I believe it. However, I think it had less to do with the transistor being a 2N3393 and pretty much all due to just having a beta high enough to allow the transistor stage to bias correctly. Another transistor type with good beta would have probably worked as well. You kept it "in the family" by choosing the 2N3393, so I prefer the route you took.
Dennis AE6C On Sun, Apr 17, 2011 at 12:33 PM, <kc9...@aol.com> wrote: > One more thing to add....the 2N3393 worked super (batter than the > 2N3394??)...in fact, at a AF gain setting of 8-9 o'clock...the audio is very > nice at at a good listening level....at 10:00 with the RF gain down on a > noisey band the R-4b really hears well. > 73, > Lee > > > > > -----Original Message----- > From: Garey Barrell <k4...@mindspring.com> > To: Dennis Monticelli <dennis.montice...@gmail.com> > Cc: kc9cdt <kc9...@aol.com>; drakelist <drakelist@zerobeat.net> > Sent: Sun, Apr 17, 2011 3:25 pm > Subject: Re: R-4B audio problem traced to low beta Q7 (2N3394) > > > Dennis - > > I think the saying is 'even a blind squirrel finds an acorn now and > then, if you put him under an oak tree!" :-) > > That's why I suggested Beta, a low B-E current just made it worse. I've > never seen a small signal BJT fail that way, but your explanation makes > sense. I have seen some of the early RF Power transistors that were > actually multiple transistors all collected in parallel and one or more > would blow open! > > I believe R141 is ALSO a form of temperature compensation, as the > transistor 'warms' up, the B-E drop lowers, increasing the bias > current. The drop across the E resistor also increases, decreasing the > bias current. Much more stable DC operating point. It DOES also add > some negative feedback for the signal, reducing distortion. It also > stabilizes the circuit with different transistors as without it the > stage gain is limited ONLY by device Beta and internal resistance. With > the E resistor, the output is the ratio of Er to Cr, swamping device > variations, for a gain of about 580. > > Very interesting exercise. Thanks very much for your help and expertise! > > 73, Garey - K4OAH > Glen Allen, VA > > > > Dennis Monticelli wrote: > >> Lee, Garey, >> OK. Mystery solved. Curve tracers are wonderful forensic tools. >> Lee, when I first looked at the 2N3394 from your R-4B it's I-V curves >> looked fine except for a decided dropoff in low current beta. At >> first I didn't flag this as a serious issue. The beta of all >> transistors is a function of collector current, tending to peak right >> where the factory DC tests are run :-) The falloff rate at low >> collector current was greater for transistors built over 40 years ago >> vs a transistor made today. The 2N3394 in your radio still meets min >> spec of 55 (barely) at 2mA. However, Drake ran the collector current >> at 1/8 that value and used a base current biasing method (in common >> use at the time) that suffers from excessive dependency upon the beta >> value, not for voltage gain but for setting up the correct bias >> levels. Your particular 2N3394 has a beta of only 25 at 250uA which >> is why it set up too low. Once it set up too low the further >> declining beta below 250uA made things even worse, effectively >> negating the compensating extra current through the 2.2M base bias >> resistor (the collector voltage rises under the starved collector >> current condition and would normally help provide addtional base >> bias). When you placed a fresh 2N3393 in there (min beta of 90 at >> 2mA), the bias problem went away. Not only was its beta higher at >> 2mA, it probably fell off a lot less at 250uA also. >> So the only mystery left is why Q7 worked well enough to leave the >> factory but not at the present time. Low current beta is dependent >> upon "surface states" in the Silicon crystal in the region of the >> > base > >> structure. Early transistor manufacturing techniques left much to be >> desired in terms of surface state control and early plastic packaging >> (think leaching of ions) compounded the stituation. So I'm guessing >> the low current beta was marginal when it left the factory and slid >> down over time. Another posibility is a reverse application of BE >> voltage (even momentary) as that is known to degrade low current >> beta. Looking at the circuit the only way that's going to happen is >> an errant application of a test probe. So most likely it was slow >> degradation over time. >> Anyway, the data supports your 2N3393 fix. Given what I've learned I >> would recommned that concerned R-4B users make a simple DC >> > measurement > >> of the voltage across R141 and if excessively low (i.e. well below >> 20mV), then do exactly what you did: replace Q7 with a fresh 2N3393 >> and recheck bias levels. R-4B owners may want to do this because >> > R141 > >> is not there to manage bias, it is there for emitter degeneration, a >> form of negative feedback that reduces distortion under strong audio >> signals. If insufficient voltage is developed across R141 the >> distortion will be greater. >> Hope this explanation helps the list. I learned something here. >> Dennis AE6C >> > >
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