Revisting My First Attempt at a Guitar Pedal

[Jhouse]

The first project I ever took on was the SLOW GEAR by General Guitar Gadgets. I was super excited about it and it was my first time ever messing with electronics. I etched my own board, spent a ton on parts, and tried to throw it all together. Well, it didn't work so I just put it in my closet and forgot about it. I found it today and I want to try to get it to work again.

The pedal's swell time is weird and only works every now and again. It also refuses to work on my B and E strings on my guitar. I don't get it.

Looking at it now, I made several substitutions that are a bit impractical. I guess it is because I ordered the wrong parts/not enough parts to get it all right.

The list of substitutions:

R1 = 1.2k
R5 = 3.9k
R12= 1.2k
R15= 230k
R19= 2 220k resistors in series (They are 1/2 watt too. Haha.)
R26= 39k
C6 is electrolytic and its negative side goes to the left.
Everything else is just the same as in the layout from GGG.

Here are my voltages

IC1
P1= 0
P2= 0.46
P3= 1.02
P4= 0
P5= 0
P6= 8.56
P7= 6.18
P8= 0

Q1
C= 8.98
B= 0.47
E= 4.21

Q2
S= 5.98
G= 0.24
D= 5.98

Q3
E= 5.98
B= 2.01
C= 8.81

Q4
E= 3.98
B= 1.14
C= 4.98

Q5
E= 0
B= 0
C= 2.68

Q6
E= 0
B= 0
C= 2.64

Z1
A= 6.00
K= 0

D2
A= 0
K= 0

D3
A= 0
K= 0

D4
A= 2.81
K= 0

Looks to me like there is something seriously wrong with my old pedal.

[Jhouse]

Bump

[PRR]

> R26= 39k

What is your "+4.5V"?

The exact value of R25 and R26 is not critical but they should be the SAME value. Within 10% or 20%. 22K and 39K is WAY-WAY off-center.

In many circuits "near center" is good enough; indeed 22K and 39K might "work".

But there are SO many other wrong voltages, and unrelated wrong voltages, that I suspect you have the wrong pinouts for your transistors or the wrong transistors for your pinouts, and possibly the IC also (did you use a dula where should be a single opamp?).

And the ever-popular bad solder joints.

[Jhouse]

Alright, Well, I replaced all of my transistors and my IC. I also changed R26 to a 22k. I'm using the BC549's, 2SK30A transistors. I also put a LM741CN in there for my IC. I made sure that everything was oriented correctly. The pedal is still not working.

[PRR]

> still not working.

Did any voltages change?

Are you having layout or solder-joint problems? Forgotten connection, next-hole-over, and unworkmanlike solder skills are the most common troubles.

[Jhouse]

The voltages changed only slightly. I'm sure it has to do with my sub par soldering skills. 

[petemoore]

  Pick a transistor, some had the same voltage on two of 3 pins.
  Test with DMM 'beep mode' if they're connected, any other connections including that all grounds are common.
  Test with DMM 20vdc range that the V-divider is providing 1/2v of supply voltage.
  Often when a group of stages are all whacky, finding a problem with just one active component can lead the way to a solution that is common to numerous stages, such as with the voltage divider which provides a common bias voltage to them.
 

[twabelljr]

Use this diagram as a guide for some voltage measurements; http://www.generalguitargadgets.com/pdf/ggg_sgo_lo_wiring.pdf
The green traces should read 9 volts. (or whatever your power supply is)
The purple/blue traces should all be grounded
The gold trace should be 4.5 volts. (or half supply)
In your voltage postings it looks like you have battery votage on pin 6 of IC instead of pin 7.
This is just a starting point. I really don't know what all of the transistor readings should be but the IC looks off.

[Jhouse]

Well, I managed to find 1 solder bridge between one of my transistors. I thought this might be the problem so I tried the pedal again. It works, but I have to pick very hard to get it to work, It crackles out notes most of the time, and sometimes it just doesn't work at all. Here are my new voltages and my 4.5 volt is at about 3.01

IC1
P1= 0
P2= 0.46
P3= 0.89
P4= 0
P5= 0
P6= 8.12
P7= 4.56
P8= 0

Q1
C= 8.98
B= 0.47
E= 3.21

Q2
S= 5.98
G= 0.24
D= 5.98

Q3
E= 5.98
B= 1.98
C= 8.81

Q4
E= 3.01
B= 0.47
C= 5.97

Q5
E= 0
B= 0
C= 3.01

Q6
E= 0.05
B= 0.02
C= 3.01

Z1
A= 5.99
K= 0

D2
A= 0.17
K= 0.41

D3
A= 0
K= 0

D4
A= 0
K= 0

[Jhouse]

In your voltage postings it looks like you have battery votage on pin 6 of IC instead of pin 7.
This is just a starting point. I really don't know what all of the transistor readings should be but the IC looks off.

Are you sure? Pins 1, 4, 5, and 8 are either grounded or not connected to anything. Pin 6 is connected to V+ with a 1k resistor separating it. It seems to me like this would be correct.

[PRR]

What kind of volt-meter are you using?

[Jhouse]

It's a 17 range analog multimeter that I picked up from Radioshack a few weeks back.

This is it.

[PRR]

> analog multimeter .... from Radioshack

Ah.

That's a fine meter, for electricians.

What was really baffling me is the low readings adjacent to correct readings. And the low readings were after HIGH value resistors.

To be blunt, this meter is no-good for many electronics. It needs 0.2mA to read full-scale, and it sucks that from the circuit it is reading.

For 60mA relays and 20,000mA wall-sockets, that hardly loads the circuit voltage.

In this case, emitters are flowing 1mA-2mA, when reading half-scale the meter only sucks 0.1mA, the readings are not far off.

But the Base and Gate resistors normally flow 0.003mA or zero mA. The ~~0.1mA of this meter is a BIG difference, and a major disruption to the actual working voltages.



VTVMs and DMMs steal far less power from a circuit (they don't have to, they eat batteries instead). Since DMMs came down from $2K to $99 to $3 (plus $6 shipping!), most folks don't use analog meters today. I do, I own, uh, four? One like yours, one much more sensitive ($99 in 1957), and a beater for truck lights. But also 2 or 3 DMMs.

I hate to send you back to the Shed, or tell you to blow $9 on eBay for a DMM. But these saggy readings (as well as the back-forth from schematic to layout to your readings) is tough.

[twabelljr]

Pin 6 is connected to V+ with a 1k resistor separating it

I'm pretty sure that is pin 7 connected to the V+ with the 1k according to that layout. +1 on a high impedance meter!

1-U-8
2----7
3----6
4----5

[Jhouse]

1-U-8
2----7
3----6
4----5

I was reading it as:

1-u-5
2----6
3----7
4----8

Whoops. 

I hate to send you back to the Shed, or tell you to blow $9 on eBay for a DMM. But these saggy readings (as well as the back-forth from schematic to layout to your readings) is tough.

I used to have one, but it just stopped working one day. I replaced the fuses and everything but no luck. Oh well. From the looks of what you said, I definitely need to get another one.

[Jhouse]

So I took another look at this pedal and made sure that there were absolute no solder bridges and cold solder joints (I literally went over every single joint). Anyway, my half voltage part of the circuit is now reading 7.21 v. Also, the voltages on all of my other components are relatively the same.

The pedal works sometimes, it just crackles out or just plays the note. It doesn't work consistently.

[wavley]

Much like when people think their Blue Box isn't working because it won't track well, your Slow Gear may be closer to normal than you think.  The slow gear is a really touchy pedal about working smoothly, your picking style, which pickup you use, these kind of things matter a lot with them.  I was just playing my clone about five minutes ago, I love that thing.  Also, have you tinkered with the 10K trim, it makes a really huge difference and is really time consuming to get set just right, this may be the most likely cause of your problem.  Once you get that right, try a compressor in front of it, that really helps to smooth out the swells.

[Jhouse]

I've fiddled with the 10k pot a little bit. I'll end up messing with it some more here soon. Thanks for the idea with the compressor!