Amplifier

[Arvid Puschnig]

Hi!

I got an old SONY amplifier where the right channel seems to be
defective. When I turn the balance to the right speaker, the volume
gets lower and low frequencies are missing completely. I am really not
sure, where I should start searching for the error. First I tried
another pair of speakers, but this was not the cause. Can anyone tell
me where I should start searching for the error? Can anyone tell me
what the integrated preamp (I found this on the schematics) is good
for?

Thank you,
arvid puschnig

 

[Arvid Puschnig]

"Tweetldee" <dgmason99@att99.net> wrote:

Posting a copy of the schematic would definitely help. It eliminates
a lot
of guesswork.

Ok, I'll put it up the next 2 days.

First thing you should do is to check voltages around the output
stage of
the defective channel. If the schematic shows voltage readings, use
them as
a guide for troubleshooting. As a backup, since the other channel is
working, you have a good source of voltages/resistances built right
into the
unit. Use the other channel as a comparison when you get into an
area where
the voltages are way out of whack.
Start at the output stage and work your way back to the input of the
power
amp. You should find a stage where the voltages are very wrong.
Concentrate there.

Do I have to check voltages with an input signal or with load (speakers
connected)?

When you have found that stage, turn it off and check all
semiconductors in
that area. If the amp stages are DC coupled, remember that the
incorrect
voltages may cascade down the chain, so keep that in mind while
you're
working.

I am not sure if the stages are DC coupled, but since I could not find
a capacitor in the signal way I would say they are. Perhaps you can
have a look at the schematics then.
There's a label at the front of the amp "All power stages direct
coupled"...

Use the diode check function of the ohmmeter in the DMM to check
semiconductor junctions. In some circuits, you may have to remove
the
semiconductors to get meaningful readings, especially if the stages
are DC
coupled or if there are very low-value resistors in the neighborhood.

Ok.

Why don't you post a decent scan of the schematic and then take some
voltage
measurements around the bad channel, and post those here in the
group. Then
we can get to work.

I'll put it up tomorrow and at the weekend I'll take voltages.
Thanks for your help!

arvid

 

[David]

First thing you need to do is try and determine if the problem is in the
pre-amp stage or power amp stage. Looks like you have a good break point at
R500/R550 for one channel and the equivalent point for the other channel.
Manually swap the pre-amp stage to power amp signal between the left and
right channel at those resistors and see if the symptom stays with the right
channel (power amp problem) or moves to the left channel (pre-amp or earlier
problem). Note I am not talking about what Sony calls the pre-amp
transistors in the power amp stage as shown on the schematic, but the actual
pre-amp before the power amp board.

I would also suspect any electrolytic capacitors in the signal path, like
c500/550 for one due to the extreme age of the unit. C505/555 would also be
suspect due to its location in the feedback path.

Due to the age I would probably start with the capacitor ESR meter as
capacitors commonly go bad with age. There are not that many to check and
it owuld rule out one common failure source.

David

Arvid Puschnig <arvid@sbox.TUGraz.at> wrote in message
news:3f3aef6e$0$32074$91cee783@newsreader01.highway.telekom.at...

"Arvid Puschnig" <arvid@sbox.TUGraz.at> wrote:

Ok, I'll put it up the next 2 days.

Ok, here it is:
http://www.sbox.tugraz.at/home/a/arvid/TA-AV501R/
It's the schematics of the Speaker Board and of the Power Board.
Please tell me if file sizes and quality are ok...

arvid