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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