BINGO!
Just as Dr. Barrell said........
Q7
Collector= 8 v
Base = .6
Emitter = .02
And Audio is the wanderful Drake audio of all time!
I guess Q7 was not blown, but had a high resistance emitter to base or
something like that.
Usually on transistors my experience is it either works or it doesn't.
I guess it ain't so.
I learned a lot shooting this bug ....and I couild not have done it
without you Garey!
So many thanks indeed.......
Thanks for hanging in there with me.
Actuallythis was fun.
Wait until my friend hears his R-4B now!
He will understand just why we love em!
I'm buying you 2 cups of coffee at Dayton (Or something better if you
want)
73,
Lee
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Tue, Apr 5, 2011 6:29 pm
Subject: Re: [Drakelist] Fwd: My R-4B audio?
Either should work fine. Just watch the lead pattern!
73, Garey - K4OAH
Glen Allen, VA
[email protected] wrote:
Garey,
I have the board out. I discovered I really did not swap the
Q7...tired and late at night Hi HiI also noticed that the cap we
discussed that is in the feedback is damaged some.
I have another...so I am replacing the Q7 with a 2N3393 I have and
that questionable cap now.
Also...I have a radio shack MPS2222A I could use instead?? any
preference?
Lee
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Tue, Apr 5, 2011 5:48 pm
Subject: Re: [Drakelist] Fwd: My R-4B audio?
Lee -
Do you have any input (tone) when you are measuring those levels? AF
GAIN must be a zero to check DC.
That collector voltage is still high. I was just playing with my
breadboard version, and I have 18.5V DC and 8 V on the Collector.
14.5
V on the Collector says the transistor is still not drawing 'normal'
current. Mine has 0.019V on the Emitter, and about 0.6 on the Base.
I
believe you have tried another Q7, perhaps just very low Beta? Have
you disconnected the input and output capacitors? Less than 1M of
'pull-up' resistor is way too low, unless there is leakage current to
ground, or the transistor Beta is extremely low.
73, Garey - K4OAH
Glen Allen, VA
[email protected] wrote:
Garey,
OK, I have not found the root cause yet..but, I paralleled a 1 meg
resistor to the 2.2 meg bias and:
Q7
Collector is 14.5 v
Base is .578
Emmitter is .013
And the audio sound pretty loud and clear...so I know the Q7 is
working.
Something is presenting some additional resistance in the cicuit. Got
to find it next
73,
Lee
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Tue, Apr 5, 2011 10:44 am
Subject: Re: [Drakelist] Fwd: My R-4B audio?
Lee -
OK.
No, the voltage MUST be a little less than half of whatever voltage
appears across C173 / top of R148. There has to be 'room' for the
Collector to go positive or negative depending upon the current
through Q7.
The transistor is turned on via current through R131. As Q7 turns
on,
the current drawn through R148 drops the voltage at the top of R131,
causing the stage to reach 'equilibrium' at 'about' 1/2 of B+, (18V).
The Emitter resistor, R141B, is to compensate for Base-Emitter
voltage
variations with temperature, e.g., if V B-E drops, the voltage across
R141B drops, again reaching equilibrium.
When AC is applied via C167, the +/- swings alternately increase and
decrease the current through R148 and R141B, causing the voltage to
swing 100 times more (assuming Beta of 100) on the Collector. The
current also goes through R141B, and this drop is subtracted
(negative
feedback) from that across R148. C169 is frequency compensation to
limit high frequency (supersonic) gain. This capacitor is left OUT
of
some of the R-4Bs that I have worked on. I think it was included,
'just
in case'.
So the problem is a very simple one at this level, and strictly a DC
question. If you take a transistor, 82, 47k and 2.2M resistors, and
connect them per the schematic, across an 18V supply, the Collector
voltage will be about 8V, the Base about 0.6V and the Emitter about
0.02V. Sources of error are leaky input or output capacitors,
incorrect
resistor values, faulty or blown transistor, poor ground
connections,
or
interconnection faults. You have the components, put them together
'in
the air' and you'll get these voltages! :-)
Obviously, in your case, there is little if any current through Q7,
therefore no drop across R148 or R141B. This is the simplest of
circuits, almost a 'it takes two wires to ring a doorbell' case.
This
can only be caused by a few things.
a) the voltage is not getting to the Collector of the transistor.
It's
on the 'bottom' of R148, so this leaves any PC trace, solder
connections
at the Q7 end, a broken lead inside the transistor. OR,
b) the voltage IS getting to the Collector, BUT the Emitter is not
getting through R141B to ground. Same sources of trouble as above.
OR,
c) those connections are all good, and there is no (or insufficient)
voltage to overcome the V B-E (~0.6V) getting to the Base. Same
sources
of trouble as above, we only need a few uAmps through R131, so
anything
other than a perfect connection, or leakage from the Base to ground,
(leaky C167, incomplete etch between traces, dirty PC board, contact
'cleaner' crap, etc,) can drag down this voltage.
R130 is decoupling, with C188A, for the BFO. It drops the voltage to
about 11 VDC from the 18V across C173.
73, Garey - K4OAH
Glen Allen, VA
[email protected] wrote:
Garey,
OK, I find no issues with land patterns.
I have continuity from each lead of the Q7 itself to appropriate
spot.
OK, NOW...
I parralled a 2.0 meg resistor to the Q7 bias 2.2meg. =now approx 1
meg
The audio is higher.
Q7 voltages:
Collector = 17 v
Base = .53 v
Emitter = 4 mv
All resistor values seem to be correct....SO
Are we sure the voltage at R148 are correct?
If they were higher, then the bias would be higher??
Getting tired, but just a thought????
Going to bed.
Thanks,
Lee
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Mon, Apr 4, 2011 11:29 pm
Subject: Re: [Drakelist] Fwd: My R-4B audio?
Lee -
AC measurements aren't going to mean much until you get the DC
biasing
correct. The key is still 7.5 - 8 VDC on the Collector, 0.02 V on
the
Emitter, and about 0.55-0.6 V on the Base with no signal. The 18 V
across C173 is correct so there has to be a problem from the bottom
of
R148 down. Perhaps a cracked PC board trace.?
It's plain ol' DC, so gotta be a break somewhere. You would also
get
similar results if the Emitter was open, cracked R141B, cracked
solder
on same, broken trace?
73, Garey - K4OAH
Glen Allen, VA
[email protected] wrote:
Never mind...there isa little less than 1 volt P-P on pin 2 of
V7......I had a bad connection.
so, just 1 volt P-P at AF gain at 2:00 is not enough, correct
I think you said we were looking for 2-3 volts P-P
I am not doing the checks you said in earlier e-mail
Lee
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Mon, Apr 4, 2011 2:20 pm
Subject: Re: [Drakelist] Fwd: My R-4B audio?
OK, you're 'well equipped'.! I never know when I start these
'projects' just what I'm working with...... :-)
I went back to my breadboard because it should have handled more
output
V before clipping. Musta been in too much of a hurry this morning!
:-) I use one of the white plastic breadboards with the rows of
connected holes, perhaps a lead wasn't pushed in good enough.
Now getting about 7.5 V Collector, 0.6 V Base and 0.02 V Emitter,
and
a
good 10V p-p before distortion begins. It doesn't clip until 20V
p-p,
and now clips on the bottom as I would expect. Max Peak Grid 1
voltage
on the 6EH5 is 3 V, so plenty of headroom. Gain is 47k / 82, so
about
575, meaning about 7 mV input will produce 3 V output.
73, Garey - K4OAH
Glen Allen, VA
[email protected] wrote:
Garey,
My HP 410c needs calibration so I have been using my DVM.
The specs say it has a 10M ohms input impedance in parralel with 2
pf.
+ - 1%.... not bad for a Radio shack instrument (Not one of their
cheapys)
I have a HP scope and a HP 8640b sig. gen.
Off to do a busines errand then I will get on this.
Thanks,
Lee
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Mon, Apr 4, 2011 9:07 am
Subject: Re: [Drakelist] Fwd: My R-4B audio?
Lee -
This is beginning to annoy me...... :-)
This is about the simplest audio amplifier circuit around, and has
been
working for 50 years. Just to cover the bases, have you measured
R141B
for 82 ohms?? Have you checked the Collector voltage with C167
disconnected entirely??
The circuit is ...
The current chosen for about 0.25 mA, and with a 47k load resistor
this
results in a maximum voltage swing of ~12V p-p while staying in
the
linear part of the transistor curves.
This idle current would result in a voltage across 82 ohms of ~ 20
mV
(0.02V). A Beta of ~100 says that we need 1/100th of 0.25 mA or
.0025
mA through R141B or ~5.5 V through R131. The voltage at the Base
MUST
be >~0.6 V to turn the the transistor 'on' and into the linear
range.
I
think that perhaps the 'noise' input to the amp is contributing to
the
Base voltage. Also, this is a very high impedance point so
loading
of
the meter could also affect the reading. What is the input
impedance
of
your meter?
I've got rehab this morning, but when I get back this afternoon
I'm
going to breadboard this circuit and see if I can figure out what
is
happening. It's an 'age-old' circuit, and has been working in
5,000
R-4Bs, so we must be missing something.
Do you have a scope? I suspect if you look at the Collector of Q7
with
the Calibrator tuned in for a ~1000 Hz tone, you'd see a sine wave
with
about the bottom third just clipped off.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
[email protected] wrote:
Garey,
I put in the new transistor...no help, audio is exactly the same.
Base is still at .48v
Thanks,
Lee
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Sun, Apr 3, 2011 10:54 pm
Subject: Re: [Drakelist] Fwd: My R-4B audio?
The 2N3393 is ok. It's Beta is 90 - 180 so should work. It has
the
same base as the 3394, E-C-B
73, Garey - K4OAH
Glen Allen, VA
[email protected] wrote:
Regarding question #1 from before I mrant R145
All I have is a 2N3393 and some other PNP ones
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Sun, Apr 3, 2011 10:42 pm
Subject: Re: [Drakelist] Fwd: My R-4B audio?
[email protected] wrote:
Garey,
More updates.
1) The Q7 is OK, if I pull one lead of 145 I get very, very
weak
audio...esentially going around Q7
What is '145'??
2) Are you sure 18.5 v is not normal on Q7's collector? From
the
voltage droppers starting at R136 = 135v, R137 = 19.8 v, R145 =
18.5
v
See previous message. 0.5 V on Base of Q7 is not enough to turn
on
the
transistor, hence no current, no drop across R148.
3) I think the gain is just low on Q7
Possible, see previous message.
4) The base of Q7 is at .48v
Not enough. Has to be closer to 0.6 - 0.7V.
5) What is c169 for..I have not done anything with it
C169 is negative feedback to flatten the audio response. Most
receivers
don't have it at all. Pull at least one lead, possibly leaky,
but
not
likely. If it was leaky, would turn Q7 on even harder.
6) There is only 9 volts different between the plate of V7 and
the
screen!!! Is that enough?
That's ok.
7) What do C173 and C90A do? I have bridged the c173 like you
said
but
not unhooked it & patched in a new one
They are both additional decoupling/filtering to clean up any
power
supply ripple. If you have 18 V across C173, it's they're both
ok.
My R-4C audio is great at 9:00...the R-4B is not even as good
at
12:00
and is a little distorted I think.
Yes, the audio signal itself is actually turning Q7 on on the
positive
peaks if you turn it up loud enough. The Collector has to be
about
half
or a little less than the voltage at the top of R148 for Class A
operation. The DC bias sets the Collector voltage 'in the
middle'
to
allow maximum positive and negative swings of the audio
waveform.
That's Class A.
This is a tough one!
Yes, we must be missing something, but I sure can't see it from
here!! :-)
73,
Lee
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: kc9cdt <[email protected]>
Sent: Sun, Apr 3, 2011 12:17 am
Subject: Re: [Drakelist] Fwd: My R-4B audio?
Lee -
The leads on the 2N3394 are Emitter - Collector - Base. Most
of
this
type are E-B-C. No way to tell if the transistor is blown with
these
voltages. If the Base voltage goes above ~ 0.7V more positive
than
the
Emitter, THEN the transistor is blown.
Yes, cans are always common negative to can. Custom is the
'straight'
bar of the symbol is '+', and the 'curved' bar is '-'. Also
you
will
almost NEVER see a capacitor larger than 5 uF that is NOT an
electrolytic.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
[email protected] wrote:
Garey, as I thought...why don't they mark it as an
Electrolytic
on
the
schematic?
I guess it is assumed if it's a can...it's Electrolytic?
OK...This is strange.....That Q7 is a standard TO-92 case
right?
Well the base has 18v on it...looks like someone may have oput
it
in
wrong??
The center connection on Q7 is the one with 18v on it and it
looks
like it is installed wrong?? Hard to believe...I checked this
several
times.
If it is true..do you think Q7 is blown?
Thanks
Lee
Lee Simmonds
Summit DCS LLC
260-799-4077 Office
260-403-6936 Cell
-----Original Message-----
From: Garey Barrell <[email protected]>
To: drakelist <[email protected]>
Sent: Sat, Apr 2, 2011 11:36 pm
Subject: Re: [Drakelist] Fwd: My R-4B audio?
Lee -
Yes, C90A IS an electrolytic, it is part of the Twist-Lok can
cap.
It
is the terminal with NO mark on it (square, half-round,
triangle
on
the
other three). The terminals are all ' + ', the can is common
'
-
'.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
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