Rotel RA-820BX3 set quiescent method?

[cpemma]

Does anyone have details for setting up the idle current on this amp?

A schematic would be welcomed too.

[jean-paul]

Let the amp warm up for at least 15 minutes before measuring. Measure the voltage on the emitter resistors of the power transistors with volume on 0 and with speakers connected. Do this for both channels. Write the values down and calculate what idling current runs through the emitter resistors. I do not know of the amp you mention but most class AB amps like 50 to 100 mA.

In general there is one potentiometer per channel near the power transistors for adjustment. Now slowly adjust the channel that deviates to the value of the channel you labelled as OK. Please turn it very carefully and do not run too high current. It may destroy the amp.

This is the "no schematic, no service manual, non-scientific approach" When you are not confident with this please have it done by a more experienced person. Or buy the service manual.

[Netlist]

In addition to Jean-Paul, a scope can be real handy to look at the crossover distortion.

/Hugo

[jean-paul]

I just knew I forgot something.....

[derguteton]

Hi,

you can ask Andrew Shaw, service manager from Rotel america to mail you a servicemanual. He is a very friendly and nice guy. If you ask him friendly, I am shure he would do it.

his mail: Ashaw@bwgroupusa.com

If you are successfull, please mail me the SM too:

derguteton@web.de

good luck Michael

[cpemma]

Originally posted by derguteton
you can ask Andrew Shaw...

Mailed him, will pass on any response.

Measure the voltage on the emitter resistors...

First checking the actual value, they look to have been running rather warm, paint a bit flakey and bands brown, brown, brown...

Thanks to all for the help.

[anatech]

Hi cpemma,
One correction to note. Bias current is never measured with speakers or a load connected. A DC offset (small even) may throw off your measurements.

Many consumer amps are set for 10 ~ 30mA of bias current. The crossover / scope method will normally give you bias currents of a couple mA (it doesn't take much) into a load. This will depend on the circuit. Normally we would use 10Khz and a 4 ohm load (fosgate car amps) with a couple volts out. Higher powers at high frequencies will burn out the zobel network.

Factory info is always the way to go if possible.

-Chris

[cpemma]

Originally posted by anatech
Hi cpemma,
One correction to note. Bias current is never measured with speakers or a load connected. A DC offset (small even) may throw off your measurements...Factory info is always the way to go if possible.

Thanks Chris, the factory info I've now got gives the procedure as

From the service manual
Prior to Bias adjustment, run about 5 minutes with rated output (8 ohm) and warm up Power Transistor and Heat Sink.
Set Volume Control to minimum.

There are test points fitted on the board.

First 2 steps: Bias testpoints are near the sink level with each "idle" trimpot. Using TP3(+) & TP1(-) set nearby VR601 to 2.5mV; same with TP4(+) & TP2(-) & VR602.

Final 2 steps: Offset adj. has testpoints 1 & E1 (VR603), 2 & E2 (VR604) located behind each appropriate speaker terminal, set zero.

Thanks to derguteton for the contact, and Andrew Shaw at Rotel for the fast response.

Should add the emitter resistors between the bias testpoints are 0R22 so 2.5mV = 11.4mA

[anatech]

Hi cpemma,
That was easy, now you're off to the races!

-Chris