This is a similar front end from an old simulation. Look at the offset voltage and its polarity as the transistors are changed in just one of the pairs. In simulation, any device of the same number is identical to every other of the same type. 100% matched in every possible way.
Ideally this amp needs a manual method of tweaking the offset or a DC servo adding.
Ideally this amp needs a manual method of tweaking the offset or a DC servo adding.
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Hi ssi,
well done, they both work again !! Congrats
according to the manual you can use this amplifier as bridged mono amplfier with 180W/8 Ohm output power. This is only valid, if the connect speaker are > 8Ohms. Do not connect speakers with <8 Ohms in bridged mode. Try to find the operation manual for the RB-970BX on the net for more detailed informations.
BR
Günni
May change 2SA1208 + 2SC2910 as they were next in line for failure after
Q621: 2SA1376
Q619: 2SC3478
from the original very burned resistors.
They seem to be getting quite warm comapared to the right hand channel. Only £3 for 2 pairs from China from the seller I purchased Q621/619. They specialise in audio and have 99.8% rating from 27K sales so happy to risk it for this project.
Q621: 2SA1376
Q619: 2SC3478
from the original very burned resistors.
They seem to be getting quite warm comapared to the right hand channel. Only £3 for 2 pairs from China from the seller I purchased Q621/619. They specialise in audio and have 99.8% rating from 27K sales so happy to risk it for this project.
If they are getting warmer than the other channel then that suggests either the DC conditions are incorrect or there is oscillation.
Make sure the DC voltage is the same across R621 and R623, and that the voltages are the same between channels. You should see around 1.2 volts across each, with each transistor dissipating around 700mW.
Also make sure those two resistor values are correct. If you have replaced them then check they are the same value as fitted to the good channel.
If the voltages agree and the resistor values agree then any temperature difference is down to either perceived differences or differences due to physical layout with one side seeing better airflow and cooling. W=I*V and so if those values are the same between channels then the heat produced is the same as well... instability and oscillation excepted.
Make sure the DC voltage is the same across R621 and R623, and that the voltages are the same between channels. You should see around 1.2 volts across each, with each transistor dissipating around 700mW.
Also make sure those two resistor values are correct. If you have replaced them then check they are the same value as fitted to the good channel.
If the voltages agree and the resistor values agree then any temperature difference is down to either perceived differences or differences due to physical layout with one side seeing better airflow and cooling. W=I*V and so if those values are the same between channels then the heat produced is the same as well... instability and oscillation excepted.
Changing transistors based on a suspicion that they may be partly damaged is not a good idea. In virtually all cases of properly designed circuits, small signal transistors either remain 99.9% OK or they are toast in the blink of an eye. Unlike large resistors or electromechanical devices, there can be no middle ground when failures occur in milliseconds. Sure, there have been some manufacturing design flaws that resulted in gradual failure of certain small signal transistors but they are well known to service people.
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