Error found on the Pastyface BOM & the volume drop

[chromesphere]

Hey everyone,
i posted on DIYSB's a problem i was having with the output of the soul bender (pasty face) being less then unity.  That problem still continues unfortunately, but i thought i would just post here for Brian that there is an error in the soul bender BOM.  Prehaps you were going blind typing those BOM's up R10 says jumper, but should actually be a 10k as per schematic.  The tone control doesnt work with R10 as a jumper, but is fixed when adding that 10k res in.  I've tried it, its definitely wrong.

Any suggestions from anyone on how to fix that volume drop would be great!  So far i have swapped R11 for a jumper which i think seemed to help a bit, but still a volume drop with the volume on full.

Any help appreciated.

Paul

[jimmybjj]

Have you tried using a larger pot? More resistance to ground means less signal is being cut. Might help a little.

You might also try playing with output cap, more bass through sounds louder but changes how the circuit sounds slightly.

[mgwhit]

I breadboarded this a month or so ago with a set of Small Bear generics starting with the Soul Bender specs and working backwards towards Vox specs, and I didn't notice a volume drop or a non-functional tone control in the Soul Bender version.  That was actually my favorite of the bunch -- I specifically noted "useable sweep on Tone control knob".  And the only other schematic I've seen for the Soul Bender (by J.D. Sleep) shows the Tone pot connected directly to the Volume pot, too.

A few other people have built this board, so let's see what they have to say.  I haven't populated my board yet, but I might do it tonight if no one else chimes in.

[madbean]

There is no 10k between the Tone and Volume pot on the Soulbender. If your tone control is not working unless there is a 10k there, then there may be another issue at play, perhaps related to the very low volume output you are experiencing. I'm quite certain of this as I compared the parts directly to a friend's Soulbender some time ago.

However, I did notice an error in the BOM just now. For the Sola Sound Tonebender, R10 should be 220k. I've made that correction on the project doc.

[chromesphere]

The schematic im looking at, persumable the soulbender, is on the last page of the project file and most definitely has r10 - 10k.  The BOM says jumper...Maybe i should have specified, im following the single sided pcb etchable layout...?

EDIT: Also, just wanted to say, i've built two of these and they both have a volume drop. Maybe theres an issue with the single sided DIY layout?  idk...

Paul

[madbean]

The schematic im looking at, persumable the soulbender, is on the last page of the project file and most definitely has r10 - 10k.  The BOM says jumper...Maybe i should have specified, im following the single sided pcb etchable layout...?

Correct, but it also say on the same page "(This layout allows you to build the "Soulbender" or similar version – component numbering is slightly different than the schematic on pg.2)".

If following the single sided layout you should be using the component numbers and values listed on the last page, not the BOM for the double sided layout. The difference came from expanding the double sided layout a little from the previous version of the Pastyface to allow one to build several types of Tonebenders---not just the Soulbender.

So, on the SS layout, yes you definitely need to use 10k for R10...a jumper there would cause problems.

[chromesphere]

Ah k...I was following the soulbender BOM on page 3.  I'll have to go over the values and check them, maybe there is another difference between the two causing the volume drop. Thanks for your time Brian and for clearing that up.
Paul

[mgwhit]

If you jumpered R10 on the etched board, you're dumping all of your high treble straight to ground and the Tone control should be acting more like a volume attenuator on the rest of the signal.  I think that's your whole problem right there.

The only other significant difference in the schematics is that etched R9 = fabbed R13, but they're both 10K in the Soul Bender specs, so you're probably good.

[chromesphere]

Yeah, that 10k on the tone pot was my mistake.  I think im just used to following the BOM, it was kind of automatic.  Anyway, tone's working well now so thats good.  

I'm just trying to hunt down the cause of the volume drop now.  Demo's on Youtube suggest that there shouldnt be volume drop.  I quickly checked over Brians pcb layout as well this morning, all checks out (as expected ).  So i've clearly stuffed something up lol.  I've checked over my boards 1000 times, but hey, hasnt stopped me from missing some detail in the past I'll have another go at it tonight.

Thank you both for your help, ill update this thread as i go incase anyone else has a similar problem.
Paul

[madbean]

Have you checked voltages on your transistors? That may provide a clue.

[chromesphere]

I think thats the next place to look.  Will post voltages tonight.
Cheers.
Paul

[chromesphere]

Ok, ive measured the voltages on the transistors.  These are my results:


q1    e     1.33
   b   1.43
   c   3.68

q2   e   1.21
   b   1.33
   c   3.68

q3   e   0v
   b   0.12
   c   1.92

Q3 looks a bit low, but i found this old diysb thread where someone else has very similar readings...are they correct?

http://www.diystompboxes.com/smfforum/index.php?topic=80871.0;wap2

Paul

[mgwhit]

Your voltages look similar to mine when I breadboarded a Soul Bender, except your collector on Q3 looks low/  Or I suppose high, since these are negative voltages.

-9V = -9.48V

Q1:E: -1.54
Q1:B: -1.63
Q1:C: -3.62

Q2:E: -1.41
Q2:B: -1.54
Q2:C: -3.63

Q3:E: 0
Q3:B: -0.08
Q3:C: -2.79

I also wrote a -2.9 next to Q3:C, so maybe that was another position on the bias trimmer.  That was probably as high as I could adjust it.

[chromesphere]

The battery i used was a bit low 8.5v approx forgot to mention.

I think i'm concluding that this pedal is functioning as it should.  I found a thread where someone is having a similar problem and a few people have confirmed that this can be part and parcel of some germanium fuzzes.  

A suggestion towards the end of the thread was to increase R10 to 33k from 10k "may result in more bass", which i tried and now when the bass is up 90ish percent, im on the doorstep of unity. I also removed the 47k on the output with a jumper, and swapped R8 & R9 around, which could have contributed to the increased output.  The biggest difference was R10 though.

It still sucks that you cant just set the volume to like...half and boost from there.  The volume control is almost redundant as im going to set it 100% and leave it.

The thread for anyone following:
http://www.thegearpage.net/board/archive/index.php/t-887046.html

Only other thing i can think of is to try some leakier Ge's when they show up.

Paul

[chromesphere]

Ok, to finialise this post...

1) Theres nothing wrong with the layout as Brian cleared up before.  If you are following the DIY etch for the soul bender / pasty face make sure you following the schematic values and not the BOM.

2)  Volume problem turned out to be a problem with my test box!!!  As soon as i boxed both of these boards up, they both worked at the proper volume...What a pita this build has been.  No fault to anyone, just a whole lot of bad luck

Thanks for all your help, they sound nice

Paul