Troubleshooting Tone Bender MK-II

[mrsage]

No real progress...

The only resistors I had laying around were 4k7, so I tried one of those:

Q1
C: 6.68
B: 0.06
E: 0.00

Q2
C: 1.43
B: 0.13
E: 0.00

Q3
C: 1.29
B: 1.43
E: 1.24


And I double-checked the LED...I definitely flipped it and wired it as shown in the positive ground diagram.

[Fret Wire]

Time to check for cold solder joints, bridged (touching) traces, and correct component values. Whip out the magnifying glass and double check the traces on the board to make sure none are touching.

You already rewired for pos ground. Did you thoroughly check the above? That also includes polarity of caps too. Assuming the LED is good, there's also a wiring problem. At this point, it's best to remove all off board wiring from the pcb. Just hook up the battery only, and check voltages. Then you can be sure if the problem is on the board, or the off board wiring and components.

[mrsage]

Yeah, I checked the traces, and it doesn't look like anything's out of the ordinary. Caps are pointing in the right direction and all component values are correct...

Okay...I unhooked the board and wired a battery with the negative going as shown on the diagram and the positive going to ground (I assume that's the way to do it?)

Q1
C: 6.94
B: 0.05
E: 0.00

Q2
C: 1.75
B: 0.15
E: 0.00

Q3
C: 7.07 (with original 8k2 resistor reinserted)
B: 1.75
E: 1.73

[Fret Wire]

what's your battery voltage?

[mrsage]

Same battery... 7.4 V

I'm getting some fresh ones tomorrow...Should I re-test and report then?

[Fret Wire]

Yes, it will be more accurate with a fresh battery. Also, now you need to go higher on the 8k2 (R6). Believe it or not, it is coming together.

[RDV]

Those are some strange readings even with a low battery like that. What value do you have at R5?

RDV

[mrsage]

Those are some strange readings even with a low battery like that. What value do you have at R5?

RDV

1K

[mrsage]

Okay...fresh Battery: 9.39 V

Q1
C: 9.07
B: 0.05
E: 0.00

Q2
C: 2.23
B: 0. 16
E: 0.00

Q3
C: 9.11
B: 2.23
E: 2.19

[Fret Wire]

What value is in R6 right now?

[mrsage]

8k2

[Fret Wire]

Q3's collector needs to be at half your supply voltage (about 4.7 with your battery), so try around 10k and check all the voltages again.

[B9 Robot]

I recently bought the mkll transistor set from Small Bear and the board from GGG. I did test the board with my meter and noticed that two of the traces were bridged by the copper around one of the mounting pilot holes. This was no big deal since lightly scraping some of the copper around the mounting hole with a razor blade quickly rectified the problem. The sheet that came with my transistors listed the same (stock) resistor values as well. My transistors have the following gains: 65, 81, 130. What were yours like? I am seriously thinking about following the great advice given on this forum and replacing the 8K2 resistor with a 20K trimpot just to be sure I will be able to bias it correctly when I finish building it. This is a newbie question but I was wondering what type of trimpot should I use for this board and where can I order one? I am about ready to place an order with Digikey and would like to order the correct part from them if possible. All of the trimpots I have found appear to have three legs and will not fit my board in place of the 8K2 resistor. Can anybody give me any suggestions, part numbers, etc? I will keep you posted on my progress. I appreciate all of the good advice.

Thank you,

Todd

[Fret Wire]

You probably won't find a trimmer to fit without drilling the pcb. What you can do, however, is wire up a 25k pot in place of the resistor. In other words, a pot or trimmer with long wires running to the resistor pads. When you are done setting the bias, unsolder the pot/trimmer without disturbing the pot, and then measure it with a DMM. Then put the closest value resistor in, and you're all set. This way you can use any pot or trimmer laying around to set the bias.

[B9 Robot]

Thanks Fret Wire. I too was considering that option but I just thought the trim pot would be more convenient. That fact that the trim will not fit changes my options   . I was under the impression that the resistor values supplied with the transistors were tested in an actual circuit. After hearing the trouble that Mr Sage is going through with tuning his, I thought I better bias it myself instead of sticking in the values provided.

Thank you,

Todd

[Fret Wire]

Those values he gave you may just work fine. Probably will. The other ckt isn't done yet. If it doesnt respond to a bias change, then it's a component or solder problem.
Fuzz type ckt's are harder to debug, because in addition to all the normal problems a ckt can have, it can add more problems with gain, matching, and leakage problems. Which can mask other problems, and make it a mess to debug.

[mrsage]

Okay...Battery: 9.38 V

R6 = 10k

Q1
C:  9.04
B: 0.05
E: 0.00

Q2
C: 2.19
B: 0.16
E: 0.00

Q3
C: 9.07
B: 2.18
E: 2.16

Something else has to be going on...The voltage barely changed...And it was nearly the same when I still had that 4k7 in there, too.

Should I reconnect the rest of the wires? Would that make a difference? I'm still testing with just the battery hooked up...

[Fret Wire]

Something more than biasing. You did hook up the battery correct for pos ground right? Time to check the traces and components again. Do you have a strong magnifying glass? You need to see if there is the slightest copper or solder bridging any traces. Plus you need to check the traces to make sure none have a small break in them. You need a mag glass for this, you eye alone won't see it.

[mrsage]

I can't see anything wrong with it. I checked the traces with a magnifying glass, made sure it was all wired properly, double-checked component values.

It's just not working. Not sure what else to try...

[RDV]

Uh...when you turned this all back to positive ground, did you turn the electros back around?

I know it doesn't seem like it right now, but if the PCB was right, and the layout was right, and the parts were placed correctly, then it would be working right now.

I've certainly been through this sort of thing....to the point where I had to put the circuit in a plastic bag, throw it in a drawer and come back to it again some other fine day.

Those voltages tell a tale of unrestrained DC all through that circuit, which is why I asked about the electrolytics.

Good Hunting

RDV