Overheating with low bias...

[EchoWars]

I have a Sony TA-F555ES integrated amp. It had problems with the protection (which is now fixed), and of course I thought I'd set the bias and put it all back together.

Found the measuring points, and basically read '0.00V'. So I check the manual, and it says to adjust to read 100mV. So I start to crank it up slowly, and even as I got to a reading of about 50mV, I noticed waves of heat rolling off the heatsink... Wha..??

I turn down the bias to read about 1mV, and power off. I suspect that the collector resistor is not what Sony says. If you look at the collector resistor in the pic (R339), Sony has it labled '1 ohm 5W', so if indeed that was the value, 100mV would result in 100mA. But on examination, I see this dinky resistor the size of a normal 1/4W film resistor, and upon measuring it, it appears to be a 0.1 ohm instead of 1 ohm. No wonder 50mV was freaking it out...was pushing 500mA through it!! Luckily, nothing fried.

OK...so if it's really a 0.1 ohm, then set to read 10mV across it to get 100mA. So I set it, and the output transistors (2SC2922 & 2SA1216) get too hot to keep your finger on after just a minute or two, and the heatsink is shedding waves of heat too. Still confused, I drop the bias to where I read 1mV across the resistor. Set here, it gets fairly warm...not blistering, but a lot warmer than what you would expect if it had only 10mA of current through the outputs.

Almost all my amps are TO-3 output devices. On most of them they want a bias of about 50mA, and with that set, the heatsinks (even fairly small ones) get 'warm', and the transistors themselves 'feel' to a finger to be not a whole lot warmer than room temp. So these MT-200 devices getting as hot as they do with what is 'supposed' to be 10mA through them pretty much shocks me.

Another thing that's wierd...the size of the resistor. If it were indeed a 0.1 ohm 1/4W, it couldn't pass more than about 1.6A.... The color code on the resistor is tough to read buried where it is, but appears to be 'brown black gold gold'. Seems that would be a 1 ohm resistor, but a 250mW 1 ohm wouldn't last very long on the collector on the PNP device.

The parts list says it's a '1 ohm, 5%, 1/4W'. No way, even though that's what it appears to be. To confuse matters more, in the text where Sony is bragging about how wonderful this amp is, they say this:

The resistance of the emitter resistor (authors note: there is no emitter resistor!!) used for the amp's bias network is 0.1 ohm, the lowest possible value which will keep the output stage stable and output distortion mimimum.

I'm about ready to set the voltage drop across the resistor to 1mV and call it a day. But is there a possibility that it is a 0.01 ohm instead? With my meters (nice Fluke ones), I don't have the kind of resolution to measure values like that...

Opinions?

(sorry for the length...this is just wierd and I wanted to at least try to be clear..)

[Jennice]

Hmmm... What an odd use of D305,D306

Anyway... Is TP2 connected to the negative supply rail, or is there more to it than can be seen on the schematic section you've scanned?

What is the potential (rel. to GND) of TP1? It should be about
"-61V" according to the schematic.

I know it sounds nasty, but have you tried to un-solder R339 and connect an ampere-meter to the solder-spots in stead?

100 mA idle current is quite high for a class B amp (as they claim it to be), and it will cause quite some heat.

You could also try to switch the resistor to a new one which you KNOW is 1 ohm.

Jennice

[Tube_Dude]

Hi Echo

I think the the resistor is effectively a 0,1 Ohm 5 w resistor.

But it can be also a 1 Ohm resistor with 0,25 W that is only used for calibration purposes and after calibration it must be shorted.

About the heat issue i suspect that the problem with your Sony is oscilation .

Regards

[Jennice]

Interesting thought, Tube Dude!

I didn't give it much thought that there is done nothing (nothing visible on the schematic shown, anyway) to prevent oscillation!


EchoWars,
Do you know someone with an oscilloscope? If so, bribe them to bring it to your house, or bring the amp to the scope, 'cause it'll really help you tracking the problem.

Jennice

[EchoWars]

Originally posted by Jennice
Hmmm... What an odd use of D305,D306

It just looks funny because of the way Sony drew it...redraw with the diodes in the vertical, and it looks more normal.

Anyway... Is TP2 connected to the negative supply rail, or is there more to it than can be seen on the schematic section you've scanned?

Yes...goes right to the supply cap.

What is the potential (rel. to GND) of TP1? It should be about
"-61V" according to the schematic.

About the same as TP2...at the moment, I have the voltage drop across the resistor set for 1mV. Heatsink still gets a lot warmer than you would expect.

I know it sounds nasty, but have you tried to un-solder R339 and connect an ampere-meter to the solder-spots in stead?

I'm thinking about this, but the resistor is bloody difficult to get to. Thought I'd see what someone else thinks first, and maybe if anyone else has opened up one of these.

100 mA idle current is quite high for a class B amp (as they claim it to be), and it will cause quite some heat.

You could also try to switch the resistor to a new one which you KNOW is 1 ohm.

How to choose a value? The circuit description text says '0.1 ohm', but the schematic and the parts list say '1 ohm', and the schematic says '5W' but the parts list says '1/4W' (no way does a 1/4W 1 ohm resistor belong in there). If I replaced, I'd be tempted to put a 0.1 ohm 3W or 5W in there...or maybe a 0.5 ohm 5W (since I have a bunch of these around).

Edit: Sure...got a nice Tek scope right here. No oscillation on the output, at least none that a 100MHz Tek can see.

[Jennice]

Ok, here's another suggestion (assuming that you have ~0V DC on your output terminal).

Connect a sine generator to the input, and monitor the output.
Initially (at zero bias) you should have some cross-over distortion.
When you reach the point where this distortion is gone, does it still get hot (not caring what the exact idle current is?)

Jennice

[EchoWars]

DC offset sits about 0.1mV (it has a servo).

I tried your idea. I have a Tek 4084 distortion analyser, which has an output that allows you to view the output of the amp minus the input signal (in essence, the distortion component). On the scope, I can superimpose the signal with the distortion, and I've done this before to view crossover distortion (superimposing just helps to see where the distortion is occuring on the wave). However, even with a very low-level signal (where the crossover distortion ought to be more obvious) and the voltage drop across the resistor set to '0.00V', I see no crossover distortion whatsoever.

Odd...

[Tube_Dude]

Have you inspected if the resistor is not shorted? (wire in the underside).

[Jennice]

I'm starting to get a bad feeling about this.

Have you checked that your transistors are intact?
I am concerned that your output stage has a faulty transistor, and is only kept in place by the DC servo.

As soon as the bias is turned up, the faulty transistor causes an abnormally high current to flow. Just a theory. Other than that, maybe a driver is shorted, making the thermal compansation inaccurate?

Jennice

[Eva]

I suggest temporarily soldering a 1 ohm 5W resistor in place of the unknown one, then adjusting bias and then placing back the old resistor

If there is enough space on the board, you may even leave the 1 ohm 5W resistor in the board and solder the old 'unknown' resistor [or a piece of wire] to its legs in orden to ease further bias adjustments [just desoldering one side of the old resistor/bridge]