[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/f...mplifier-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.

[Richard Crowley]

"EADGBE" wrote ...
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/f...mplifier-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.

Can't see what parts external to the circuit shown could
cause any of your symptoms. The input and output are
decoupled with capacitors. Furthermore, you confirm
that the internal, DC side of the input and output are
at the prescribed voltage. You have also confirmed that
the supply voltage is OK. There is nothing else externally
that could affect the circuit. I am assuming that the
node at the lower left corner finds its way back to
ground somewhere.

The question remains why the collector of Q205 is at
131mV rather than the predicted 7V. Q205 is simply
not conducting and that is causing all the rest of the
symptoms.

One of the marvelous things about stereo is that you've
got two identical channels. So you have a good one to
compare it to.

What is the base voltage of Q205 on the working side?
And what is it on your broken side?

[bz]

EADGBE wrote in news:7a5f62ce-00ec-4fac-84f3-
:

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/f...mplifier-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.

remove ch100-5 to avoid spam trap

[bz[_2_]]

bz wrote in
98.139:

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.

remove ch100-5 to avoid spam trap

[Gareth Magennis]

"EADGBE" wrote in message
...
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/f...mplifier-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.

[Gareth Magennis]

"Gareth Magennis" wrote in message
...

"EADGBE" wrote in message
...
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/f...mplifier-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.




As an example: BC184 and BC184L have different pinouts.



Gareth.

[James]

You need to get a scope or at least a signal tracer and see where the signal
stops on the bad side. It's a stereo amp so it should be a no brainer to
compare signals as you move through both sides. If all you have is a meter
then you just don't have enough equipment to do repair work.

"EADGBE" wrote in message
...
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/f...mplifier-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.

[Eeyore]

Gareth Magennis wrote:

"Gareth Magennis" wrote
"EADGBE" wrote

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/f...mplifier-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)


As an example: BC184

Centre base ('TO-18 style').


and BC184L

Base at one end.


have different pinouts.

Gareth.

[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/f...mplifier-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.

[James]

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.

"EADGBE" wrote in message
...

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

http://i240.photobucket.com/albums/f...mplifier-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[_2_]]

"James" wrote in
:

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.

"EADGBE" wrote in message
...

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

http://i240.photobucket.com/albums/f...mplifier-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.

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.

remove ch100-5 to avoid spam trap

[Ol' Duffer]

In article e79927cb-9552-41f4-81e9-0c380ce099b0
@f36g2000hsa.googlegroups.com, says...

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

http://i240.photobucket.com/albums/f...mplifier-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.

You don't need a scope for this one, just DC meter and brain power.
For the output transistors, Q207 base should be about one Volt higher
than Q208 base. If not, either Q206 is shorted or Q205 is not sourcing
current. If I can believe the voltages you show on Q205, it probably
has an open collector.

[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.

[Arny Krueger]

"EADGBE" wrote in message

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/f...mplifier-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.

Have you followed the recommended procedure for debugging direct-coupled
circuits like this?

Step 1 - remove all transistors and check them for shorts and opens out of
the circuit. An ohmeter usually suffices.

[John Phillips[_2_]]

On 2008-04-22, EADGBE wrote:

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

http://i240.photobucket.com/albums/f...mplifier-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.


--
John Phillips

[Don Pearce]

Richard Crowley wrote:
"EADGBE" wrote ...
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/f...mplifier-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.

Can't see what parts external to the circuit shown could
cause any of your symptoms. The input and output are
decoupled with capacitors. Furthermore, you confirm
that the internal, DC side of the input and output are
at the prescribed voltage. You have also confirmed that
the supply voltage is OK. There is nothing else externally
that could affect the circuit. I am assuming that the
node at the lower left corner finds its way back to
ground somewhere.

The question remains why the collector of Q205 is at
131mV rather than the predicted 7V. Q205 is simply
not conducting and that is causing all the rest of the
symptoms.

One of the marvelous things about stereo is that you've
got two identical channels. So you have a good one to
compare it to.

What is the base voltage of Q205 on the working side?
And what is it on your broken side?

All points to Q205 collector being open apart from one thing. On the
revised schematic, the output voltage at the junction of the two 3R3
resistors is measured as being correct. That doesn't add up at all. I
could do with seeing this voltage checked.

I've put the circuit into Pspice and played around with various fault
conditions to see what the volts do. Q205 is the only thing that seems
to reproduce the symptoms.

d

[EADGBE]

On Apr 22, 10:08 am, John Phillips
wrote:

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[_2_]]

On 2008-04-22, EADGBE wrote:
On Apr 22, 10:08 am, John Phillips
wrote:

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.

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?

--
John Phillips

[GregS[_3_]]

In article , EADGBE wrote:
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/f...mplifier-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]

On Apr 22, 10:34 am, (GregS) wrote:

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[_3_]]

In article , EADGBE wrote:
On Apr 22, 10:34 am, (GregS) wrote:

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.

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

greg

[Don Pearce]

GregS wrote:
In article , EADGBE wrote:
On Apr 22, 10:34 am, (GregS) wrote:
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.

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

greg

Q204 is in a pretty simple DC feedback path with Q205 and the 207/8
combination. Its emitter currrent flows down through Q208 to ground.

The amplifier is non-inverting from the base of Q204, but inverting from
its emitter, which is where both the DC and AC feedback end up.

d

[EADGBE]

On Apr 22, 11:29 am, (GregS) wrote:

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[_3_]]

In article , EADGBE wrote:

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[_2_]]

On 2008-04-22, EADGBE wrote:

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.

--
John Phillips

[Richard Crowley]

"EADGBE" wrote ...
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.

[James Sweet]

"James" wrote in message
...
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.


Last swap meet I went to I saw at least half a dozen scopes under 10 bucks.
A couple of them were even decent semi-modern looking solid state things,
probably 10-15 MHz, so this stuff does turn up.

[EADGBE]

On Apr 22, 12:44 pm, "Richard Crowley" wrote:

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.

Richard:

Sorry about not posting that information. I wasn't ignoring this
request, I promise. Just hadn't gotten around to it yet.

I will post the same schematic, but this time with the measurements
taken from the "good" channel.

And I will also take measurements at the node(s) you inquired about.

[EADGBE]

OK, here is the schematic of the "good" channel, with voltage
measurements indicated.

I realize that some of the voltage indications are redundant, but I
decided to "over-label" in the interest of clarity....

http://i240.photobucket.com/albums/f...od-channel.jpg

[EADGBE]

.....And I should point out that ALL measurements taken in both
channels were obtained using chassis ground.

[David Farber]

"EADGBE" wrote in message
...
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/f...mplifier-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.

I haven't read every follow up message but have you tried removing the
electrolytic caps and retesting the d.c.? I've seen the d.c. stability been
thrown off in many amps due to bad coupling caps.

Good luck.
--
David Farber
David Farber's Service Center
L.A., CA

[GregS[_3_]]

In article , EADGBE wrote:

.....And I should point out that ALL measurements taken in both
channels were obtained using chassis ground.

I completely missed the star explanation. Why not put measured voltages there.
What is the voltage at collector Q204 on other channel.
Q205 doesn't seem to be conducting much.

greg

[Eeyore]

EADGBE wrote:

OK, here is the schematic of the "good" channel, with voltage
measurements indicated.

I realize that some of the voltage indications are redundant, but I
decided to "over-label" in the interest of clarity....

http://i240.photobucket.com/albums/f...od-channel.jpg

Are those the voltages you are NOW getting ?

Those look correct.

Graham

[EADGBE]

STOP THE PRESSES!

THE AMPLIFIER PROBLEM HAS BEEN *SOLVED* !!!!!!!!!!!!!!!!!!

SEE MY POST REGARDING THIS!!!!!!!!

MANY, MANY THANKS TO EVERYONE WHO OFFERED HELP, TIPS, AND
SUGGESTIONS!!!!!!!!!!!!!!!!!!!!!!!

[GregS[_3_]]

In article , EADGBE wrote:

STOP THE PRESSES!

THE AMPLIFIER PROBLEM HAS BEEN *SOLVED* !!!!!!!!!!!!!!!!!!

SEE MY POST REGARDING THIS!!!!!!!!

MANY, MANY THANKS TO EVERYONE WHO OFFERED HELP, TIPS, AND
SUGGESTIONS!!!!!!!!!!!!!!!!!!!!!!!


Maybe we can improve the design ?

greg

[jakdedert]

EADGBE wrote:
STOP THE PRESSES!

THE AMPLIFIER PROBLEM HAS BEEN *SOLVED* !!!!!!!!!!!!!!!!!!

SEE MY POST REGARDING THIS!!!!!!!!

MANY, MANY THANKS TO EVERYONE WHO OFFERED HELP, TIPS, AND
SUGGESTIONS!!!!!!!!!!!!!!!!!!!!!!!


And the winner is...????

jak