Sans Amp GT2 (sin amp) Tonepad PCB issues...

[seagurt]

So this is my second attempt at building the Tonepad Sansamp GT2 circut. The first one, I had an issue with a faulty soldering iron, and trace lifted, so I bought a second one. Got a new good quality Hakko station, so I populated the pcb, drilled the enclosure, wired all the off board stuff. Plugged it in for sound test, the adaptor when plugged in, and the 3pdt is engaged lights up the LED, but no sound. Bypass works, so I decided to turn my clean channel volume all the way to 10, gain at 1.5 (using a Fender 5150 III EVH 50 watt) and I sure enough, am getting the tiniest signal. Now this is where things get interesting. The Mic switch and and Mod switch seem to be working, but the amp selector switch doesnt, it only seems to give one sound, when the toggle is at the bottom (I believe this is the fender emulation). I am wondering what component would be responsible for such a volume loss, and what I should be looking at in the switch issue. I am also currently debugging, but thought that because my experience and pedal based knowledge was rather limited, that someone more knowledgeable may have some insight. The switch seems to have continuity from the unit itself to the board, so no break in the connection.

I will entertain all questions, and request for pics, etc.

Thanks in advance.

[Govmnt_Lacky]

Check out the debug thread here:

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

Read, heed, and post the good info for us to help you solve this dilemma 

[seagurt]

1.What does it do, not do, and sound like?
It turns on, the issue is the sound. I got more output out of it, the 2N5088 wasn't properly seated in the socket. The output is now loud, and the unit whistles with the gain past a certain point on certain switch settings, or whistles when I increase the treble. The whistle changes in frequency when the gain and or treble knob is turned. I may swap the transistor for a BC549 or BC550, as I have them available, and from what I am reading, they are low noise.

The main issue is the switching, It seems that my MIC switch (I am using the ALCO switches from small bear http://smallbear-electronics.mybigcommerce.com/dp3t-on-on-on/ )isnt working in the first position (Off axis, there is no audible difference, so I am going to hook it up to a spectrogram to be sure) and there seems to be an "in between" switch position, between position 2 and 3 that sounds pretty good to my ears, a little beefier. The same issue with the MOD selection, I get ridiculous squealing here especially with the gain above 50%. The hot wired, hi gain and clean. Another issue is that it seems position 1 to me sounds pretty much like position 2, still pretty distorted, but less bass and a little less volume. Position 3 is ballsier, but then there is another "in between switch position that sounds 10x beefier!". The biggest issue here, with the switches though, is that my AMP selection switch (S3) only outputs signal on the 3rd position (Tweed), there is also an in between position, between the 2 and 3 positions that sounds even cleaner, so maybe its only the Boogie emulation I am getting (if its backwards?)

2.Name of the circuit =
Sans Amp GT-2, Analog Amp Simulator (or sin amp)

3.Source of the circuit (URL of schematic or project) =
http://www.tonepad.com/project.asp?id=22
http://smallbear-electronics.mybigcommerce.com/pcb-tonepad-san-samp-gt-2/

4.Any modifications to the circuit? Y or N
cosmetic change, my switches are ordered S1 (MIC) S3 (AMP) and S2 (MOD) from left to right.

5.Any parts substitutions? If yes, list them.

Yes,
IC1 & IC4 are TL072CP
IC2 & IC3 are TLC2272CP
I also have NE5532 IC's available, if it would help

The transistor is a 2N5088, and socketed ( I have BC549, BC550 )

6.Positive ground to negative ground conversion? Y or N

Tip negative jack, and receptacle. No need.

7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? =>

This unit is only setup for an adaptor at the moment.


Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages as follows:

Q1
C =9.64
B =4.85
E =4.25

IC1 (or U1)
P1-4.85
P2-4.85
P3-4.40
P4- ( - )
P5-4.31
P6-4.74
P7- ( - )
P8- starts at .77 and drains

IC2 (UI2)
P1-4.85
P2-4.90
P3-4.79
P4- ( - )
P5-4.85
P6-4.90
P7-4.85
P8- 9.65

IC3 (UI3)
P1-0
P2-4.82
P3-4.85
P4-4.85
P5-4.81
P6-4.85
P7-4.85
P8-9.64


IC4 (UI4)
P1-0
P2-4.84
P3-4.85
P4-4.85
P5-4.85
P6-4.85
P7-4.85
P8-9.64

If any more info is needed, I will be happy to oblige.

Regards.

[Govmnt_Lacky]

Right off the bat I see problems with all of your ICs except for IC2.

All of them should read 0 or ground on pin 4 and full or close to full voltage on pin 8.

Pins 1-3 and 5-7 on all of them should read about 1/2 the power voltage on your pin 8s.

Look around these areas for shorts,  wrong component values, etc.

Good Luck

[PRR]

> All of them should read 0 or ground on pin 4

It seems unlikely he could have all of them wrong on a "bought" board (twice).

Somewhat more likely he is counting pins counter-wumpus to the way the chip maker counted them.

This is the standard DIP-8 pin layout:


(Why not numbered like a V-8 engine? Chips used to be round. The pins were numbered clock-style like the tubes which came before. When rectangular chips came out, they were pinned-out and numbered so you "could" bend-up the pins and jam them into the round hole-layout.)

> whistles with the gain past a certain point on certain switch settings, or whistles when I increase the treble.

This will happen when the gain is too darn much, or the output wires run too close to input wires. (Same as acoustic fedback when the PA mike is too close to the speakers or/and the gain or treble is turned up.) Pictures--- let's see how the wires run. Before you load the camera, try pushing wires around with chopstick (or since this is all lo-voltage, your finger). If squeal gets better OR worse as wires are moved, that is A Clue.