JC1 bias setting

[Aubi]

Is it difficult to adjust the bias setting on my JC1's? Well I know how to adjust it, but how is it done correctly?
The reason I want to know this, is that one of them is a bit warmer in low position than the other, and it seems to me as a good idea to check this out since they are few years old now.

[Glen B]

Setting bias is not difficult, but you will need to find out the factory recommended value by contacting Parasound to obtain this information. You will need to measure voltage across one of the output transistor emitter resistors with a digital multimeter set to the mVDC scale, and slowly adjust the bias trimpot to obtain the recommended value. Parasound should have other recommendations such as allowing the amp to warm up for a set period of time before commencing to make bias adjustments, and allowing time for the output stage to settle after every adjustment that is made.

[Aubi]

Thank you! I will contact Parasound to obtain this information.

[Glen B]

That is, provided they're willing to release that information to you. See the bias trimpot and emitter resistor locations pictured.

[Aubi]

What I will do first, is to do the mesurement. If both amplifiers are have the same value, maybe I will settle with that.
Is this mesurment a good way to reveal if any of the transistors are not working anymore?

[Glen B]

When an output transistor "stops working" it usually fails suddenly and completely. If you had that problem, you would know it -- the amp would have gone into protection mode or blown the rail fuses. If you can't get the bias information and the amp sounds fine otherwise, IMO just leave it alone.

[Aubi]

The reason why I started wondering about this in the first place, is the fact that I resoldered the output transistors a week ago. Before that, one of them had started making noises, and the other one was more or less cold. The solder condition was bad, and it seems to me that a lot of transistors were in "resting mode"

Now I am wondering if one of the previous owners have done any adjustments on them based on the bad solder state. But then it would not matter, since the bias adjustment is done from mesuring the output of one of the transistors. Am I right?

[Glen B]

That is interesting information you did not share before. Bias level is checked by measuring the voltage across one of the emitter resistors. This tells you how much current is being drawn by using Ohm's Law (V/R=I), where V is the voltage read across the emitter resistor, R is the value of the emitter resistor, and I is the current drawn by the transistor. I personally like to check voltage across each transistor on the rail versus a single device, because of slight differences in current draw, and adjust my final bias figure accordingly.

Measuring the voltage across each emitter resistor will help to identify any transistor that is bad and not drawing current. If you have any bad transistors, it is highly recommended that you replace all of them on the same rail with a gain-matched set. Don't replace individual transistors because gain differences will result in unequal load sharing among them, which is bad for device longevity.

You can't just go out and buy gain-matched transistors. Only Parasound would have matched sets available. To gain-match, you would need to buy a bunch of transistors, test them individually, and keep only the ones that are close. Its a time-consuming and costly process. You may also need to install new mica or silpad insulators, and thermal grease between the transistors and heatsink. Its all a lot of work that needs to be done right. IMO, if you haven't done this work before, a JC-1 is not the amp you want to practice on.

If you discover any dead transistors, it may be best to just send the amp to Parasound for service.

[Aubi]

I have measured the voltage across each emitter resistor. It seems to me that all the transistors are working, but not as well matched. The difference between transistors on the same rail is up to 5mV in low level.

More interesting is the difference between the two amps. The low level is 10-13mV, and the high level 15-19mV on one. The other has a low level of 15-20mV, and a high level of 22-27mV.

It prooves my initial conclution that they work more or less the same having one in high position and the other in low.

[Glen B]

5mV difference among as many transistors as you have on the same rail is actually quite good. More important than the range between the lowest and highest readings is the sum total each, for the low and high level settings of both amps. Sum up the individual readings of all the transistors in each amp on the low and high settings, and report the results. Also, what is the value of the emitter transistors in the two amps ?

[Aubi]

Also, what is the value of the emitter transistors in the two amps ?

Do you mean the transistor emitter resistors?

[Aubi]

The sum of the readings in low position are 206,5 and 286,5 for the other one. I did not take measurments of all positions in high mode.

[Glen B]

Do you mean the transistor emitter resistors?

Yes. Do they have the same color bands as in the pictures above ? Those are 0.1 ohm, but manufacturers can change values during the life of a production run.

The sum of the readings in low position are 206,5 and 286,5 for the other one. I did not take measurements of all positions in high mode.

The sum of you low position readings should be around 180mV and high position around 396mV. Your 206.5 in low position seems fine, but 286.5 is out of range.

Curiousity got the better of me, so I e-mailed Parasound Service and was sent the JC-1 biasing instructions by Tony. PM me and I will send it to you.

I also received a separate e-mail from Parasound president Richard Shram advising that "there is a significant risk of injury from electrical shock, and one slipped probe can wipe out an entire bank of transistors and emitter resistors and damage the PCB beyond repair. We won’t repair an amp or supply replacement parts for an amp that has been destroyed in this fashion."

He is correct about the damage that is possible if your probe slips and touches the wrong point. Work on the amps at your own risk.

[Aubi]

That's fine, because Tony at Parasound would not give me that information. I suppose they are under pressure by the local importers not to give any support to european customers.

The resistors are the same as in the picture.