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 Post subject: poof there goes a rare transistor
PostPosted: Jan Sun 23, 2011 1:30 pm 
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Joined: Nov Thu 22, 2007 11:31 pm
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Location: Johnston, Iowa
All,
With help from this forum I've spent the last couple of years restoring test equipment so I could work on my vintage automobile test equipment. Last night I started working on a 1965 Sun tach and Dwell unit. I broke the meter on this particular piece of test equipment a year ago and I recently found a replacement meter. I installed the new meter hooked the tach up to one of my tach simulators and I started the re-calibration procedure. After adjusting all the pots I got this thing almost perfect but the readings were off slightly when switching between the rpm ranges. This can be corrected by padding one of the capacitors. So I hooked up my newly acquired vintage Cornell-Doubler Decade capacitor box up to the calibration cap and started fine tuning the tach. I got a bit carried away because I wanted to see the range of adjustment I could get with my Decade box. All of a sudden right before my eyes I see a puff of black smoke come out of the only transistor in this machine. The problem is I can't seem to find a cross reference number or any data for this transistor. The transisitor is labeled N372A 7136 I believe it was made by Texas Instruments. Some of my other Sun equipment used Military spec transistors which may be why I can't find this part number. Here a picture of the transistor. Can anyone out there help me find a replacement?
Thanks in advance,
KeithImage


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 Post subject:
PostPosted: Jan Sun 23, 2011 3:22 pm 
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Location: Albuquerque, NM 87123
If it supposed to be a 2N372 it crosses to an NTE126.


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 Post subject:
PostPosted: Jan Sun 23, 2011 3:46 pm 
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On the part and on the parts list it's listed as a N372A, could that be the same as a 2N372?
thanks,
Keith


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 Post subject:
PostPosted: Jan Sun 23, 2011 3:58 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
It's possible; the NTE126 comes in that case style.


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 Post subject:
PostPosted: Jan Sun 23, 2011 4:10 pm 
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Joined: Oct Fri 16, 2009 11:10 pm
Posts: 440
Location: Bergen The Netherlands
Hi,

2N372, made in week 36 of year 1972

Manual GE of 1964 :
Germanium PNP . Pmax=80mWatt . Imax=20mAmp
hfe=60 . Vcemax=24Volt


RCA manual 1967:
GE p-n-p alloy-junction drift-field type for use as an rf mixer in AM
and shortwave receivers, indentical to 2N370 (in europa: AF185)


Jard N.


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 Post subject:
PostPosted: Jan Sun 23, 2011 4:32 pm 
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Location: Long Island
Sounds like the "magic smoke" that makes semiconductors work got out! 2N372 is a PNP germanium transistor classified for RF or IF applications, but it could certainly work in something like a dwell tach.

I checked several old ECG and GE replacement semiconductor cross reference books; the numbers skip right over N372, and 7316 corresponds to a much larger audio output transistor. Therefore, if it is not a 2N372A, it may be a proprietary Sun house number.

Do you have a schematic? It would be helpful to know if a PNP transistor is indeed used. If no diagram is available, a little detective work is in order.

If you follow the emitter lead of the transistor (the one next to the little tab sticking out the side), it will probably connect to a resistor or two, and as you follow the path, you will eventually get to the positive battery terminal if the transistor is PNP. If it is NPN, the emitter will eventually find its way to the negative side of the power supply.

Should it turn out that the transistor is apparently PNP, you might as well give the NTE 126 a try.

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 Post subject:
PostPosted: Jan Sun 23, 2011 5:13 pm 
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Are there 4 leads on the transistor (the 2N372 had 4 leads emitter, base, collector and case) so that it could be used in shielded RF applications. If so a replacement for the 2N372 could be considered. I don't think that a replacement transistor in a dwell meter would be critical.


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PostPosted: Jan Sun 23, 2011 6:20 pm 
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Location: Southern NH, 03076
The 7316 certainly could be a date code as it matches the era of the other components shown.

Id start with looking for a close match to a 2N372A by scanning the NTE list before paying a likely high price for a real NTE. Also check the Japanese 2SC numbers.
The specs listed above look very generic and Ive even used a silicon device in many low and even moderately high voltage circuits with no other component change.

Carl


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 Post subject:
PostPosted: Jan Sun 23, 2011 6:37 pm 
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Location: Johnston, Iowa
All, Unfortunately the emitter connects to ground and the schematic indicates that it is a NPN. This transistor also has 3 legs. It has what looks like the outline of Texas printed on the transistor so I'm thinking Texas Instruments made this thing and would they put different marking on this if it was made specifically for SUN? What should I do next, if a post the entire schematic would someone be able to recommend a replacement?
Thanks,
Keith


Image

Image


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 Post subject:
PostPosted: Jan Sun 23, 2011 7:52 pm 
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Location: Mt. Airy, Maryland
It's not in the least critical. Try any low-power NPN silicon transistor you have lying around. All it appears to be doing is squaring off the input pulse.


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 Post subject:
PostPosted: Jan Sun 23, 2011 8:30 pm 
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Location: Monterey California USA
TI probably made more house-marked transistors than they did generic-marked, which is what they did here. A poor choice of numbering because it would be easy to conclude that there should be a "2" before the "N."

Most of my transistorized equipment is full of house marked parts, the bane of the repairman. Most of the time the only purpose is to make you buy replacement parts from the manufacturer or send it to them for repair.

As JN has pointed out, this application looks so non-critical that almost anything would work.

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 Post subject:
PostPosted: Jan Sun 23, 2011 10:03 pm 
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Location: Long Island
Actually, you are lucky you do not need a 2N372. That was a drift field transistor, an early kind of high frequency device that went into the first all solid state auto radios of the 1960s, and other products. They have not been made in decades. The NTE 126 might have similar enough characteristics to be recommended as a substitute, but there's a good chance it doesn't work out in all cases.

If you look at the circuit, they are bringing the pulses from the points of the ignition system in through a RC filter section with a protection diode, then applying it to the base of the transistor and the 1-k bias resistor. The load resistor on the transistor appears to be 680 ohms. The transistor should probably have a minimum hFE of at least 20 for the circuit to work; more than that would be icing on the cake. Since nearly all small signal transistors have much higher hFE's than this, it follows that you could use practically any small NPN transistor you have handy, as others have said. If you don't have any spare transistors kicking around, NTE 123A is perhaps one of the most common and widely available small signal NPN replacement transistors.

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 Post subject:
PostPosted: Jan Sun 23, 2011 11:21 pm 
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Joined: Nov Thu 22, 2007 11:31 pm
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Location: Johnston, Iowa
Thanks everyone, I was worried when my transistor numbers didn't match any of the cross reference tables. Unfortunately, I'm new to this hobby and have working almost exclusively with tube gear. I did have a single new NPN transistor, a 2N3053 in my possesion. I put this in and everything works but the calibration was way off from the old transistor. After re-adusting the pots it looks like this one will work. If this transitor is a bad choice or if someone can recommend a better replacement please let me know.
Thanks again, as usual this forum is a life saver.
Keith


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 Post subject:
PostPosted: Jan Mon 24, 2011 1:28 am 
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A 2n3053 will work, but a switching transistor such as a 2n2222 or 2n2222a would work better.


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 Post subject:
PostPosted: Jan Mon 24, 2011 2:50 am 
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Location: Albuquerque, NM 87123
I think it is best to determine if the original transistor is germanium or silicon, and use only that type.


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 Post subject:
PostPosted: Jan Mon 24, 2011 2:51 am 
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Location: Long Island
2N3055 is a big NPN power transistor that was and still is used in power supplies, audio amplifiers, and other equipment. Its hFE (small signal forward current gain) is only around 20 to 40. That's why it worked, but the calibration was off. The original transistor almost certainly had more gain.

The good news is, you now know nothing else is wrong with the tach.

If the 2N3055 will fit in the case and it works to your satisfaction, there's no reason you couldn't use it. But the 2N2222 and NTE 123A (they're equivalent) have a hFE of around 150 and they're the same size or smaller than the old transistor. Either one would make a better replacement.

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 Post subject:
PostPosted: Jan Mon 24, 2011 3:11 am 
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Chris108 wrote:
2N3055 is a big NPN power transistor


He used a 2N3053

Dave


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 Post subject:
PostPosted: Jan Mon 24, 2011 6:20 am 
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 Post subject:
PostPosted: Jan Mon 24, 2011 12:36 pm 
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Joined: Nov Thu 22, 2007 11:31 pm
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Location: Johnston, Iowa
All, thanks for all the advice. I ordered a dozen 2N2222a's from ebay and they were cheap. How can one tell if the original was germanium or silcon?
Keith


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 Post subject:
PostPosted: Jan Mon 24, 2011 3:26 pm 
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Location: Minneapolis, MN
I have found that most germanium transistors in that case style when looked at from the bottom the case looks as if it is filled with either a green or reddish epoxy.

The silicon ones USUALLY have a metal bottom with insulated areas for the base and emitter leads to come thru, and the collector is connected to the case.

I am sure there are exceptions.

If one junction is still intact you might measure its front to back resistance with a diode check on a digital meter

Ge .2 to .3 forward
Si .5 to .7 forward


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