how to find value of an unmarked transistor?

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dustbro

Well-known member
Joined
Feb 1, 2006
Messages
665
Location
New York - USA
Hey guys,
I need some help trying to find the value of some unmarked transistors on an old RM57. Unfortunately no schematcs exist for this compressor, which would have made my life a bit easier. What do I need to test these values? And when I get them, how do you figure out what model transistor to replace them with?
thanks for all.
 
try to draw a circuit
establish NPN or PNP
if they still work you may want to risk taking one out to put in into a test circuit ... even if it just a standard transistor test found on DVMs .. HFE

a good schematic will go a long way to help find a sub ... typical modern transistor substitutes can be suprising good in non-critical situations

having said that, people do sift through many transistors to find just the right one to put in there TS9 / 808 stomp box.
 
I need a little help establishing NPN or PNP. I fould this from another site on testing transistors with the Diode setting on a multimeter...
Test each pair of leads both ways (six tests in total):
The base-emitter (BE) junction should behave like a diode and conduct one way only.
The base-collector (BC) junction should behave like a diode and conduct one way only.
The collector-emitter (CE) should not conduct either way.


So I did the test and came out with the following results.
transistor-test.jpg


What exactly is this telling me?
 
Thanks Dave... the problem is I'm trying to reverse engineer a piece of gear that doesn't have an easily found schematic. So if the transistor is messed up, then I'm pretty much screwed.
 
although I must say that the compressor is functioning fine, so the transistor must be working.... maybe I messed something up in the test?
I tested all 14 transistors in this piece of gear and get similar results.... 3 give nothing, and 3 give results. only 2 of them give me 2 open and 4 values.
 
[quote author="NewYorkDave"]You're testing the transistors out-of-circuit, right?[/quote]
Well, of course I am... what do you think, I'm stup..... wait a sec. I have to take it out to test it? ;) hmm. didnt say that anywhere on the site I was referencing, so that must be were the flaw is.
 
do try to draw the circuit
take it slow and just re-create the pcb on paper
start with the components around the transistor in question

I bet the guys here will work things out even if you do get a couple of tracks or signal paths wrong
 
Thats an NPN.

Set up a 10K bias pot from base to neg millamp meter terminal.
Pos millamp meter to plus 9 volt battery.
Then put a 10K from collector (pin 3 to your right if looking at flat side) to + 9.

Put a milliamp meter in between the emitter (pin 1) and ground (minus batt term).

Draw a chart of base current vs collector current.

Then you can match it with the charts seen in data sheets.

Over your head?
Then use a DMM cheapo beta test.
 
I'm definitely in over my head... but that's why I"m here to begin with. gotta learn some how.
thanks for the suggestions guys! I'm going to try the test and see what I come up with. I'll also draw a quick schematic to see if you guys can figure it out from that.
you guys rule!
 
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