"71" Coliseum Bass amp... Need trasistor pinout for 2n4304 small signal FET

[1ampman]

It's the first input transistor. It's round with a black dome top with white band around it with numbers that don't seem to make sense. 3 pins with a Teledyne insignia and NO indexing marks. I can't find any data sheets with the pinout? It was mounded on socket that was broken. I was gonna solder it directly on the board? Any comments are welcome.

[Jazz P Bass]

About all I could find.
Link:2N4304 Semiconductor (Transistor, diode, IC) Cross reference

[1ampman]

This transistor case is completely round and doesn't flat side. That's the problem no indexing indicator.... It looks to be an original transistor. The pins do have a triangler patteren (only 3 pins) maybe that will help?

[Jazz P Bass]

How to test FET's-Jfet and Mosfet Electronic Circuits-Circuit Diagram-Electronics Projects-Design
Meter check of a transistor : JUNCTION FIELD-EFFECT TRANSISTORS

[1ampman]

Thx for the info. It appears the transistor is bad. I also fed a signal from my signal generator traced it right to the transistor but nothing is being passed. The voltage is a little high on one side (instead of 23vdc, it's 30). But the transistor has proper voltage. The schematic is at www. richbriere.com/the_sunn_shack.htm. It's the schematic of the month. Any subsistutes?

[Jazz P Bass]

Are you sure the JFet is installed correctly?
The drain to source should be normally on.
The gate will "pinch" it off.
Jumpering D to S should get you going, temporarily.
I do not know the function of this device in the circuit.
It appears to be an input limiter.
Try a 2N3819 as a replacement.

[1ampman]

It's a FET (3 pins) not a 4 pin J-FET. I'll look for a replacement. I appreciate your help....

[Jazz P Bass]

I think any 30 voltJfet will work.
Against my best instincts, the 2N4304 crosses to a NTE133.
Link:NTE ELECTRONICS|NTE133|N CHANNEL JFET, -25V, TO-106 | Newark.com

Added: Fairchild crosses the 2N4304 to a PN4303
Link:PN4303 Fairchild Semiconductor JFET

[1ampman]

I'm really green at knowing what to sub and your help is great. I did see a reply from Enzo that he said, like you, that several subs would work. He was recommending a Motorala 2n5458. I have to put in an order to Mouser this week. What would you choose?? Not to put the pressure on you... But this in not my in data base (brain). I better test the other transistors before I order, eh? You up for this project??

[Jazz P Bass]

I would go with Enzo on this one.
2N5458.
Link:FindChips.com Online Electronic Component Distributor Inventory Search
This site will find the part for you.

[loudthud]

I just pulled a JFET out of a Concert Bass I have open. Looking at the bottom of the part where the leads come out, position the part so the two leads that are farthest apart are at the top and bottom and the third lead is to your left. The Gate is the top lead and the other two are Drain and Source. You can interchange the Drain and Source, the part will work fine either way. Between the Drain and Source you should measure about 500 ohms and the Gate will give you a diode indication to either Drain or Source.

[1ampman]

Thx, Very well explained. Between the source and the drain I got 140 ohms with either lead polarity. Gate to source and drain was 10meg in 1 direction and open in the other... I tried 2 meters to see if any difference was had.... your conclusion?

[loudthud]

10meg in 1 direction and open in the other...

If that's what you normally see with a diode, then the part is good. Usually, a blown JFET will show a low reading on the Gate. The Drain to Source resistance can vary quite a bit so the 140 ohm reading is fine.

[gbono]

Try a PN4302 from Fairchild - I curved traced a few original "glob -top" parts and these matched nicely. The 2N5457 also worked too.

[1ampman]

The transistors are OK with the basic mulitmeter checking. So I'm looking at my 23 volt supply/dropping resistor. But CR101 & CR102 little glass diodes with NO markings on them could be the problem. Anybody know what they are and what would sub for them??? I get 32v at the correct point but the 23v is 32v (not dropping down).... This is all on the first transistor.... ???

[loudthud]

The little diodes near the input JFET are for input voltage protection. Sunn specifies 1N458. Any signal diode like 1N914 or 1N4152 should work fine. The input stage is an N channel JFET driving a PNP 2N4250 transistor with feedback from the collector of the PNP back to the source of the JFET. The collector of the PNP should be about +1 to +2 volts if everything is working. Since it seems like almost no current is being drawn, I would guess that no collector current is flowing and the collector voltage is -15V.

[1ampman]

Hey good news! I replaced the 2 small signal diodes and resolder the fet back in and the signal is going thru thanks to your help.... Now some where on the preamp board the lower half of the sign wave is being cut off when i turn up the volume. The circuit board is not marked at all. So I'll have to slowly track down the circuit. The volume control is dual which is different from what I would think. This is great. I'll be posting my problems/progress every day or so.... thx again!

[1ampman]

It's done! A bad cap, the usual pre-out/power amp in jack had dirty contacts, and that was it. It lives! Thx to everyone for the help.