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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