Amplifier Problem - Revised Schematic

[James Sweet]

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]

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/ff58/eadgbe123/amplifier-good-channel.jpg

 

[EADGBE]

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

 

[David Farber]

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.

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.

 

[GregS]

.....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]

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/ff58/eadgbe123/amplifier-good-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]

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]

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