What's Happening?

[rickosound]

... No, not the Byrds song.

I have a fuzz pedal called the Yardbox that is meant to emulate the Sola Sound Tone Bender Mk1. Normally, it has lots of volume boost (enough to where I can set the volume at 9:00 and get unity gain), and sounds good. But, lately, when I switch it on at my normal settings, there is no sound. When I max the volume, there is a little bit of sound, but it sounds really weak and muddy, almost like the battery is dying. This has been going on for a month or so. I tried a new battery, but it didn't affect the sound. Same guitars, amp, cords, place, everything.

Does anybody know what could be going wrong? Thanks for any responses.

Matthew

[beatlefreak]

Possibly a blown transistor.

[rickosound]

Possibly a blown transistor.

Thanks for the reply. How do you fix a blown transistor? Do you just have to swap it out for a new one?

Can anyone confirm that it is a blown transistor? How does this happen?

Matthew

[jingle_jangle]

Possibly a blown transistor.

Thanks for the reply. How do you fix a blown transistor?

Verrry carefully, and with tiny little tools!!!

Seriously, though, there will be a number on the transistor, which you can plug into google and get a source and/or cross-reference.

Remove the old and solder in the new, using a heat sink on each lead in turn.

[rickosound]

The names on the back of the transistors have been rubbed off, either because they are old transistors or because they want to conceal the circuit. Still, I was able to make out most of the letters on one and googled it. Apparently, they are silicon NTE181's. I thought it was a germanium fuzz, but I guess not. If anyone can direct me to a place to buy them, it would be much appreciated.

No one at our house is very handy, so I think I'll find someone who is comfortable working on the circuit. I can already think of a few people we know who can probably do it.

How do I tell which one is blown?

Thanks,
Matthew

[jingle_jangle]

The standard source for these would be Newark Electronics. How many of these are there in the circuit? They can be checked, but I usually just replace all of them on a board when I'm working on one of my keyboards, provided the number is less than a dozen or so.

I'm not an electronics guy; perhaps someone can chime in with a test procedure, or you can try googling it.

[johnallg]

Matthew, what is the number you got off the transistor?

[rickosound]

Matthew, what is the number you got off the transistor?

NTE181, and there are three of them in the circuit.

[beatlefreak]

A small signal transistor can be generally checked with an multimeter set either to ohms, or the diode checker (if available). The transistor needs to be out of circuit to do this. There are three leads on the transistor - They are Base, Emitter and Collector. If the transistor is a metal can type, the emitter is designated by the tab, the base is offset next to it, and the collector is furthest away from it. If it's a plastic package, the base ususally the middle lead, but it's best to look up a spec sheet on the part to be sure.

NOTES: Be sure to determine how the transistor is positioned in the circuit. It's important to put it back (or install a new one) the same way. Don't mix up the leads. Paul's advice about using a heat sink on the leads when you solder a small signal transistor into a circuit (or remove one from a circuit) is wise. Heat is usually the culprit that kills transistors.

With the part out of circuit, measure the resistance from the base to either of the other two leads. It should measure a relatively low resistance (a few thousand ohms) in one direction, and a high reisistance (100k ohms or more) when you reverse the multimeter leads. The same holds true for measuring the base to the other transistor lead. When you measure from collector to emitter, there usually is a moderate to high reisistance in both directions. If you find a short between any two leads, the transistor is bad, although you might find what looks like an open on a good transistor.

If the multimeter has a diode checker, even better. The base-emitter junction will drop ~0.7V in one direction, and will not drop voltage in the other direction for a silicon transistor (~0.3V for a germanium transistor). The base-collector junction will usually exhibit the same voltage drops as the base-emitter. The collector-emitter junction usually drops ~1.4V either in one or both directions.

[jingle_jangle]

Thanks, Kris. I've copied this and it'll go into my shop notebook!

[whojamfan]

Thanks for that post Kris, it's nice to read something that doesn't contain 8 million mathmatical calculations to fix a problem.

[beatlefreak]

Keep in mind that these are general checks, and small signal transistors can vary greatly, but no SS transistor should show a short (or even just a few ohms of resistance) between any two of it's leads.

Transistors can be circuit configured in any of three different ways: Common emitter, common collector, and common base. By far, the most prevalent configuration is common emitter (Base-emitter input, collector-emitter output). The emitter is usually connected to ground, although it may do so through a resistor. There are inline resistors on the base (from the input or previous stage), and on the collector (usually from the power supply). These are there to limit the current going through the transistor and to bias it. There may also be a resistor from the base to ground to set a low input impedance to the stage. capacitors in the stage are there to remove DC supply voltages from the signal path, and to add stability to the stage.

A diode checker on a multimeter (many of them have this feature) is a better way to check transistor junctions, as it measures the voltage drop (safely) across any two leads. Transistors are two types: NPN and PNP. NPN types will drop ~0.7 V (Ususally between 0.6 and 0.8 V) between the base and emitter on a silicon transistor when the positive lead of the multimeter is on the base, but not drop voltage when the negative lead is on the base. The Reverse is true for PNP types. Germanium transistors will drop ~0.3 V (usually between 0.2 and 0.4 V).

[FretlessOnly]

Thanks, Kris. I've copied this and it'll go into my shop notebook!

That made me smile. We can't be experts at everything. Unfortunately, I'm not an expert at anything being discussed here. I don't think I'd even qualify as a novice. Vive le difference indeed.

Nice read even if I didn't quite lock in on the nuts and bolts. Good to know we've got many great minds to draw upon here.

[whojamfan]

It is indeed awesome to have such knowledgeable people here on this forum that are happy to share it with us!

[beatlefreak]

Paul W. has shared plenty with the rest of us, I'm just glad I can return a little bit.