Marantz SR14 idling current issues

[iMac]

Have an issue with setting the idling current on my SR14, then I set it to 4mV (30 m warm up according SM) and wait for say 10 m then it can be from 2-6mV.. If I blow gently on the circuit board then it will drop from 4 to 2mv in seconds!

The same if I put the lid on and wait 10 m then it will rise from 4 to 12mV. Should it be so sensitive or is it some problem with the amplifier? It's the same on all five channels..

[djoffe]

The Vbe of bipolar transistors has a temperature coefficient of about -2.1 mV/degree C. If you cool down just one transistor in the output loop by 1 degree centigrade, you change the bias across the resistor by 2.1 mV...

So yes...blowing on a transistor will do that...the "fiction" in these amp design is that everything is kept at the same temperature by thermal coupling...but as you can see...it doesn't take much to upset the apple cart.

[iMac]

OK, so it should be like this, but the next question comes then. How shall I set the idling voltage then, with the top cover on or off!? The SM doesn't say anything about that..

Have found out that it's the tail voltage source (transistor and 2 diode) that is the most sensitivity for temperature change..

[djoffe]

I'm not sure about the quality of my guesses without more info...since you have the service manual, can you scan the schematic and upload it? Then you could point out the device that seems most sensitive, and we could think if it really makes sense, and if so...is there a way to make it less sensitive, and if there isn't, what's the right way to make the adjustment (cover on, cover off?)

[iMac]

It's the D701, 703 and Q705 that I talking about. If blowing on Q705 the current decrease and on D701, 703 it increase.

[djoffe]

Your description makes perfect sense with the schematic. Let's analyze what's going on...Q705 has a collector current of about 0.7/270=2.6 mA. Its collector to emitter voltage is about 63 volts, so it is dissipating 2.6 mA*63 Volts=163 mW. The thermal resistance is 150C/watt, so its temperature rise is about 24 C above ambient. That's a pretty good local heat source. Their hope is that D701, D703 and Q705 are about at the same temp, but that's probably not going to happen owing to the wide variation in dissipation.

Simplest thing to try is place a resistor between the collector of Q705 and the
common emitters of Q701 and Q703. If the R soaks up most of the dissipation, and can be kept away from Q705, D701, and D703, their will be much less thermal nonsense going on. Try a value of 45/0.0026=17 k Ohms.

In addition, you could replace D701 and D703 (which I assume are plain vanilla diodes (is this so?), with a short (for D701) and a green LED for D703. That will put about 1.85 Volts across the LED, so you'll have to raise R713 from 270 to around 1.1/0.0026=423 Ohms. Round it up to 470 or so for safety.

If you make the mods, be sure to dial the bias pot to the low current side before you power up...my calculations are all kind of back-of-the envelope...

Good luck...no warranty expressed or implied...

P.S. - you could use two LEDs instead for D701 and D703...and a higher value of R713, trying to keep the nominal bias current the same. There are pluses and minuses, but it's too long a discussion for now...

[iMac]

But if I put the diodes beside Q705, will the result be like adding the resistor? The diodes is an standard glass type. I was also thinking about changing them to LED, but maybe adding the resistor will do the best result..

As I understand adding this resistor will also give a positive effect on the performance of the amp except the temp improvement, I'm right or?

I had been more sceptic if you had done the calc on a paper napkin

I hope to have some time tomorrow to try this out, it's some job to get the board out of the amp.. I'll test it on one channel first..

[djoffe]

The goal is to get the diodes and the transistor at the same temperature...that will still leave some tempco, but much less, and probably in a safer direction.
You could get everything at the same temp if they were all in intimate thermal contact. You could also get things to stay at the same temp if you could eliminate the heat source that is changing things.

So...if you add the resistor, you eliminate the heat source in the transistor (assuming the resistor doesn't end up on top of the transistor and diodes).
Further, if with the heat source removed, you still could thermally couple diodes and transistor, you'd have the best of both worlds.

[iMac]

It's in place, the resistor, and I have about 34V across it, 10V less than your calc.

It's still sensitive for temperature changes, but seem to handle increasing temp better the before! So I will go for the LED also..


About the question, top cover on or off. What is a normal bias for this kind of amplifier?

[djoffe]

Thanks for the feedback...

What value of resistor did you use, and what was the voltage across it? That will help us nail down the operating current. That's good so we can duplicate it if we change to an LED instead of diodes.

As to cover-on/cover-off...hopefully it shouldn't really matter, but sometimes these amps aren't that stable for bias point...It makes me wonder if we're missing something, or if that's just the way the design is...

I guess you could bump up the resistor value a bit, to get a bit more dissipation out of the transistor...just make sure you leave the transistor at least 5 volts from collector to emitter.