Overheating with low bias...

[EchoWars]

Output trannies, drivers, and pre-drivers are good. Trust me on this one, I know. Plus, output looks just too good to be the result of an amp with transistor problems. When the only symptom is excessive heat and there's no oscillation, bias has to be the issue: just have to figure out where to set it.

I see no wire under the resistor, and yes, the Fluke does tend to tell me that it's a 0.1 ohm. I mean, it doesn't measure as a short...just close to one. And, of course, both channels measure the same in all respects...resistance values to power-on behavior.

Eva... What's the possibility that...the resistor in the amp now is a 0.1 ohm, and proper bias is when I get 10mV across it, and that the amp just runs really damn hot? I'm not used to output transistors in a Class B amp running almost hot enough to burn a finger (maybe a little bit of exaggeration, but they are certainly hot by any measure). What I think I'd find if I did what you suggested is that, with 100mA through the outputs, they'll be blazin' hot. Do these MT-200's just run that hot?

I dunno...maybe I should stick with the original plan...set for 1mV (what seems to be 10mA) and call it good. I cannot see or measure any crossover distortion, so I'm not ready to begin losing sleep over it.

Speaking of which, I badly need some. I'll check back later, but I really appreciate your inputs. Thanks guys!

[Jennice]

Hmm....

I don't like your description of the amp's temperatures. It doesn't sound healthy to me.

Then again, Sony Amp's do tend to run hot. A friend of mine had a sony amp which worked perfectly well. I never really trusted it, though, as it was (even at idle) hot enough to smell that certain scent of hot air when breathing in over the amp (through the enclosure openings).

Could it really be the case that Sony Amp's are rice cookers?

Jennice

[Eva]

I think that brown-black-silver is 0.1 ohms and brown-black-gold is 0.01 ohms. Could anybody confirm this?

This resistor may be actually a fuse resistor to prevent disasters in case of shorted output devices

Heatsink overheating is obviously due to bias current being higher than expected, 100mA with 61V rails makes just 6.1W per output device. The only way to know the actual bias current is to temporarily replace the ?fuse? resistor with one of known value

[Tube_Dude]

Let's put some perspective in all of this.

The resistor according to the schematic is a 1 Ohm resistor.
According to the color bands Brown -1....Black- 0...Gold- X0.1...Gold-5% tolerance.
So the resistor must be 1 Ohm.

As the resistor, (as i stated in a early post) is only for measuring purposes. To be or not in circuit doesn't change the bias setting (because is in the collector circuit) .
So Sony ad NAD usually shunt this resistor after adjustment (for lowering the power supply impedance and limit the dissipation in the resistor)

So what Echo is measuring is the resistance of the short...0,1 Ohms.

If there is no wire under the resistance , then must exist a solder bridge or some point where the short is done.

If Echo take the resistor out of the circuit ,i bet that the resistor is a 1 Ohm one...and even with the resistor out of the circuit the short continue .

Cheers

[EchoWars]

In order to solve this mystery once and for all, I found a way to disconnect the negative power supply lead that runs to R339 and insert an ammeter. I also connected the voltmeter across the test points. On power up, there was 1.5V across the test points, and about 450mA (!!!!!!!!!) of current flowing!! I dropped the current to 100mA, and the voltage across the test points now measures about 0.3mV. Sure, that calculates to a resistance of about 0.03 ohms, but measuring a voltage that low isn't too accurate, so I suspect that the resistor is a 0.01 ohm.

Wierd. Certainly no wonder the amp was trying to go up in flames with it set to 10mV. Also pretty funny that the manual says to set the voltage across the resistor to 100mV.

[Aquarius72]

Hi,

though this is an old thread, in case someone is fighting with this problem, I'll reveal the secret of the mystery bias resistor:

Tube Dude was right. There is a 1 ohm 1/4W resistor which is shorted during normal operation with a tiny jumper PCB soldered to the solder side of the main board across 2 poles near the bias resistor. So if you measure the voltage at the test points, you're actually measuring voltage loss over this wire!

When the jumper PCB is removed, the bias voltage can be adjusted to 100mV. My amp runs pretty cool after I set the bias correctly.

Before I found the jumper PCB, I set the voltage to 1mV. After removing the jumper, the actual voltage over this 1 ohm resistor was 250mV!

[Tube_Dude]

Originally posted by Aquarius72
Hi,

though this is an old thread, in case someone is fighting with this problem, I'll reveal the secret of the mystery bias resistor:

Tube Dude was right.

Thank you, Aquarius.

Like a Portuguese proverb say:

- Is better latter, than never ...

Cheers

[pliedtka]

very interesting how Sony sets the bias...
I got JVC RX-665 receiver with 1blown rear channel that use Sanken 2SB1560/2SD2390 Darlingtons (all 5 Ch) and the bias in the rear Ch is at around 1mA per trans and there is no way to adjust them.
The front Ch bias was @ 5-7 mA per trans. I run freq. resp. with 8 ohm spkr. using Clio and I couldn't believed what I found out:
@0.5 W there was a big dip of 4dB between 50-150Hz leveling to -0.5dB @ 400Hz
@2W the same thing but the dip was 2dB

Finally at around 5W things started to sort of look normal.
So I started to play with bias, running freq resp/2nd, 3rd Harm Dist. to see if I can get it flat. I almost cocked the sucker when the bias hit 100mA I pulled back to around 25-30mA per trans, checked things again and the big dip was almost gone.
Still I have to see if the rears behave like fronts , it is one weird design . I copied the rear Ch onto CAD from the faulty PC board - hope its OK. What do you people think of it ?

Paul

[the amp guy]

Originally Posted by Aquarius72

Hi,

though this is an old thread, in case someone is fighting with this problem, I'll reveal the secret of the mystery bias resistor:

Tube Dude was right. There is a 1 ohm 1/4W resistor which is shorted during normal operation with a tiny jumper PCB soldered to the solder side of the main board across 2 poles near the bias resistor. So if you measure the voltage at the test points, you're actually measuring voltage loss over this wire!

When the jumper PCB is removed, the bias voltage can be adjusted to 100mV. My amp runs pretty cool after I set the bias correctly.

Before I found the jumper PCB, I set the voltage to 1mV. After removing the jumper, the actual voltage over this 1 ohm resistor was 250mV!

Hey, Aquarius72, thank You for clearing this mystery. You really helped me.
I know this is old thread, but anyway, can I ask You something? Can You also help solving the mystery about the V-I board DC BALANCE adjustment? Because, following the procedure from the service manual does not give any results. I believe there are also some steps omitted in the procedure, like it eas with the BIAS setting (omitted was a step for removing the jumper board before the adjusting to 100mV).
Here I put a thread:
Vintage SONY TA-F555ES DC adjustment question
Thanks once again!