Amplifier Problem - Revised Schematic

[EADGBE]

In the interest of clarification, I have added some information to the
schematic of the tape deck headphone amp that I'm having problems
with.

Some circuit paths are now labeled:

http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier-1.jpg

As always, the points at which voltage readings are marked with a star
are NOT producing the voltage indicated. The +7V location is
measuring 131mV instead. The other two points where a star is
indicated are measuring between 20mV and 30mV (fluctuating slightly).

I am starting to think that a circuit OUTSIDE of this circuit might be
at fault, because NOTHING inside of this circuit seems to be
faulty...but the darn thing still WILL NOT work.

 

[bz]

In the interest of clarification, I have added some information to the
schematic of the tape deck headphone amp that I'm having problems
with.

Some circuit paths are now labeled:

http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier-1.jpg

As always, the points at which voltage readings are marked with a star
are NOT producing the voltage indicated. The +7V location is
measuring 131mV instead. The other two points where a star is
indicated are measuring between 20mV and 30mV (fluctuating slightly).

I am starting to think that a circuit OUTSIDE of this circuit might be
at fault, because NOTHING inside of this circuit seems to be
faulty...but the darn thing still WILL NOT work.

You do NOT indicate the voltage on Q205's base, also Q204's collector.

If it is 11.9, then Q204 is not conducting and Q205 has no forward bias,
will not conduct, and all other bad values follow from there.

Since the OTHER voltages around Q204 are correct, the collector of Q204
should be LOWER than the 11.9 rail by half a volt or so.
It should thus be 11.4 on Q204 collector and Q205 base.

If it isn't, then Q204 is probably bad, open on the base/collector
junction.




--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap

 

[bz]

Since the OTHER voltages around Q204 are correct, the collector of Q204
should be LOWER than the 11.9 rail by half a volt or so.
It should thus be 11.4 on Q204 collector and Q205 base.

If it isn't, then Q204 is probably bad, open on the base/collector
junction.

OR, the Q204 collector is not connected to to the 12 k resistor and the
base of Q205.

You said you replaced Q204. You could have a bad transistor(new parts can
be bad), a part soldered poorly, or a transistor soldered in wrong.
Check for broken traces and make sure that the orientation is correct on
Q204's replacement.




--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap

 

[Gareth Magennis]

In the interest of clarification, I have added some information to the
schematic of the tape deck headphone amp that I'm having problems
with.

Some circuit paths are now labeled:

http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier-1.jpg

As always, the points at which voltage readings are marked with a star
are NOT producing the voltage indicated. The +7V location is
measuring 131mV instead. The other two points where a star is
indicated are measuring between 20mV and 30mV (fluctuating slightly).

I am starting to think that a circuit OUTSIDE of this circuit might be
at fault, because NOTHING inside of this circuit seems to be
faulty...but the darn thing still WILL NOT work.

Make sure your transistors are oriented the right way round, i.e. B,C & E
really are where you think they are. I remember once being stuck with a
repair where the lead out of a replacement transistor was not the same as
the original. Couldn't work out what the hell was going on for quite a
while, but C and E were reversed. (don't just use a diode check for this)

Gareth.

 

[Eeyore]

"Gareth Magennis" wrote


As an example: BC184

Centre base ('TO-18 style').

and BC184L

Base at one end.

 

[EADGBE]

I have revised the schematic further by adding the voltages at the
base of Q205 and collector of Q204....

http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier-2.jpg

I guess I don't have enough equipment to fix this problem. I can't
afford a 'scope right now. This will have to go on a shelf until I
can deal with it.

 

[bz]

You might want to look at pc scopes. Or ebay. If all you are doing is
audio and possibly lower rf frequencies there are lots of decent used
scopes on ebay.

You might download a program such as waveview or spectragram (demo versions
of many programs are available), make yourself a resistor divider probe and
look at the waveforms with your computer.

http://www.waveviewdaq.com/
http://www.neasmn.org/ben/software/gram517.zip






--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap

 

[EADGBE]

What's driving me crazy is that, for the sake of learning, I replaced
Q205, Q206, and Q208, all with exact replacements that are all known
to be good, and still got exactly the same voltage readings.

 

[EADGBE]

And I also tried replacing Q204.

 

[John Phillips]

I have revised the schematic further by adding the voltages at the
base of Q205 and collector of Q204....

http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier-2.jpg

I guess I don't have enough equipment to fix this problem. I can't
afford a 'scope right now. This will have to go on a shelf until I
can deal with it.

If I understand this correctly the nominally 5.8 V point on Q208 base
is close to 0 V but the output node is OK at +6.4 V.

If that's right, Q208's VBE plus the drop across the lower 3.3 ohm output
resistor is about 6.4 volts rather than about 0.6 V. That can't be
so unless the lower 3.3 ohm resistor is open circuit, or it has 6.4 V
across it and is boiling hot, or Q208 is open circuit B-E.

 

[EADGBE]

If I understand this correctly the nominally 5.8 V point on Q208 base
is close to 0 V but the output node is OK at +6.4 V.

Actually, I am measuring +7.3V at the junction of the two 3.3 ohm
resistors between Q207 and Q208.

 

[John Phillips]

Actually, I am measuring +7.3V at the junction of the two 3.3 ohm
resistors between Q207 and Q208.

And +20 - +30 mV at the base of Q208 from a previous point if I am
correct?

What's the voltage across the B-E of Q208 and what's the voltage across
the lower 3.3 ohm resistor?

 

[GregS]

In the interest of clarification, I have added some information to the
schematic of the tape deck headphone amp that I'm having problems
with.

Some circuit paths are now labeled:

http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier-1.jpg

As always, the points at which voltage readings are marked with a star
are NOT producing the voltage indicated. The +7V location is
measuring 131mV instead. The other two points where a star is
indicated are measuring between 20mV and 30mV (fluctuating slightly).

I am starting to think that a circuit OUTSIDE of this circuit might be
at fault, because NOTHING inside of this circuit seems to be
faulty...but the darn thing still WILL NOT work.

Where did you get this diagram. Is it completely from scratch ?


greg

 

[EADGBE]

Where did you get this diagram. Is it completely from scratch ?

Greg:

No, it was copied, carefully, from the original schematic. I would
have scanned the original schematic, but it is very fuzzy. I did
check my schematic against the other channel's schematic, which was a
bit clearer but did not have the expected voltages indicated, and was
cluttered with the actual component values.

I have checked, re-checked, and RE-checked the accuracy of my hand-
drawn schematic, comparing it to the original, and there are no
mistakes.

 

[EADGBE]

I have just discovered that the voltage measured directly at the
emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere
from -300mV to -16mV.

 

[GregS]

Greg:

No, it was copied, carefully, from the original schematic. I would
have scanned the original schematic, but it is very fuzzy. I did
check my schematic against the other channel's schematic, which was a
bit clearer but did not have the expected voltages indicated, and was
cluttered with the actual component values.

I have checked, re-checked, and RE-checked the accuracy of my hand-
drawn schematic, comparing it to the original, and there are no
mistakes.

The emitter of Q204 has a strange DC path. The amplifier appears
non inverting. What is the voltage on Q204 collector ?

greg

 

[EADGBE]

The emitter of Q204 has a strange DC path. The amplifier appears
non inverting. What is the voltage on Q204 collector ?

greg

Greg:

The voltage at the collector of Q204 measures +11.09VDC.

 

[GregS]

I have just discovered that the voltage measured directly at the
emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere
from -300mV to -16mV.

Thats no good. It seemed most was OK, and I imagined you have no scope.
I was going to suggest injecting a signal at the bases of the imput stage. Holding
a metal probe with your hand should give equal sounds from left and right on corresponding
points of each amp.

You are going to have to check the Q208 more thoroughly.

greg

 

[John Phillips]

I have just discovered that the voltage measured directly at the
emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere
from -300mV to -16mV.

By subtraction from the 7.3 V you said was present at the junction of the
two 3.3 ohm resistors there's nominally 7.3 to 7.6 V across the lower one.

So it looks like:

- that lower 3.3 ohm resistor is open circuit; or

- it's dissipating about 16 watts because of another fault and presumably
getting rather hot which may lead to it going o/c soon (depending
on its power rating which I would guess doesn't support a 16 watt
dissipation); or

- it isn't actually connected to the emitter of Q208 so check the
soldering.

 

[Richard Crowley]

I have just discovered that the voltage measured directly at the
emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere
from -300mV to -16mV.

You show the collector of Q208 as being ground.
Is that same ground not the reference for all your
reported voltage measurements. How could it
possibly be negative? There are no sources of
anything below ground shown in your schematic.

What is the voltage on the "to volume control pcb"
node at the bottom left corner of the circuit? Is it
ground/zero? Is that node *really* disconnected
from the rest of the ground reference (collector of
Q208, bottom of the 680 resistor, etc.)? It seems
like it ought to be ground, but you show it as a
separate node.

It seems suspicious that the base of Q204 is measured
to be the proper voltage, but the collector seems to
be pretty much up at the supply rail. That indicated to
me that Q204 isn't conducting properly and preventing
proper bias on Q205.

We have asked you several times about measuring the
equivalent nodes in the other (working) side, but you
seem to have ignored this valuable source of information
about how a proper circuit behaves.