LSTR Tone Pot

[squirrels]

I've been working on an LSTR tonight and unless the tone control is at 0, the effect doesn't work. With the tone at 0 all other pots function normally. I tried changing all the parts in the tone stack with no luck. Right now there are a few substitutions due to lack of parts. I tried MPSA13 and 2N5089 transistors, the 68k resistor was replaced with 120k and 150k resistors in parallel (66k) and the collector resistor on Q4  was replaced with an 18k instead of 15k (used 15k initially but ran out).

I used Mark's layout: http://tagboardeffects.blogspot.com/2012/11/black-arts-toneworks-lstr.html?m=1

My voltages are:

Q1
C: 3.6
B: 0.54
E: 0.01

Q2
C: 3.65
B: 0.60
E: 0.03

Q3
C: 3.62
B: 0.59
E: 0.04

Q4
C: 0.61
B: 1.10
E: 0.53

My voltages for Q4 were the same using the 15k resistor. I replaced all the resistors for Q4 with no change. Any help would be great and much appreciated. Thanks.

[squirrels]

Also used a A250k tone pot but replaced with a B500k (no B250k right now).

[squirrels]

Maybe pics might help.

[Ciaran Haslett]

Your problem is definitely in the tone recovery section.  Can you take better pics?  These one's are too small to see much of anything.  Don't scale them when uploading on this forum.  The compression is awful.  May a good pic of the wiring too

[squirrels]

Thanks Ciaran. I'll take some when I get home.

[PMowdes]

Here's the schematic if it helps

http://music.codydeschenes.com/wp-content/uploads/2013/10/Black-Arts-Toneworks-LSTR.png

[PMowdes]

Like Ciaran said the pin voltages suggest that you have an issue at Q4.  Id check the 15K that supplies 9v to the collector (next to D5) and the 470K that runs to the base.

There's also some suspect solder on the 68k to the left of Q4

[squirrels]

I had to use an 18k. There was a 15k originally but I ran out. The 68k is a 120k and 150k in parallel to give me 66k. I'm out of 68k's too. I'll check to make sure the solder isn't touching anything though.

[PMowdes]

Have you got a signal passing from Tone 2 (on the far right of the board) through the 100n into the base of Q4??

[squirrels]

Honestly I'm not sure how to even check if signs is passing through. I swapped out the 100n on Q4's base as well as changed the collector resistor on Q4 (now 12k) and the parallel resistors for a 56k. I have no signal passing through. On the 9v side of the resistor I get 8.47v and 0.8v on the Q4 side. No signal passed through now.

New pictures. I pulled the transistors for a better view. I had to resize them or they wouldn't post.

[PMowdes]

This is where an audio probe is really useful

If you look at the schematic you can see the signal path into tone 3 via the 100n cap and out of the tone pot via lug 2 into the base of Q4 and out through the collector through C13 (100n) into vol 3 and to the output via vol 2.



If you follow the signal path through the vero with an audio probe you should be able to find where it goes missing and determine where your error lies

[PMowdes]

I'd also be tempted to take a thin hack saw blade and run it between the tracks in this area



There's definitely a voltage issue going into the collector at Q4 so perhaps there is a solder bridge or a dry joint particularly around the 15k running into the collector.  Perhaps measure the voltage either side of the resistor and see if it makes sense.  You should have pretty much 9v on one side of the 15K and i'm guessing around 3.6 on the transistor side.

[squirrels]

Reflowed all the joints and cut the tracks. No luck. I also checked the collector resistors and Q4 is definitely no good regardless of what I swap it out with.

[PMowdes]

Hmm,  um running out of ideas,  what about the 3 cuts,  maybe go over them again with a drill tip,  maybe there's a short there

[Ciaran Haslett]

If you're absolutely positive your placement/values/wire connections/vero tracks are all correct then that leaves the only thing you haven't checked.....the socket.

I've had at least one instance where a burr from a cut got trapped under an IC socket and bridged to another row.  Took a few days to find that one! Use your DMM to check for any continuity where there shouldn't be.

What I would do now....using a DMM of course

With the board comp side up, check for continuity on all component legs on the same row.  It will help find dry joints doing it this way rather than from the solder side.

Do the same with pot wires...from a component on the board to the lug on the pot.

Lift the 100n leg from Q4C row and measure DC on the collector.  

Reluctantly remove everything from the tone pot 2 to the output and replace on at a time measuring at each stage.

.....and finally after all that.....set it on fire and nuke it from orbit!

But maybe just check the socket first

[squirrels]

The extra wire was a for the LED. I removed it with no luck. I swapped everything surrounding Q4 except the socket so that's worth a shot. If that doesn't work I'll scrap it.

[squirrels]

Built up a probe and the signal died at the 100nf between tone 2 and the base of Q4. Works great now. Thanks so much for your help everyone. I'll build a better probe after this  I've always read about people using them to debug. Thanks for the suggestion PMowdes.