Debugging Sweet Thing

[ward]

Hey all,

I'm building a Doug Hammond's Sweet Thing http://home.cfl.rr.com/dbhammond/sw_sch.gif
But it needs some troubleshooting.
if someone wants to have a peek, greatly appreciated ..

What does it do, not do, and sound like?

Sound comes through but with a volume drop,
The pots react as expected.
The sound is not nicely distorted, kinda harsh ?

Any modifications to the circuit? Y or N

No modifications

Any parts substitutions? If yes, list them.

Fets used are BF245a,
they're pinout is the reverse of the J201.

BF245a:
G  ! )
S  !   )
D  ! )

The tonepot is 100k instead of 50k.

I breadboarded it, here's my layout:

the connection Q4-source to Q3-drain I made in the air.
vol1 goes via C11 directly to tone2 and is not on the breadboard.


the voltages:

Q1
G= 0,03
S= 0,22
D= 4,5

Q2
G= 2,83
S= 4,5
D= 8,5

Q3
G= 0,0
S= 1,34
D= 5,70

Q4
G= 2,83
S= 5,70
D= 8,50

Q5
G= 5,70
S= 6,78
D= 8,50

[ward]

Strange fact that I noticed,

When pushing my finger on the solderside of the breadboard somewhere between Q3 and Q4 it sounds how it 'I think' should sound.
More volume and nice sound. 
Apart from strange effects like radio interference and hum.

So I thought, I have a bad solder joint somewhere, but I can't make that phenomenon happen with a plastic stick pokin around.

[Doug_H]

Probably a cold solder joint. Reheat all of the pads until the solder flows and let them cool.

[ward]

Thanks Doug,

You were right,
This morning after reflowing some solderjoints it worked.

Haven't had the change of really listening though, 
my first impression was that the sound is nice but the volumegain is maybe a bit too much.
I'm gonna play with it some more when I get home, ...
Can I customize the drive- and volumeboost range ?

Thanks for making this effect available for the diy'er to all who contributed to the design !

[Doug_H]

You can always put a voltage divider on the output (before the volume control) to pad down the volume if you need to. You might  check out Ed Guidry's BSIAB and BSIAB2 too as they are a further evolution of this concept (amzminitubes->booster2.5->ST->BSIAB->BSIAB2).

[ward]

Thanks Doug,
the pedal works,
good sound long sustain,
though when I bypass it and set the pots at extreme settingts it acts strange.
With the drive somewhere near max, the pedal starts oscillating, and the volume pot changes pitch.
Is there a way to make it more stable ??
If I'm the only one having these problems, maybe it's because I used other Fets ?  I used BF245a .

Second problem:
with my power supply it hums, well hum and an allmost beelike buzz.
I checked with batteries, then the noise is gone.

Strange, the powersupply works good with other high-gain pedals ?
It's a simple 7809 regulator with two large caps.

I thought maybe I'm having a ground problem.
Is there something wrong with my layout ?

[ward]

Out of some other threads here I think of trying:

* maybe an extra capacitor close to the efx-boards.
Does that make sense ??

* or I saw this schem
http://www.generalguitargadgets.com/pdf/two_in_one_wiring.pdf
maybe that's a better layout ?

[ward]

Bump, noone ?



The tychobrahe works good, in the same box.

I was thinking, the jacks both have hot plus ground connections, but the box gives ground too.
Would that cause a groundloop ?
I think I need to do some more reading on groundloops ...   

Cheers

[aron]

though when I bypass it and set the pots at extreme settingts it acts strange.

Are you muting the input when bypassing? That's probably it.

[ward]

Are you muting the input when bypassing? That's probably it.

I'm sorry, but I'm not following you ?   
What do you mean by "muting the input" ?

The oscillating is happening at about halfway the volume when I bypass the effect.

The noise sound happens all the time unless the effect is bypassed,
when I roll the volume down on my guitar, the buzz disappears, but the bee-like sound stays at the same volume.

[ward]

Is this what you mean ??

The version of true bypass with the DPDT that leaves an extra throw to which the input of the effect is connected when the effect is bypassed has been used in some pedals to good advantage to ground the input of high gain distortion effects, which produces some decrease in capacitively coupled noise.

from GEO: the technology of bypass switching

[Doug_H]

The ST is high gain and many yrs ago I became so frustrated with an osc problem with it I pulled it from its box and put something else in there. Turns out it was a crosstalk problem with a cheap DPDT switch I was using. Have not had these kinds of problems with the Carlings or blue 3pdts. That may/may not help you.

All I can say is it is mostly likely a layout issue with the board or external wiring. And you are going to exacerbate any layout issues by putting 2 separate circuits in the same box. I would not have recommended that. High gain circuits can be tricky enough in & of themselves with osc/noise without combining them with another in a single chassis.

[ward]

Thanks Doug and Aron,
I rewired the switch to bypass with grounded efx-input.
The oscillations are gone,

But I still have noise issues, I'll take out the tychobrahe to rule that out.
And use shielded wire for the long wireruns.

Thanks for all the help.

[ward]

Grmbl 

I can't get it to quieten down.
I used shielded wire for in/outputs, removed the tychobrahe completly.
...
Noise

Took the switch out off circuit,
...
Noise

How can you post mp3 files over here ?
Maybe hearing it would help narrowing down the problem?

Maybe I should probe it, never done that before?.