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 Post subject: Could someone tell me why this transistor reads like this..
PostPosted: Feb Mon 12, 2018 12:03 am 
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Here is a photo of a transistor I needed to replace in a Channel Master 6519, which is the 1st IF amplifier transistor...

I do not know what the extra line is and what looks like a diode that was displayed on my tester...

Maybe someone else knows...thanks..

Kenny KE4HVE


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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 12:44 am 
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It's the substrate diode.


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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 1:03 am 
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It's in there to prevent reverse voltage spikes, if you use it with a relay or such, or any kind of inductive load.

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 1:14 am 
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Ok...but the same transistor I took out(the first one..that I thought was bad)...did not have that extra diode...same model number transistor,, a 2SA202..and it came out of the exact same model radio..in the exact spot which was the 1st IF transformer amplifier..

weird right???

Kenny


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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 1:38 am 
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Nope. The diode is inside the Transistor package; it's not a discrete component.

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 1:39 am 
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So fifties why did one transistor(the one I was replacing) have no diode in it..and the replacement have the diode...??

They were both 2SA202

KNY


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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 1:43 am 
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I can't answer that, Kenny. What makes you sure that the Transistor you replaced did not have the Diode?

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 3:30 am 
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Im not sure...but it has exactly the same numbers on it....do you think the one I took out that was bad did not show it because it changed internally, and that is why it was bad???


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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 5:26 am 
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It could be an unitentional pn junction thru the substrate as flipperhome suggested. It could also just be the reverse breakdown voltage, as its never intended to see voltage in that direction. I doubt that its a deliberately made diode. The measured "forward drop" ( note the quotes)is too high. This transistor is a Japanese made germanium small signal PNP. Not really intended to drive a relay coil. And if used for such an application, it would be a relatively low current relay coil.

The 2SA202 was first made ~1958. That model radio came out in the mid 60's. Two transistors could have come from two different production batches, or maybe one is slightly defective.

Never take your test instrument results as gospel. Best test instrument is the one between your ears. Mine is a tad defective.

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 6:08 am 
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Kennynva wrote:
Ok...but the same transistor I took out(the first one..that I thought was bad)

Is the radio now operating correctly with the replacement, or has there been no change?

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 7:09 am 
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With the transistor change..it took care of what I think it should have..the volume stays ok..but there are multiple problems with this radio, I recapped it thinking it would take care of it, and it did help with the volume..BUT...the meter is, while the switch on the front is in the normal position, goes up and down with the volume control, and not with a signal...and pegs out to full or to the max the meter will read...I have spent alot of time on this radio..and after getting really peode at the fact that every time you flip it over to install any part the tiny wires start breaking..again and again..glad I took photos of it..like I always do.


Kenny


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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Mon 12, 2018 5:25 pm 
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Kennynva wrote:
Here is a photo of a transistor I needed to replace in a Channel Master 6519, which is the 1st IF amplifier transistor...

I do not know what the extra line is and what looks like a diode that was displayed on my tester...

I suspect the transistor is leaky and as a result has confused your tester into thinking that there is a protection diode between the collector and emitter. The Vf of 2.64 volts is evidence of this, as a germanium transistor of that age cannot possibly have a semiconductor junction with a 2.64 volts Vf. (Unless someone has opened the case and inserted an LED :-)

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Tue 13, 2018 12:11 am 
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The 2SA202 is a germanium alloy transistor medium gain IF transistor that was put on the market in 1959 by Sanyo and used in a large variety of Japanese transistor radios. It has no substrate, substrate diode, or embedded damper diode. Those things were not invented until much later.

The component "guesser" is showing you a reversed diode across the CE junction with a forward voltage of 2.64 volts, which would certainly be a problem since the spec sheet gives 15 volts as the breakdown voltage. If you have access to a standard transistor tester you might try that, or you could try the usual back-to-back ohmmeter test (i.e. test the CB and BE junctions like two back-to-back diodes). It is probable you will find that one of the junctions is leaky. Leakage would likely cause the transistor to break down or turn on during the reverse part of the test cycle and the tester is reading it as a spurious diode.

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Tue 13, 2018 2:33 am 
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Chris108 wrote:
The component "guesser" is showing you a reversed diode across the CE junction with a forward voltage of 2.64 volts, which would certainly be a problem since the spec sheet gives 15 volts as the breakdown voltage. If you have access to a standard transistor tester you might try that, or you could try the usual back-to-back ohmmeter test (i.e. test the CB and BE junctions like two back-to-back diodes). It is probable you will find that one of the junctions is leaky. Leakage would likely cause the transistor to break down or turn on during the reverse part of the test cycle and the tester is reading it as a spurious diode.

Isn't the 15 volt breakdown spec for the forward breakdown voltage and isn't that 2.64 volts shown by the tester for a reverse voltage? Transistors generally have a much lower reverse breakdown voltage then their forward breakdown voltage. Though usually not as low as 2.84 volts.

Many years ago at the place I worked we found a transistor ( 2N3638 ? ) that had a very predictable reverse breakdown. We sometimes used it as a cheap 6 volt zener diode.

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Tue 13, 2018 12:41 pm 
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Breakdown voltage in transistors is a nebulous thing since it can be forward or reverse, with base open, connected to the collector, or to the emitter. Data sheets--especially those written in Japanese in 1959 :wink: --seldom give all the permutations. That's why I was careful to say that the component tester sees it as a diode conducting in the reverse direction, not that the transistor is going into reverse breakdown. We don't know if it is actually breaking down or where, but -15 volts is given as the maximum voltage for the 2SA202 and the tester puts about 4-1/2 volts open circuit across it, so it sure looks like the transistor is not dealing with it. The OP also mentioned that he tested another transistor of the same type and did not see the diode symbol, so whatever is going on presumably has to do with this one particular transistor.

One thing you have to be careful of with these PIC testers is, certain versions of firmware will occasionally mis-identify transistors with the C and E leads reversed. You'll note an unusually low hFE (gain) number when it happens. This does not appear to be the case here since the hFE reading of 62 seems reasonable for this transistor type. If the tester mis-read it, the hFE would be around 3. Re-testing the transistor once or twice will usually cause the tester to read correctly.

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 Post subject: Re: Could someone tell me why this transistor reads like thi
PostPosted: Feb Wed 14, 2018 6:01 am 
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Chris108 wrote:
Breakdown voltage in transistors is a nebulous thing since it can be forward or reverse, with base open, connected to the collector, or to the emitter. Data sheets--especially those written in Japanese in 1959 :wink: --seldom give all the permutations. That's why I was careful to say that the component tester sees it as a diode conducting in the reverse direction, not that the transistor is going into reverse breakdown. We don't know if it is actually breaking down or where, but -15 volts is given as the maximum voltage for the 2SA202 and the tester puts about 4-1/2 volts open circuit across it, so it sure looks like the transistor is not dealing with it. The OP also mentioned that he tested another transistor of the same type and did not see the diode symbol, so whatever is going on presumably has to do with this one particular transistor.

I believe that the breakdown voltage specified in the data sheet is for voltage applied in the forward direction, that is the polarity normally applied to the collector when the transistor is used normally. The breakdown with the voltage reversed is not normally specified and is not normally important, but is usually more than 2.64 volts. I think that the tester ignores more normal reverse breakdowns but sees this unusually low voltage as a diode.

As mentioned above, I am familiar with transistors with a forward breakdown of around 30 volts but have a reverse breakdown of 6 volts.

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