Determining pin layout of transistor once it's already on the PCB

[Ocoaibass]

I want to remove the transistors in my Big Muff to swap out with BC184C trannies. I know the pin layout on my new trannies is EBC, but I can't figure out by looking at it what the layout on the board is. Any help on this?

[R.G.]

This is a GREAT place to start learning how to relate schematics to PCBs. The parts and connections on the PCB are the same as the schematic, just that the parts on one are in different relative places than the other. The lines on the schematic and the copper on the PCB both represent the metal connctions between pins of the parts.

Take the schematic. Find a transistor. See what parts it connects to. Then refer to the schematic. The connections will match when thought about in the right way. The schematic then tells you which hole is E, B, and C.

[Perrow]

My first, second and third advice is to listen to R.G.

A lazy way to solve this would be to identify the current transistor and search the interweb for information about it.

[Ocoaibass]

This is a GREAT place to start learning how to relate schematics to PCBs. The parts and connections on the PCB are the same as the schematic, just that the parts on one are in different relative places than the other. The lines on the schematic and the copper on the PCB both represent the metal connctions between pins of the parts.

Take the schematic. Find a transistor. See what parts it connects to. Then refer to the schematic. The connections will match when thought about in the right way. The schematic then tells you which hole is E, B, and C.

Ok, cool. I definitely want to learn to do this stuff without running for help every other day. So will you tell me this? How do I know which direction the flow runs, relative to the emittor, base, & collector? If the signal is coming from part x & hitting the first pin, does that mean its hitting the collector, & that the last pin is the emittor?
To ask another way, can the pin layout be determined by the run of the signal, if I can figure out the
direction of signal flow from the physical board?

[Ocoaibass]

My first, second and third advice is to listen to R.G.

A lazy way to solve this would be to identify the current transistor and search the interweb for information about it.

I agree that it will be better to learn this for future reference.

As for the lazy way, I can't identify the manufacturer of the transistor. It is a BC550 from a USA Big Muff, but who knows if its a Fairchild's or any number of parts manufacturers'.

[R.G.]

A lazy way to solve this would be to identify the current transistor and search the interweb for information about it.

I agree that it will be better to learn this for future reference.

As for the lazy way, I can't identify the manufacturer of the transistor. It is a BC550 from a USA Big Muff, but who knows if its a Fairchild's or any number of parts manufacturers'.

Actually, those are two different skills, and both of them are useful. It is true that some parts have the same number, but different pinouts from different manufacturers.

However, for this exercise, you're in luck - as far as I know, all "BC550" parts from different makers have the same pinout. So a web search for "BC550 datasheet" should work fine.

So will you tell me this? How do I know which direction the flow runs, relative to the emittor, base, & collector? If the signal is coming from part x & hitting the first pin, does that mean its hitting the collector, & that the last pin is the emittor?
To ask another way, can the pin layout be determined by the run of the signal, if I can figure out the
direction of signal flow from the physical board?

It can be determined that way, but it's a lot more complicated. The complication is that in all normal transistor circuits, the input and output are two of the three pins, and the third pin is a reference pin, not part of the signal flow. But input can be either base or emitter, and output can be collector or emitter, depending on the circuit. In fact, in some circuits BOTH the base and emitter can be used as inputs. It gets confusing until you have a lot more circuit theory under your belt.

For learning PCB vs schematic, I'd suggest following the ground and power supply wires. Ground will be marked on every schematic. From there, you can trace it to certain parts. The easy way on a PCB is to use your multimeter set to measure ohms. Clip one wire to a known ground spot, like the bushing of the input or output jacks, then use the other wire to probe places you think are connected to ground. It'll show you when you hit one that really is by indicating zero ohms. Same for power supply. Any two points which ought to be connected together will show 0 ohms on this kind of test if they're really connected. That plus some eagle-eyeing of the parts markings will get you along way. It's a PITA at first, just like learning to read and write was, but with time and practice, it becomes almost instantaneous, like reading and writing, and for the same reasons.

[Ocoaibass]

Sincere thanks, all around. I fear a cold solder joint on a replaced cap is still the issue with my "in progress" Big Muff project, but now I can mount these trannies correctly & focus on the other work I did that may or may not be the reason I'm getting no sound...

[PRR]

> How do I know which direction the flow runs

You can't see current.

Also there are two ways to account current flow (electon and conventional).

I understood R.G. to be saying: if the schematic shows E connected to R13, then find a wire from R13 to transistor, that's probably E. Likewise C47 might be coming off C, look for the copper-trail from C47 to transistor.

If you don't know which R or C is which, go by value. If you know C47 is 1uFd, find all the 1uFd caps. On anything old-school pedal-size there will be only a few, maybe just one. Likewise if R13 is 47K, look for the Yel-Vio-Org resistors.

Geek tip: Base and Emitter are 0.6V apart when transistor is happy. If you know the transistor is NPN, the Emitter will be most negative. Collector may be any voltage down to Emitter (nearly) up to 50V higher; though in 9V work it is more often 1V to 7V higher than Emitter.

[Perrow]

PRR that's the kind of geek tips I like. Obvious when you read it, but hadn't ever thought about it.

Thanks

[Ocoaibass]

Nice. I'll update when I do some work next week.