2n2646 in hummingbird clone

[cnspedalbuilder]

Hello, I just got together the earthquaker hummingbird circuit on vero (layout from tagboard effects page: http://tagboardeffects.blogspot.com/2013/04/earthquaker-devices-hummingbird.html). When I use hooked it to jacks and power (no enclosure or switch) I get a clean tone, no effect. For now I'm not writing to ask for help just yet, but I am writing to ask about the 2n2646 transistor. When I hook it up to my fleabay transistor tester, it comes up as two diodes (1.06 and 1.01v), not a transistor. The tester had no problems recognizing the other two transistors (5089 & 5457). Do you think the 2n2646 is dead or is that how the transistor should read? I believe it is a silicon unijunction transistor if that helps.

[TejfolvonDanone]

That's because unijunction transistors aren't BJT. If you measure one diode from emitter to each base it's working.

[whoisalhedges]

That's because unijunction transistors aren't BJT. If you measure one diode from emitter to each base it's working.

Dumb(?) newbie question: I'm not sure how unijunction transistors work. Can you tie two diodes (with appropriately-measured forward voltages) together at the anodes and drop that in? I have a couple 2N2646s (for Vox Repeat Percussion, natch) on hand, but can get 1N914/1N4148 for pennies per unit, at Radio Shack.

[TejfolvonDanone]

If you measure the forward volteges of a regular BJT you get about 0.6V for base to collector and 0.6V for base to emitter. But you can't replace a BJT with 2 diodes because the internal structure of the transistor is made to be able to amplify while two diodes just can't. Same works with UJT-s. Little information about these UJTs:
http://www.allaboutcircuits.com/textbook/semiconductors/chpt-7/unijunction-transistor-ujt/
https://en.wikipedia.org/wiki/Unijunction_transistor
I think you can find the same relaxation oscillator in the first link as you are trying to build.

[whoisalhedges]

I'm not the OP, not trying to build anything with my UJTs just yet... thanks for the links, I pretty much know how BJTs work (better than I understand any other active components, at least) but still have a lot of questions about other transistors. Will read those links.

[TejfolvonDanone]

I'm terribly sorry for not reading the names.
UJTs aren't like BJTs or FETs in the sense that they aren't suitable for amplification. I (with my limited experience) havn't seen any other use for them besides relaxation oscillators.

[PRR]

If you could replace a (BJT -or- UJT) with two diodes, why would they make transistors?

The transistor action happens when the junctions are "intimate", closely coupled, interacting at the crystal level.

A UJT is a totally different kind of transistor than a BJT. In the most common uses, a UJT conducts no current up to a certain voltage, then conducts like crazy and pulls-down to zero volts. You can do stuff like this with BJT, but you need two of them. In theory a UJT is a cheaper way to get this function. In practice the high production of BJTs drove the price down below low-production UJTs, and UJT circuits largely faded except in DIY work where the possible extreme simplicity outweighs the half-buck cost difference.

I'm mildly surprised the PEAK does not read a UJT. But if it does not recognize the connection, yes "two diodes" is what it should say. The low-current voltage should be 0.6V; the 1V reading suggests the PEAK hits it hard trying to find some transistor or SCR action.

[cnspedalbuilder]

Thanks these are helpful explanations. Also, it is good to know that I don't need to replace the UJT, though that also means I now have some troubleshooting to do! 

I'm guessing that since I get a totally clean sound from the circuit, that I might have a short between the input and output wires. Another possibility--I did not hook up level pot and instead ran two of the three wires straight into the same breadbord line. I figured this would just give me maximum level, but could this have affected the output?

[theehman]

It's possible the UJT you have just won't work in that circuit.
When I was developing the RonSound Trem-O-Matic, I couldn't get the circuit to work either.  I checked over all my parts and connections numerous times.  Finally, in desperation, I removed the UJT from an original Vox Repeat Percussion and tried it in my circuit.  Imagine my surprise to find that it DID work.
I found that some UJTs just don't like the Vox circuit.  I ended up building a tester to try out a batch of 2N2646 I bought online and weeded out the ones that were either too slow at maximum Rate or wouldn't work at all.  I have a lot more of those than ones I would use.

[R.G.]

The UJT was so variable that it was replaced in industry practice by the Programmable Unijunction Transistor - that's right, a PUT. 

PUTs are much more consistent when you figure out how to set them up. When I did the PCBs for replacing the entire preamp in Thomas Vox Beatles, I put in alternate population setups so that either the original 2N2646 UJT or the 2N6027 PUT could be used.

PUTs are a little tricky to set up, but once you do that, they work.

The industry didn't like the process of buying many more than they needed and then throwing away a lot of rejects, either.

There's a lot of techie detail about why which one does or doesn't work, but having a different horse to ride cures a lot of problems.

[digi2t]

The UJT was so variable that it was replaced in industry practice by the Programmable Unijunction Transistor - that's right, a PUT. 

PUTs are much more consistent when you figure out how to set them up. When I did the PCBs for replacing the entire preamp in Thomas Vox Beatles, I put in alternate population setups so that either the original 2N2646 UJT or the 2N6027 PUT could be used.

PUTs are a little tricky to set up, but once you do that, they work.

The industry didn't like the process of buying many more than they needed and then throwing away a lot of rejects, either.

There's a lot of techie detail about why which one does or doesn't work, but having a different horse to ride cures a lot of problems.

This is taking me back to the Ludwig Phase II project.

[R.G.]

Yeah, I felt a little twitchy there too.   

[Keppy]

The first Ludwig clone didn't work until I discovered the bases on the UJT were reversed on the factory schematic. It's easy to think "a base is a base," but the pinout matters. You double-checked, right? Then tried it with the bases reversed, just in case?

[cnspedalbuilder]

Thank you this is great. Yes I have tried it with UJT (and the other trannies) both ways, but no luck. But there are other things to try. I realized that I found a short between the endpoints of the 39R resistor (and therefore between the ground line and the line that links up with B1. It's weird because the endpoints of the 39R show 39 ohms of resistance, but they also show continuity on the meter. Is that just because 39R is a very small resistance? Could that be a/the problem?

[TejfolvonDanone]

It's weird because the endpoints of the 39R show 39 ohms of resistance, but they also show continuity on the meter.

Meters have a threshold which tells you when will it show continuity and when it won't. For most meters it's around 50 Ohms.

[cnspedalbuilder]

@T.D. Got it, so it will show continuity even though there is technically no short b/w the two strips. In that case, I think there is one more problem to try to address.

@R.G.: Thank you for the advice about using the 2N6027 PUT . I saw that you had mentioned this in other posts, and in fact, I was thinking of building the Moosapotamus Skippy, which uses the PUT but I ended up going the UJT route. You mentioned that PUTs are a little tricky to set up. Does this mean it would be necessary to modify the circuit to replace the UJT w/a PUT?

[TejfolvonDanone]

Got it, so it will show continuity even though there is technically no short b/w the two strips. In that case, I think there is one more problem to try to address.

A soldering joint and piece of wire can range from few mOhms (it's m not M! SO it's 0.001 Ohm) to several tens of Ohms if the joint isn't good or the wire is corroded. I once had a speaker cable so corroded that it had 4 Ohms resistance to a 4 Ohm speaker. So you have to know what you are dealing with.

[PRR]

> it will show continuity even though there is technically no short

What is a "short"?

When biasing a tube, or a condenser mike, 1Meg even 1Gig is "near enough" to a "short" for the bias to get through.

My tractor starter-motor is 0.1 Ohms but is not a "short", real work happens (Diesel engine cranks) when I put a large battery on it.

As TD says, "continuity" is for very hasty checks on circuits with "simple" faults. Checking trailer-light wires. Usually the wire is less than 1 Ohm (good copper) or over 100K (broken wire with damp mud). A beeper lets you find a simple fault quick.

But we are Electronics Technicians. (Or have to act like one when things aint right.) We may use a beeper as a first step, when we still hope the fault is a connection/no-connection. But we know that we face very-low and very-high resistances which may be "normal".

In low-current electronics, under 100 Ohms is usually "pretty short". However you found a 39 Ohm which actually needs to be about that value. A "beeper" which beeps on <50 or <100 Ohms isn't telling what you need to know. The good beepers also read Ohms, look at the meter to see if "beep" really means an Ohm or less, or if it is false-beeping for 39 Ohms.

It may be useful to know what your beeper threshold is. A handful of 39, 47, 68, 100, 150 Ohm resistors will probably tell you. A 5K pot turned to the end will give variable <50 to >>100 Ohm tests.

[cnspedalbuilder]

Hi everyone, I fixed a short in the circuit, and tried it out again with all pots and mode switch hooked up. I am definitely getting sound through the transistors, but no effects. Actually, I pick up radio signals from some weird talk radio station, but otherwise it's just an effected guitar (this gets very loud if I put my finger on the 2646). The level pot doesn't seem to be having much effect.

I've got the transistors socketed for now, and will test them tonight to make sure that signal is passing through sockets.

In the meantime, I seem to remember reading that the 2646 does not work with sockets--could that be a problem?

Thanks, CR

[cnspedalbuilder]

Another question for y'all:
So I fixed some bugs (including a missing 27K resistor) and tested the sockets (good continuity b/w trans and socket pin), but still all I get is a clean sound. The level pot works, but no effect of depth and rate. Also, I know the 2n2646 is a prime suspect, but I found that the sound and tone remain even if I pull out the transistor altogether. Does that make sense in this circuit or does it mean I have a hidden short lurking somewhere?