WANTED: Good Source of Electronics Theory

[EADGBE]

I am a home hobbyist who enjoys restoring vintage stereo equipment. I
have a good working knowledge of electronic components and what they
do. I have decent test equipment and good soldering skills, and most
of the time I am able to track down and repair a particular problem.

However, I do feel that I need to know more about the theory behind
electronic circuits in order to improve my troubleshooting skills.

Sometimes I find myself looking at a schematic and thinking, "Boy, I
wish I knew more about what is supposed to be happening in this
circuit!"

Can anyone point to any good books or online sources where I can learn
more about how to analyze schematics and recognize what is supposed to
be happening?

CASE IN POINT: I am attempting to repair a tape deck. One channel
(the right channel) of the headphone amplifier doesn't work. The tape
deck has perfect output through the line out jacks.

Here is the schematic for one of the channels of the tape deck's
headphone amp:

http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier.jpg

I have reproduced the schematic in my own handwriting because the
downloaded schematic was too fuzzy to scan. I *think* I have copied
everything correctly.

I cannot figure out why this particular circuit isn't working
correctly. I have checked each component--and even replaced a few
transistors even though all transistors tested fine--but nothing is
improving.

I have tested all of the electrolytics with my ESR meter and they all
check out OK.

I have checked all resistors and none of them have opened up or "gone
high".

Notice that there are some "typical" expected voltages indicated at
certain points. These "typical" voltages have come right out of the
service manual. All of the voltages marked "OK" are...well...OKAY.
But the three voltages marked with a star (*) are NOT okay...they are
all just a few millivolts each--around 20 to 30mV.

All capacitors are electrolytic with the exception of the 4.7pF
capacitor, which is ceramic.

All resistors are 1/4 watt. If there is no "K" after the number, then
that is the amount of actual ohms it has.

For the record, I replaced Q204, Q206, and Q208 but still got exactly
the same results.

What am I overlooking?

 

[hr(bob) [email protected]]

I am a home hobbyist who enjoys restoring vintage stereo equipment.  I
have a good working knowledge of electronic components and what they
do.  I have decent test equipment and good soldering skills, and most
of the time I am able to track down and repair a particular problem.

However, I do feel that I need to know more about the theory behind
electronic circuits in order to improve my troubleshooting skills.

Sometimes I find myself looking at a schematic and thinking, "Boy, I
wish I knew more about what is supposed to be happening in this
circuit!"

Can anyone point to any good books or online sources where I can learn
more about how to analyze schematics and recognize what is supposed to
be happening?

CASE IN POINT: I am attempting to repair a tape deck.  One channel
(the right channel) of the headphone amplifier doesn't work.  The tape
deck has perfect output through the line out jacks.

Here is the schematic for one of the channels of the tape deck's
headphone amp:

http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier.jpg

I have reproduced the schematic in my own handwriting because the
downloaded schematic was too fuzzy to scan.  I *think* I have copied
everything correctly.

I cannot figure out why this particular circuit isn't working
correctly.  I have checked each component--and even replaced a few
transistors even though all transistors tested fine--but nothing is
improving.

I have tested all of the electrolytics with my ESR meter and they all
check out OK.

I have checked all resistors and none of them have opened up or "gone
high".

Notice that there are some "typical" expected voltages indicated at
certain points.  These "typical" voltages have come right out of the
service manual.  All of the voltages marked "OK" are...well...OKAY.
But the three voltages marked with a star (*) are NOT okay...they are
all just a few millivolts each--around 20 to 30mV.

All capacitors are electrolytic with the exception of the 4.7pF
capacitor, which is ceramic.

All resistors are 1/4 watt.  If there is no "K" after the number, then
that is the amount of actual ohms it has.

For the record, I replaced Q204, Q206, and Q208 but still got exactly
the same results.

What am I overlooking?

Do you have an oscilloscope?? If you do, it is a simple matter of
tracking the signal on the good side and comparing it to the "lack of
signal" in the bad side, stage by stage.

The training source given by the 2nd poster looks to be great!!!.

Bob Hofmann

 

[Eeyore]

Notice that there are some "typical" expected voltages indicated at
certain points. These "typical" voltages have come right out of the
service manual. All of the voltages marked "OK" are...well...OKAY.
But the three voltages marked with a star (*) are NOT okay...they are
all just a few millivolts each--around 20 to 30mV.

There's something amiss with the biasing of the output emitter-follower
stage. The voltage on the base of Q208 should be about 1.2V less (not 0.2V
less I think that 6.8V should be 5.8V) than the voltage on the base of
Q207. That suggests possibly zero or very low current in Q205's collector
circuit. Suggest you measure the voltage across the 12k connected to
Q205's base-emitter.

Graham

 

[EADGBE]

In your tape circuit, the big clue to the problem is lack of any voltage at the
collector of Q205. Q205 must be turned on, at least partially, in order for any
voltage to appear at its collector (which is showing no voltage). It is the
only path that can supply the current to Q203 is through the E-C circuit of
Q205. If Q205's B-E is properly forward biased, current should flow through the
C-E circuit.

Could you repeat this part, please?

There is no Q203 in this circuit.

Thanks for your help!

 

[EADGBE]

There's something amiss with the biasing of the output emitter-follower
stage. The voltage on the base of Q208 should be about 1.2V less (not 0.2V
less I think that 6.8V should be 5.8V) than the voltage on the base of
Q207. That suggests possibly zero or very low current in Q205's collector
circuit. Suggest you measure the voltage across the 12k connected to
Q205's base-emitter.

You are ABSOLUTELY CORRECT. I took another good, hard look at the
service manual schematic (very fuzzy), and the base voltage of Q208 is
indeed +5.8V.

How did you deduce this?

I'm not sure about the 12K resistor across Q205's base-emitter. I
will take a voltage measurement across that resistor and post it here.

 

[EADGBE]

FWIW, the base voltage of Q205 is +11.09V. This was measured at the
junction of the base terminal and the 12K resistor.

 

[EADGBE]

Don:

I'm not sure, but you might be reading my schematic incorrectly.

There SHOULD be +7V at Q205's collector...but right now, there is only
20 - 30mV.

The +7V figure is what the service manual indicates is the "proper"
voltage for Q205's collector.

Regarding Q206, the actual voltages for the base and emitter are both
currently reading just a few millivolts, just like the collector of
Q205.

The indicated voltages for Q206 are, once again, merely from the
service manual, and the figure for the emitter was written
incorrectly. The proper voltage for the emitter of Q206 is +5.8V.

 

[Eeyore]

You are ABSOLUTELY CORRECT. I took another good, hard look at the
service manual schematic (very fuzzy), and the base voltage of Q208 is
indeed +5.8V.

How did you deduce this?

Because there should be 2 Vbe's difference betwen those 2 points (to turn on the
complementary emitter follower stage).

I'm not sure about the 12K resistor across Q205's base-emitter. I
will take a voltage measurement across that resistor and post it here.

That will be helpful.

Graham

 

[Eeyore]

FWIW, the base voltage of Q205 is +11.09V. This was measured at the
junction of the base terminal and the 12K resistor.

That suggests a Vbe of 0.81V in which case Q205 *ought* to be well
turned on and conducting strongly but it isn't.

Replace Q205.

Graham

 

[Eeyore]

Q205 is biased just fine. It has nearly 5V emitter to collector,

NO. Those are the service manual values. Q205's collector is actually almost at ground
potential, i.e. not conducting.

Graham

 

[Eeyore]

I am a home hobbyist who enjoys restoring vintage stereo equipment. I
have a good working knowledge of electronic components and what they
do. I have decent test equipment and good soldering skills, and most
of the time I am able to track down and repair a particular problem.

However, I do feel that I need to know more about the theory behind
electronic circuits in order to improve my troubleshooting skills.

Sometimes I find myself looking at a schematic and thinking, "Boy, I
wish I knew more about what is supposed to be happening in this
circuit!"

Can anyone point to any good books or online sources where I can learn
more about how to analyze schematics and recognize what is supposed to
be happening?

In order to troubleshoot *discrete* transistor circuits that may be a tall
order. They don't even teach this stuff much at college or Uni any more !

Graham

 

[e-micro]

I am a home hobbyist who enjoys restoring vintage stereo equipment. I
have a good working knowledge of electronic components and what they
do. I have decent test equipment and good soldering skills, and most
of the time I am able to track down and repair a particular problem.

However, I do feel that I need to know more about the theory behind
electronic circuits in order to improve my troubleshooting skills.

Sometimes I find myself looking at a schematic and thinking, "Boy, I
wish I knew more about what is supposed to be happening in this
circuit!"

Can anyone point to any good books or online sources where I can learn
more about how to analyze schematics and recognize what is supposed to
be happening?

Visit http://www.e-micro.ro/index.php?topic=57.0

 

[EADGBE]

OK, the plot thickens somewhat...

Some of you have told me that, in my non-functioning amplifier
circuit, the most likely culprit was Q205.

I have replaced Q205 and the circuit STILL doesn't work. It is
exactly the same as before.

There is only 131.1mV on the collector of Q205, despite +11.09V on its
base and +11.59V on its emitter.

Aaargh!

 

[Eeyore]

OK, the plot thickens somewhat...

Some of you have told me that, in my non-functioning amplifier
circuit, the most likely culprit was Q205.

I have replaced Q205 and the circuit STILL doesn't work. It is
exactly the same as before.

There is only 131.1mV on the collector of Q205, despite +11.09V on its
base and +11.59V on its emitter.

And yet you continue to refuse to *directly* measure it's Vbe which is
what's important.

One other thing. Do you know how to test a transistor ?

Graham

 

[EADGBE]

And yet you continue to refuse to *directly* measure it's Vbe which is
what's important.

According to the datasheet for Q205, the Vbe is:

MINIMUM: -0.58
TYPICAL: -0.62
MAXIMUM: -0.68

These figures were arrived at using the following test conditions:

Vce = -6.0V
Ic = -1.0mA

As I have said, I am a hobbyist. Not sure how to do "real world"
testing for Vbe or why it's important. I do know how to test NPN and
PNP transistors using the diode function of a multimeter, that's it.

 

[Mr.T]

According to the datasheet for Q205, the Vbe is:

MINIMUM: -0.58
TYPICAL: -0.62
MAXIMUM: -0.68

These figures were arrived at using the following test conditions:

Vce = -6.0V
Ic = -1.0mA

And from your own measurements there is only 0.50V Base-emitter, so the
transistor is not correctly biased.
Possibly a fault in a preceding stage, or even an O/C resistor or possibly a
dry joint.
Keep looking and measuring other voltages.

MrT.

 

[EADGBE]

William:

That was very good advice. In a way, I'm trying to do the same thing,
but from a "functional" standpoint as well.

In other words, I know what the expected voltages should be, but I
can't help thinking that if I knew how this circuit actually amplified
the signal, I would be in a better position to troubleshoot it, hence
my original request for good sources for learning electronic theory.
(Some of the sources that have been recommended so far seem to be
excellent, by the way.) Where exactly does the signal go?, what
should happen when it gets there?, etc. I believe that these are
questions that I should be able to answer.

So far, what I have is a collection of components that all seem to
test perfectly OK, but don't work, for some as-yet unknown reason.