General biasing procedure when no documentation is available

[bsco]

Good afternoon to everybody.....since this pandemic has hit I have had people dig out their vintage stereo equipment and looking at getting the stuff repaired.....I do realize that there is not much money in it but it will help keep me afloat during these uncertain times....anyway, due to the lack of documentation what would be the best way to approach preforming bias adjustments on old stereo equipment...I have checked on youtube but that is questionable...I figured that using a signal generator and scope there should be a way to get me in the general ballpark without blowing up output transistors as most of those are no longer available and the suggested replacements for them are pretty much obsolete as well.....any thoughts, comments etc is greatly appreciated.....and STAY SAFE out there....as this virus is getting worse instead of better....
Cheers,
Bernie

[xtian]

Are we talking about tubes or SS?

[glebert]

Are we talking about tubes or SS?

He said output transistors so I think it is safe to assume SS.

[bsco]

Sorry about that...I am referring to Solid State units....
Cheers

[Enzo]

Send a sine wave through it, set it to coldest setting, and scope the output. See the crossover distortion notch? Now adjust the bias warmer until the notch JUST disappears. And just the tiniest pinch further. There you go.

Another cruder method, monitor mains current draw, start at coldest, and slowly advance the bias. At some point the mains current will start to rise as you adjust. At that point, stop, and back off the tiniest bit.

[bsco]

Thanks Enzo.....I just found a bunch of 50W 8 ohm resistors that I had poked away...I can use them for a dummy load....I have one stereo there..a Sony...it had no sound on one side...a driver transistor was bad....I had replaced it with something that I had thought would be a suitable replacement but as soon as the unit was turned on, the bias for that channel skysocketed immediately and took out on of the outputs....so I have to dig through my old 70's electronics that I have collected over the years,(I don't throw anything away), and I managed to find an original driver for the unit....now I have to find the original output or a suitable sub from back in that era.....Thank you for the advice...I will check out that procedure tomorrow when I am over to the shop as I have another vintage stereo over there..it is working but I wanted to check the bias.....and see if it needed to be adjusted....
Cheers

[xtian]

Good point! For a SS amp, do you have to have a load connected to set the bias properly?

[The Dude]

Generally not. Different manufacturers have different procedures, but most SS amps are biased without a load.

[bsco]

Good point! For a SS amp, do you have to have a load connected to set the bias properly?

I would think so....you would need to draw some current I guess to scope the output....I could be wrong and I am sure that someone here will correct me.....correction...the Dude just posted...looks like I might not need a load after all....I will try it that way first and then check with a load and see how it all pans out...

[Helmholtz]

As long as there's no DCV at the output (which would indicate an amp problem) it actually makes no difference for bias adjustment if a load is connected or not.
You definitely don't want any bias current to run through the speaker.

IMO it is safest and most reliable to adjust bias without load as even a DCV of 0.1V at the output would cause a bias imbalance with a load connected.

[bsco]

Thanks Helmholtz.....I wasn't quite sure about it... and, where these amps are so old and in storage for so long, they need to have some of the more critical caps replaced, like output,power supply/protections ckts....(it probably wouldn't be feasable for a full recap as these units are not all that valuable..... but a partial one would probably be ok and it would keep the cost reasonable if the unit could sound good.... but will not be like new again)........if there is even a small amount of dc offset or imbalanced bias voltages, then more than likely leaky caps in the power amp section would be a logical suspect....these people don't really want to spend a lot of money so I won't probably make much from a repair like this, but right now something is better than nothing..I'll check it out tomorrow and get back here...
Cheers

[Mick Bailey]

I initially set the bias cold and then re-check after the amp is warmed up - often there's some drift when warm despite temperature compensation. Sometimes this doesn't track the temperature change to well.

[bsco]

Getting back here...I took some time off....set up the bias after and ran it for an hour or so while I did some paper work...worked out good...looks like I will have a couple of more to do if they get dropped off to me....Thanks for all the advice everybody......
Cheers