Bias Check As Preventive Maintenance?

DreadPirate · 1st June 2006, 06:59 PM

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?

Netlist · 1st June 2006, 09:16 PM

Unless you have the manual or know how to set the bias I'd leave it alone if the amp plays fine. If you plan to give it a major maintenance you could consider replacing the pots and some or more caps.

/Hugo

anatech · 1st June 2006, 09:25 PM

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

Original Burnedfingers · 1st June 2006, 09:32 PM

I would have to say to stay clear of it because I have repaired to many amplifiers that some one thought they were going to tweek the bias and the pot was bad. I don't need to tell you what happened then.

anatech · 1st June 2006, 09:38 PM

The current meter went up and they turned it off in time?

Original Burnedfingers · 1st June 2006, 09:42 PM

The current meter went up and they weren't expecting that to happen. Therefore by by output stage.

I told them not to trust an old bias pot. Did they listen to me? Nope....

anatech · 1st June 2006, 09:52 PM

Hi Joe,
That's how all young people learn.

-Chris

Original Burnedfingers · 1st June 2006, 09:57 PM

Chris,

Not sure if us older people can take the strain.

DreadPirate · 1st June 2006, 11:33 PM

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?

anatech · 2nd June 2006, 01:05 AM

Hi DreadPirate,
Go for it. Carefully.

Hi Joe,

Quote:

Not sure if us older people can take the strain.

Yeah, I'm all burned out.

-Chris

1st June 2006, 06:59 PM	#1
DreadPirate diyAudio Member Join Date: Jan 2006 Location: Northern Virginia, USA	Bias Check As Preventive Maintenance? 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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1st June 2006, 09:16 PM	#2
Netlist diyAudio Moderator Join Date: Jan 2003	Unless you have the manual or know how to set the bias I'd leave it alone if the amp plays fine. If you plan to give it a major maintenance you could consider replacing the pots and some or more caps. /Hugo

1st June 2006, 09:25 PM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

1st June 2006, 09:32 PM	#4
Original Burnedfingers diyAudio Member Join Date: Nov 2002	I would have to say to stay clear of it because I have repaired to many amplifiers that some one thought they were going to tweek the bias and the pot was bad. I don't need to tell you what happened then.

1st June 2006, 09:38 PM	#5
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	The current meter went up and they turned it off in time?

1st June 2006, 09:42 PM	#6
Original Burnedfingers diyAudio Member Join Date: Nov 2002	The current meter went up and they weren't expecting that to happen. Therefore by by output stage. I told them not to trust an old bias pot. Did they listen to me? Nope....

1st June 2006, 09:52 PM	#7
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Joe, That's how all young people learn. -Chris

1st June 2006, 09:57 PM	#8
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Chris, Not sure if us older people can take the strain.

1st June 2006, 11:33 PM	#9
DreadPirate diyAudio Member Join Date: Jan 2006 Location: Northern Virginia, USA	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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