Sony STR-DE597 -B voltage problem
 

Hello,

I'm trying to service this unit (I have three units with the same problem)
and the -B supply for the pre-drivers starts out at -59 (+59 is ok) and then
right before the spk relay closes the voltage drops to -31 volts.

I did trace it to the overload protect circuit. The schematic states it's
the -B switch. One of the pre drivers gets very warm. The unit does not go
into protect mode.

The outputs check out. At this point, I don't know if it is the 7v line or
if it is one of the pre drivers pulling it down after a few seconds.

Any ideas?

Thanks

They do this on purpose to trigger the protection circuit.

It's a "feature", not a defect..

You will find that if you feed a sine wave to 2 (or was it 3?) channels
simultaneously via the 5.1 inputs, you can run to full power into 8 ohms
with no problem. Add one more channel, and it will consistently go into
protect WAY before it gets full power on all channels. I suspect this is to
get away with a bit smaller power transformer.

If you read between the lines in the power specs, you'll see that the full
power specs aren't given with all channels running simultaneously.

I already chased this one myself.

Mark .Z


wrote in message ...
Hello,

I'm trying to service this unit (I have three units with the same problem)
and the -B supply for the pre-drivers starts out at -59 (+59 is ok) and
then
right before the spk relay closes the voltage drops to -31 volts.

I did trace it to the overload protect circuit. The schematic states it's
the -B switch. One of the pre drivers gets very warm. The unit does not go
into protect mode.

The outputs check out. At this point, I don't know if it is the 7v line or
if it is one of the pre drivers pulling it down after a few seconds.

Any ideas?

Thanks

Hello again,

The +60 and -60 volts to the outputs is correct. It does not change. The
volume is at min and no input signal.

The 30v zener that is on the collector of Q692 (BN2F4M) should be 6.3 volts
but it's -31 volts(it seems it starts out about 4.5 to 5v then goes to
-31v).
The emitter of the same transistor starts out at about 4.5v and then
goes down to about 3.5v. According to the Service Manual it should be 6.3v.
The base should be -5.8v but it is -3.5v.
The other voltages on the pre-driver pins seems to jive with the SM specs.

I did trace the 7v back to the 7v regulator. The output of the reg. drops to
3 to 4 volts. The 15v going to the input on the reg stays around 17v.

I am not sure if it's the overload detect amp and Q691, 692 or the 7v line
that has the problem or if it is the upc2581's that have a problem.

Would I be able to desolder all of the outputs and remove the upc2581 pre
drivers and then try to power it on.

Do you think it would destroy anything? I have desoldered outputs before,
but not the pre drivers with out problems.

Note: If I disconnect the Surround Back board from the main board the -59 is
-31v almost instantly at power on.
With it connect to the main board, you get a about 3 sec before the voltage
drops from -59v to 31v.

I know on other STR-DE units I have repaired, the voltage did not drop.

Thanks for your help!
Jimmy

The voltage drops on the one rail to the drivers, not the outputs. If you
remove the 2581 driver(s) you'll just get no DC drive to the outputs, hence
no sound, no bias, no nothing.

Mark Z.


wrote in message ...
Hello again,

The +60 and -60 volts to the outputs is correct. It does not change. The
volume is at min and no input signal.

The 30v zener that is on the collector of Q692 (BN2F4M) should be 6.3
volts
but it's -31 volts(it seems it starts out about 4.5 to 5v then goes to
-31v).
The emitter of the same transistor starts out at about 4.5v and then
goes down to about 3.5v. According to the Service Manual it should be
6.3v.
The base should be -5.8v but it is -3.5v.
The other voltages on the pre-driver pins seems to jive with the SM specs.

I did trace the 7v back to the 7v regulator. The output of the reg. drops
to
3 to 4 volts. The 15v going to the input on the reg stays around 17v.

I am not sure if it's the overload detect amp and Q691, 692 or the 7v line
that has the problem or if it is the upc2581's that have a problem.

Would I be able to desolder all of the outputs and remove the upc2581 pre
drivers and then try to power it on.

Do you think it would destroy anything? I have desoldered outputs before,
but not the pre drivers with out problems.

Note: If I disconnect the Surround Back board from the main board the -59
is
-31v almost instantly at power on.
With it connect to the main board, you get a about 3 sec before the
voltage
drops from -59v to 31v.

I know on other STR-DE units I have repaired, the voltage did not drop.

Thanks for your help!
Jimmy

Transistor Q691 is turned OFF by the overload detection circuit. This puts
R691 and R692 in series with the B- for the driver IC's. The resulting
voltage drop causes an imbalance of the power supplies to the drivers. When
the signal swing bumps up hard against that rail, the signal is then
asymmetrical enough to trigger the DC detect circuit. Kind of ass-backwards,
but it gets the job done. In practice you would really have to get all those
speakers jumping (in surround mode only, really) to trigger the thing.
Like I said, it's a "feature" , not a malfunction.

Mark Z.


"Jimmy" wrote in message
...
How would you go about finding the voltage drop on the one - rail?

I didn't follow you on: "The voltage drops on the one rail to the drivers,
not the outputs."

My thoughts on removing the driver(s) was just to check and see if the
voltage would still drop.

Thanks
Jimmy

Q692 is a "digital transistor" so the 0.6 volt rule does not apply.

Is Q692 turned on ALL the time, or only when more than 3 channel are run up
hard? If it's on all the time, then IC691 might be bad.

You could always swap around your suspect 2581 driver IC with one of the
others and see if it still warms up that way.

If you like living on the edge, you could just jumper R691 and R692 out of
the circuit.


Mark Z.


"Jimmy" wrote in message
...
I haven't figured it out yet. I am still working on the beast.

I disconnected the outputs, but still have the voltage drop. So it is not
related to the outputs.

One interesting note: The base of Q692 is 5.75v. The SM shows -5.6. The
base
of Q691 is -28v, it should be -58v.
With -28v on the base, is the transistor turned on(Emitter= -59.5,
Collector= -59.5v)?

I know .6v is enough to turn most bi-polar transistors. ( I believe that
is
correct)

I guess one of the drivers may be bad, because IC601 gets warm or hot
kinda
fast(depends on how long unit stays on),
but IC701 does not. Maybe a surrounding component.

This POS is driving me insane.

Any ideas?

Many thanks for your help/info!

Jimmy

Do you think living on the edge would cause the magic smoke to escape? ;)

Please keep in mind, there are no speakers hooked up and volume stays at
min. Plus, at the moment, the outputs are disconnected.

Q692 has 5.75 volts on the base. The SM states it should be - 5 volts. Let's
say the Emitter and Collector are 6 volts and the base is at -5 volts,
that's turned on, right? Or would a neg voltage cause it to be off? I don't
know much digital transistors. I guess that is obvious.

I would think if it is + 5 volts it would be turn on.

IC691, pin 5 measured 6.20v, but the SM states it should be -5.8. I mention
this pin because it's not the same as the SM.

If I recall correctly, the voltages on the 2581 are in spec with the SM
(with the outputs in circuit). Still gets hot though.

I did notice that pin 2 of the 2581s is like -32 volts. That could be
because the outputs are not in the circuit. If I leave the unit on for about
3-4 minutes, IC601 (2581) gets hot. One would think it might be bad.


Thanks again,
Jimmy

You need to see if Q691 is turned on, with about .6 volts across the
emitter-base. Hard to say for sure about 692 - it could be leaky. You could
pull it out of circuit and see if the - rail comes back. I'm betting it
will.

Disabling this circuit's ability to lower the - B line won't smoke the
thing, but at consistently high volumes the power transformer might be at
more risk.

It's not generally a good idea to remove the output transistors from a
circuit. The drivers etc will sometimes flame out if you do. This particular
circuit it isn't a problem.

The 2581's normally run pretty warm, but as I said, swap that one with one
of the others and see if it still gets hot, or the other one does.

Mark Z.
wrote in message ...
Do you think living on the edge would cause the magic smoke to escape? ;)

Please keep in mind, there are no speakers hooked up and volume stays at
min. Plus, at the moment, the outputs are disconnected.

Q692 has 5.75 volts on the base. The SM states it should be - 5 volts.
Let's
say the Emitter and Collector are 6 volts and the base is at -5 volts,
that's turned on, right? Or would a neg voltage cause it to be off? I
don't
know much digital transistors. I guess that is obvious.

I would think if it is + 5 volts it would be turn on.

IC691, pin 5 measured 6.20v, but the SM states it should be -5.8. I
mention
this pin because it's not the same as the SM.

If I recall correctly, the voltages on the 2581 are in spec with the SM
(with the outputs in circuit). Still gets hot though.

I did notice that pin 2 of the 2581s is like -32 volts. That could be
because the outputs are not in the circuit. If I leave the unit on for
about
3-4 minutes, IC601 (2581) gets hot. One would think it might be bad.


Thanks again,
Jimmy

Well, it has been awhile, I have been ill.

I finally got this unit to work like it should last Thursday.

First off, Q691 was leaky. That was not the problem, may have been caused by
the original problem, I'm not sure.
I think I did replace both 2581's before I found it had a -60 volt problem.
It could be the other drivers caused this part of the circuit to fail.

I was really tired of taking this thing apart over and over again. So I
replaced the (Q691 Did this first then) Q692, 30v Zener Diode, op amp, and
the other diode that's off the emitter leg of Q692. I don't have the diagram
in front of me.

Now, the Q691 has .6 volts between E-B. It was 0v before. Now those pesky
resistors are not sucking down the voltage.
The 30v ZD shows -29v across it. May be where the -30 volts was going. The
voltage drop across this zener was 0v before replacing the other stuff.

After changing these few components, I really think the 30v Zener may have
been breaking down. But still, it could have been Q692 or the op amp.

I'm accustomed to seeing the zener between ground and some voltage source.
Seeing the zener in this circuit makes my brain hurt.

I would still like to know which of these item(s) was causing the problem.

What would your educated guess be?

Again, THANK YOU very much for your time and help!

Jimmy

If the voltage across the zener was zero before, then the diode was shorted,
probably Q692 was bad too. The zener is just there to drop some voltage
going to the base of Q691.

This whole thing smacks a bit of a lighting or electrical surge problem, but
didn't you originally say you had 3 receivers all alike and with the same
problem?

Mark Z.


wrote in message ...
Well, it has been awhile, I have been ill.

I finally got this unit to work like it should last Thursday.

First off, Q691 was leaky. That was not the problem, may have been caused
by
the original problem, I'm not sure.
I think I did replace both 2581's before I found it had a -60 volt
problem.
It could be the other drivers caused this part of the circuit to fail.

I was really tired of taking this thing apart over and over again. So I
replaced the (Q691 Did this first then) Q692, 30v Zener Diode, op amp, and
the other diode that's off the emitter leg of Q692. I don't have the
diagram
in front of me.

Now, the Q691 has .6 volts between E-B. It was 0v before. Now those pesky
resistors are not sucking down the voltage.
The 30v ZD shows -29v across it. May be where the -30 volts was going. The
voltage drop across this zener was 0v before replacing the other stuff.

After changing these few components, I really think the 30v Zener may have
been breaking down. But still, it could have been Q692 or the op amp.

I'm accustomed to seeing the zener between ground and some voltage source.
Seeing the zener in this circuit makes my brain hurt.

I would still like to know which of these item(s) was causing the problem.

What would your educated guess be?

Again, THANK YOU very much for your time and help!

Jimmy