Bias Check As Preventive Maintenance?

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Should I check and adjust the bias (via voltage across emitter resistor) on my amps as a preventive measure or leave alone until I have a problem? I don't want to tamper with the adjustment pots if not absolutely necessary, I am guessing this could be a problem with pots that have stayed set for 30 years (ie vintage receivers), is this the case? Are these pots cleanable like other controls using Deoxit, is this a good thing to do while I have the unit open doing other repairs on a typical unit?
 
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Hi DreadPirate,
I'm with Hugo on that. It would not hurt to record the values and leave them on the inside. Turn it on and let it idle for 20 min. with no load or signal first.

Yes, if you need to adjust the old control, you can clean the slip ring and contact with deoxid. Work the control a little and it should be fine. There will be a small indent you can feel in the position it has sat for so long in. Start there if you need to adjust it.

-Chris
 
There is alot to be said about not screwing with things that appear to be working and I wish I heeded my own advice a thousand times in the past, but shouldn't I at least measure the bias and see if it is close to what the manual says? Or am I just tempting myself into disaster?
 
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Eleven years ago, I did this once to check the bias and readjust the DC offset of an old PS-Audio IIc.

My hand slipped, and somehow I shorted the probe to something else, blowing out the output stage. It now immediately blows the power supply fuse.

I still haven't fixed it, 11 years later. Partly because I don't really like that amplifier much anyway.

If it ain't broke ...

-- mirlo
 
If it ain't broke ...

If it ain't broke don't fix it.

My thoughts also. Its very easy to slip with a probe or to have a bias pot that suddenly goes open.

It not the same thing as checking the oil in a car. Once the pot is set at the factory it just isn't going to magically move itself for the hell of it.

Some thing are better left the hell alone and this is one of them especially for those that lack experience.
 
I probably ought to check my oil...

As a beginner in this field, I feel it is something that one must know how to do, sounds like a basic skill that should be acquired. I am going to test it on one of the not so nice receivers (well, that narrows it down to all the ones I own) that I have good documentation on.
 
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Hi DreadPirate,
Insulate your leads.
Do not be distracted by anything short of blood or flames.
Look for alternate test points if yours are hard to get to.
Accept that some test points need to be connected to with power off first.
Use your head, think at all times with your eyes open.

-Chris
 
Over the weekend I replaced the cheesy speaker output terminals on my Adcom535, which I had bought new in 1986, and when I checked the bias it was almost 50% less than the service manual recommended. Not surprising, since I found the amp very cool running over the years, now with the corrected bias, it sure runs warmer...still sound great after all these years.
 
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Hi AcidJazz,
The bias checking / setting procedure for Adcom involves running th e amp until the heatsinks are hot and then letting it cool. Then you check.

I'll bet your bias is high now. As a warranty service tech I did set the bias current on the low side of normal, but within range. Even with a lower bias, your distortion and sound quality shouldn't change on these unless the output transistors are not matched properly.

-Chris

Hi Joe,
Turning the bias too low is only wishful thinking on the tech's part. A destructive customer will kill the amp even with the bias turned right down. ;) An improper repair will blow up regardless. Life is like that.

You are probably right though.
 
I check bias calculating time (from switch-off to click or fart in speakers) and reservoir capacitance. When there is no output relay this method gives some approximation, when I calculate 30mA, there is probably 0 bias, when I calculate 300mA the bias is a bit too high. Nothing to brake using just mains switch.
 
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