Bias adjusting on Threshold SA4/e

[Mywing]

I finally managed to buy a SA4/e used and in excellent condition. I only have a small problem: the temperature of the two heatsinks differ by about 4 degrees: 49 to 54 left and right sides. I should probably adjust the bias but the instruction manual in my possession does not say how ...
Among other things I do not know at what point precisely measure the temperature ... I did that in the screw holes of the top cover; would be right?

All the advices about the exact procedure of bias adjusting will be welcome.
Thank you 😎

 

[Nelson Pass]

Screw holes in the top, 50 deg.

There are a couple of potentiometers on the front end board,
one controls the bias, the other controls the offset. It won't
take long for you to figure out which.

😎

 

[Mywing]

Thanks,
...but if the potentiometer to bias adjusting is unique for both channels, how can I remedy the temperature difference between left and right channel?

 

[Nelson Pass]

I would not adjust the channel which is at 49 deg. You would
probably want to reduce the bias of the other channel slightly.

😎

 

[Mywing]

I think I have identified them.
Should be two small blue trimmers one for each channel.
I would not be wrong, but to decrease the temperature I should UNSCREW, right?

Thanks again

 

[Nelson Pass]

You should not touch either pot unless you are monitoring
the current draw. Usually counter-clockwise will reduce the
bias, but don't assume anything.

😎

 

[Mywing]

I thought it was enough to decrease the bias of the channel with the warmer heatsink...
Btw, what is the correct procedure of bias adjusting? My owner manual does not say that.
Also does anyone know where I can find a more comprehensive manual?

Thanks again

 

[Nelson Pass]

There is a lengthy procedure, but it is simple enough to put
a voltmeter on the power Emitter resistors (one for each output)
on the channel and look at the DC voltage across it. You want to
turn it down slightly and then wait a couple hours with the lid on to
see what temperature you get. When you get 50 deg for a
room temp of 25, you are done.

😎

 

[Mywing]

Ok,
which voltage value I would see on the voltmeter?

 

[MashBill]

Ok,
which voltage value I would see on the voltmeter?

That which gives you 50 degrees.......

 

[Nelson Pass]

Check out the values on the other channel which operates
at 49 deg.

😎

 

[Mywing]

Then, I can only identify the resistor to take that measurement.
Could you help to do that? Where is it?
Thanks again

 

[Nelson Pass]

There's a big fat power resistor, usually 1 ohm, next to each
of the output devices. Pick one near the middle and measure
the DC voltage across it. It will be less than 1 volt.

😎

 

[Mywing]

I don't see resistors near output devices... Maybe it need demount heatsinks?

 

[mgm31]

I don't know about the SA4e but I have a SA3 optical bias upgraded to e version with the two blue pots.

I measure 190mV between the 1 ohms resistors. You can see pictures here : www.thresholdlovers.com • View topic - Threshold SA3e.
Your picture is too small to see anything.

I don't know exactly but the 4e is the mono version of the 3.9e, so I suppose 160mV / 190mV to be the good bias.

 

[Mywing]

Thanks for answer mgm.
After patients lenghty attempts through the blue pots, I obtained a temperature at 51 degrees for both heatsink. Now I would to check dc voltage on emitter resistor (I found that it would be 165-170 mV) but I can not see them close to the output devices. Maybe they are on the rear and I must remove the heatsink to see them?
Here is a picture more visible

An externally hosted image should be here but it was not working when we last tested it.

Where are the emitter resistors?


Happy Easter

 

[Nelson Pass]

Forget about the emitter resistors. Use an AC ammeter (maybe
a Kill-a-watt (tm)) to watch the current draw of the whole amp
while you trim the bias slightly downward. Readjust after you
note the temperature after warm-up.

😎

 

[Mywing]

Which value I would see on the meter?

Anyway, adjusting bias only through the heatsinks temperature I obtained 51-52 degr. on both channels... Can I be satisfied or does it is so important to check the current draw?

Thanks again

 

[Nelson Pass]

I would be satisfied.

😎

 

[MrAcoustat]

I would be satisfied.

😎

And if it's good enough for the one and only, it should be good enough for Mying.