Dennis,
Very good explanation...I understoof about 30% of it...but then I am a
tech., not an engineer. Hi, Hi.
Since my friend had loaned the R-4B out for a few years, he does not
know when the problem started, or if it was gradual or all of a sudden.
When we were working on it...Garey rightly so kept telling me the
voltages are wrong on Q7...it proved to be correct.
What made this tough is that Q7 was kind of working...so I was
checking, replacing everything BUT Q7.
When Q7 was correctly replaced the R-4B sprang to life. (Another
lesson...don't change parts out at 1:00AM! Hi, Hi)
One thing for everyone here to understand is:
The audio was still wotking, just not well....some may have just
thought well, we have a weak tube or something and used it the way it
was! The thing is at the 12:00 position of the AF gain...it was
useable...just not right. Noe at 12:00 it will blow you away in
volume...and when it was not correct there was some distortion
appearing especially after 12:00 on the gain.
Thanks again go out to Garey for hanging in there with me to get this
fixed, and to you Dennis for letting us know what was actually wrong
with Q7.
So, to everyone in Drakeland, watch your P's & Q7's
73,
Lee, KC9CDT
-----Original Message-----
From: Dennis Monticelli <dennis.montice...@gmail.com>
To: kc9cdt <kc9...@aol.com>; Garey Barrell <k4...@mindspring.com>
Cc: drakelist <drakelist@zerobeat.net>
Sent: Sun, Apr 17, 2011 2:52 pm
Subject: R-4B audio problem traced to low beta Q7 (2N3394)
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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