Hi guys,
Iv built one of the tonepad triangle Big Muffs but have run into a little trouble.
the circuit was working fine till I went to pop it in a box, then all the voltages dropped and now there is very little effect on the clean tone going into the circuit.
Here transistor values im getting at the moment.
BATTERY = 8.74v
Q1
C 4.33v
B 3.49v
E 4.18v
Q2
C 0.69v
B 0.08v
E 0.67v
Q3
C 4.24v
B 0.69v
E 0.12
Q4
C 3.81v
B 1.65v
E 1.06v
and some voltage reading off the power rail.
(http://s10.postimage.org/f4x5zbawp/big.png)
If it stops working when the circuit is boxed up, check for shorts to the casing, or pinched wires.
Two things are immediately obvious:
- Q2 definitely looks wrong, it should have similar voltages to Q1.
- Q1's Emitter voltage is too high. It should be below the voltage at the base.
The red trace should have the same voltage at any point along the trace, but you're showing a 1.3V drop at the 12K resistor next to Q2. Could you please post photos of both sides of the PCB?
Double post
Sorry you're right the voltage is the same all along the red line....measured the wrong spot!
May be hard to post photos as my cam is playing up.
I have fluxed resoldered and checked each connection but still low voltage across the transistors?
Any other mesuments I can take to help debug?
Make yourself an audio probe and start probing from the input on, when you loose the signal it's a good area to start looking for shorts, opens etc.
I have a audio probe, but the trouble is I don't lose audio at any point....it just sound flat and lacking fuzz or amplification all the way round.
Quote from: carboncomp on June 22, 2012, 12:01:37 PM
I have fluxed resoldered and checked each connection but still low voltage across the transistors?
Any other mesuments I can take to help debug?
The voltages you already took should narrow it down. You should be checking connections and making sure you have the right values around Q1 and especially Q2. You'll need to look at the schematic.
That's what I have been doing but all the valves are right :-(
Is the voltage supposed to drop down so low at the 12k resistor just before Q2 (drops to 1.25v)?
Quote from: carboncomp on June 22, 2012, 06:29:22 PM
Is the voltage supposed to drop down so low at the 12k resistor just before Q2 (drops to 1.25v)?
It's the 9V rail, so no. The other side of the 12k resistor is the Q2 collector which should be around 4V. FWIW, I measured my Russian Big Muff, and the collectors are all around 4V.
Using Isopropyl alcohol (rubbing alcohol/tape head cleaner) and an old toothbrush, scrub all the flux residue away.
CHECK AGAIN with a magnifying glass for solder bridges.
Check that Q2 is the right way around!
Yep Q2 is the right way round, even just took it out and tested it in case it was broken.
really is a clean soldered board and i can't see any shorts when using a jewellers loop.
Here are the voltage readings from that area if anyone want to help me out as im clearly at a impasse
(http://www.freeimagehosting.net/newuploads/aj13b.png)
What are the values of the following resistors supposed to be according to the BOM:
R4
R10
R20
Look at the color codes or measure their resistances to see if they match the BOM.
There's too much current flowing thru R10, which is causing the large voltage drop across it. There must either be a short between tracks or an incorrect resistor near that area. Recheck the value of R10 , R11 and R9.
great thread title
good luck man .
Hey guys, checked the resistor values and they seem to be right
R4 22k
R10 12k
R20 390k
R11 150R
R9 390k
You mention in your first post that the circuit was working fine until you boxed it up, I know this is kind of stating the obvious, but maybe that is a hint.
What changed during the boxing up process ? Did you rewire the pots ? Are they all wired up as before ?
Are there any bad solder joints to the pots and on the various ground points and ground connections to jacks etc? Buzz them out with a meter.
Do you have your input and output jacks ground connected together or are you relying on a ground connection through the enclosure ?
Did any slivers of solder fall on to the the board........etc.
This is a weird one. Good luck trying to fix it.
-Alan
Looks like its time for photos of both sides of the board and the off board components. Could you please post some photos?
Here are some photos, guys (thanks for your help with this).
http://postimage.org/image/v6tjju5bb/ (http://postimage.org/image/v6tjju5bb/)
these are really large one, so thought id pot a link rather then in the tread!
Thanks. I'm still reviewing, but the first thing that jumps out at me is that R8 is missing from your board. Another thing is I would use some isopropyl alcohol and a tooth brush or nylon bristle brush and clean all of the flux from the board. Use a hair dryer or heat gun on low to dry the board before firing it up.
There may be more too - without R8 Q2 won't bias properly, so that's a big hint -but: Are you sure C1,3,4,7 etc are 1uF or .1uF? They look an awful lot like ceramics to me, suggesting much lower values.... This would kill anything but the upper register, so if you get Q2 to bias and are left with very thin sound, the cap values are up for review.
The caps in question are 100nf ceramics, and on the layout it has R8 omitted on the rams head.
I'm certain the missing R8 on the schematic you're looking at is an error. I've looked at several schematics for the Ram's Head and they all have a 100k on Q2 where your R8 is. Here's one example. The resistor on this schematic is labeled R16.
(http://i1160.photobucket.com/albums/q485/jdansti/fbbb75ca.jpg)
If you drop a 100k in the R8 holes, that sucker should work.
put in the 100k and the Q2 collector has jumped from 1.2v to 3.5v, but there is still no noticeable effect on the output, just a little more gain :(
Ok. I went to the Tonepad site and it appears that you're doing the "triangle" muff. You're correct that there is no R8. I don't understand how Q2 is supposed to bias without R8, but apparently it's supposed to work. I noticed that this version also omits C2, but it looks like you've decided to keep it. The other difference between the stock and triangle versions is many of the resistors and capacitors have different values.
(http://i1160.photobucket.com/albums/q485/jdansti/4dd365c7.jpg)
Going back to one of your first statements that it was working and it stopped working after you boxed it, I think the next thing I'd do is reflow all of the solder joints including the pots and jacks, clean off the flux, and look for solder bridges. If that doesn't help, I'd replace Q2 (I can't recall if you said that you've already done this. This is a tuff one!!!
Quote from: Jdansti on June 26, 2012, 01:12:00 AM
Ok. I went to the Tonepad site and it appears that you're doing the "triangle" muff. You're correct that there is no R8. I don't understand how Q2 is supposed to bias without R8, but apparently it's supposed to work.
The base hopefully gets pulled up to bias by the 470k resistor between the base and collector. It lets a very small current flow into the base of the transistor, and out through the emitter (and emitter resistor to ground) which lets the transistor turn on.
The bias point is arrived at from a lot of assumptions:
Transistor gain, estimating the base current from that, then guessing at a voltage drop across the resistor from those...
The problem with this way is that the bias point becomes dependent on the transistor's Hfe which can vary a lot (the Art of Electronics has a schematic [pg. 72] illustrating this way of biasing with the caption "Don't do this!").
Adding in the 100k to ground makes the bias point the same regardless of what transistor you use.
ok, going back to your voltages -
Q1
C 4.33v
B 3.49v
E 4.18v
Q1 is wrong, the base should be higher then the emitter. You also have a 3.6k resistor on the base and schematic says it should be 3.3k, but I doubt that would make such a big difference.
I noticed your using solid core wire, this can put stress on the copper pads and can cause hairline cracks where the pad joins the rest of the track, so it might be worth checking continuity from the pots to the next component in the schematic. i.e dont just check continuity from the pot to where the wore goes into the board, but check it to the adjacent component.
When using an audio probe on it, there should be a noticeable increase in sound on the collector of Q1, it's configured as a clean boost stage, if the volume is not getting boosted from Q1 then the problem lies there, again Q1 voltages are not correct - unless you mixed up the B and E voltage readings when you first took them.
What transistors are you using ? 2N5088's ?
There are also a few dodgy looking solder joints, base and emitter of Q1 look like they could do with a reheat, and the 100k resistor ground connection on the base of Q4 looks suspect.
c'mon man get this working - you can do it.