Mystery transistor

[John Lyons]

This is a C236AB transistor.  "D"



I would think it's a FET doing some switching function but
I can't find any datasheet or specs on it.
I can find reference to it but no data.

I assume it's a generic part. What can I use in it's place??

Thanks

John

[R.G.]

I can usually guess at these things, but there are just enough ambiguities in that to keep any of the guesses from fitting.

Yes, I agree that it seems like it would fit with a JFET used for signal routing. However, the schematic fragment shows the ?signal? presumably coming to the device through that 1uF cap and going  to the case of the device?!? If it's switching that signal, it should go to one of the three or four pins. I say "or four" because some vintage MOSFETs did have four pins, the extra pin being a connection to the case or a second gate. Some devices do use the case as a contact, but they're all metal can devices.  So where does that 1uF really go?

The diode/resistor/cap/switch is what makes this seem like a JFET switch, but that is missing a pullup resistor to a higher voltage (V+?) that would make it be a decent gate switching signal.

1M and 47K to the ?? emitter ?? of C might be pullups to a bias voltage if what connects to the base of C (if that's a base) sets up a bias voltage. Maybe.

The "C236ab" might be a "2SC236", which did exist at one time, but it's unlikely because that's a very old part number. Could be a "BC236", as they're sometimes abbreviated by dropping the "b" as well, but I can't find any BC236 datasheets.

I have noticed that when you put these circuit fragments here for help, you tend to put too little of them here to guess, and the next question tends to be "what's the rest of that circuit?" So, in line with tradition - John, what's the rest of this? Then maybe we can help.

[John Lyons]

Ok RG, you asked for it.  



The schematic is a mess but I just drew it as I traced it.
The Transistor says C236AB so that's all I have to go on.
I do have the transistor here though if testing it will reveal anything.
EDIT: Just realized that the battery + Leg of the switch was left off the
first schematic posted. Sorry.

The A,B,C...points go to BJT gyrators. The circuit is a semi fixed
graphic EQ.

Thanks

John

[R.G.]

Yep, that helped. It's a JFET. Don't know what the "C236" means, maybe house number, maybe a device I can't find.

The pullup to battery on the switch makes that certain. JFET signal switches are about the only thing that can use that. Well, MOSFETs maybe, but I've never seen them used that way. They would not need the series diode.

What the thing does, from the input, amplify a bit in device B, then follow that with C to unload the collector of B. C's output is a low impedance signal point, which goes through the 47K and 1M resistors to the mystery device and to the input of the second opamp. The second opamp is misdrawn; pin 7 is likely an output and pin 6 an inverting input. Then the signal goes to the first opamp. The first opamp's output goes through the 1uF cap to the mystery transistor. The mystery transistor is getting the emitter follower output on its drain and source both, both of which are DC blocked by capacitors from anything else. So they sit at the same DC voltage as the emitter of C. When the switch is in a position to let the JFET be open, then the signal from the opamp is blocked by many megohms in the JFET and the signal through the 47K, as divided by the 47K, 27K?, and 62K appears on the blue/out. When the JFET is on, the few-ohms impedance of the opamp output easily swamps the signal through the 47K, and substantially only the opamp output goes out.


It's likely an N-channel JFET. You could tell by using an ohmmeter to check the gate for diode forward/backward to the channel. Does the 1uF go to the center pin as it seems like it might from the drawing?

[John Lyons]

Thanks for the insight RG! I appreciate it.
The 1uf does connect to the center pin.

Bah! You're right, 6/7 on op amp 2 are reversed
I need to fix that, 

I'll pull the transistor and check the forward/backward readings.

[PRR]

Most Japanese devices, you want to put "2S" in front of what's printed.

I found a 2SC236, just one line in a short-form listing which said nothing useful (not even which polarity). I also got a very odd IE "error", which I suspected was bogus.

Yeah, the "EQ" makes no sense until you flip an in and out.

[R.G.]

Bah! You're right, 6/7 on op amp 2 are reversed
I need to fix that, 

No biggie -- you should see some schematics I've drawn. ACK!!

[John Lyons]

Ok, popped out the transistor and this is what I get.



+ = red probe
- = black probe
Diode fuction om DMM

[R.G.]

You need to take six measurements to define it - each pair of pins, each way.

A JFET will show resistance both ways on the channel pins (drain and source) and a diode (open vs diode forward voltage) from the gate to both the drain and source.

A bipolar will show no conductance either way from C to E, and a diode from base to both of the C and E.

A MOSFET will show no conduction from gate to either D or S, but a diode from S to D.

[John Lyons]

Ok, I think it's a Jfet. I get resistances and diode voltages.
Thanks again RG!

[R.G.]

Ok, I think it's a Jfet. I get resistances and diode voltages.
Thanks again RG!

You're welcome.

You can tell N-channel from P-channel by which way the gate conducts. If the gate conducts to the channel when it's positive, then it's an N-channel JFET. If it conducts to the channel when it's negative, it's a P-channel. But you'd already figured that out.

[waltk]

I found this really simple method of determining the pins of an unknown transistor somewhere on the internet (don't remember the source), and it always works for me...

Answer: First you have to find the Base, with your millimeter set to diodes play around with the probes between the three pins until you get a reading between one pin to both the other two pins that pin will be your Base, if your red probe is on that pin then it is a NPN transistor, if it is the black probe then it is a PNP, there will be a slight difference in the reading, the pin with the smallest reading will be the Collector and the larger is the Emitter

If it's a JFET, I guess you can just substitute "Drain, Source, and Gate" for "Collector, Emitter, and Base", although you would expect to have some conduction between the drain and source with a smaller voltage drop than Drain/Gate and Source/Gate.

If this is way off base (no pun intended), I'm sure somebody will correct me.

[zack-w]

I know this is a fairly old post but I thought I might be of some help to others. I've been looking for the same part. I'm repairing an Ampeg bass amp with a couple in it (one of them is definitely bad) and I managed to get the schematic for it.

The transistor in question has a couple strange symbols and the markings sort of look like this:

$$J
175
C236AB

It turns out this device is a J175 P-channel JFET switch. Digi-Key has them for about 35 cents apiece.

Zack