up and biased perfectly.  Not really sure what the problem was but the changes have sorted it.

Thank you jon for all your help.  You've taught me that my DMM is for more than making beeping noises with!

Build report to follow...