I am very sad, my rotel RA-931 with over 20yrs service decided to blow its power supply fuses.
I tested the amplifier and found that the LH channel Output transistors had failed. I replaced all output stage transistors and replaced the capacitors in the power supply stage.
When I fired it up there was hum on the LH stage so i switched it off before it cooked itself. I did check the bias and found this to be at 1.5VDC when it should have been 4mvdc. Adjustment of the Bias pot made no difference.
Not sure where to turn next, I thought maybe I replace the pre-amplifier stage, any pointers would be greatly appreciated.
I tested the amplifier and found that the LH channel Output transistors had failed. I replaced all output stage transistors and replaced the capacitors in the power supply stage.
When I fired it up there was hum on the LH stage so i switched it off before it cooked itself. I did check the bias and found this to be at 1.5VDC when it should have been 4mvdc. Adjustment of the Bias pot made no difference.
Not sure where to turn next, I thought maybe I replace the pre-amplifier stage, any pointers would be greatly appreciated.
The first thing to do is to always make sure you use a DBT (dim bulb tester) to limit current and protect the output transistors while you are fault finding on it.
Driver transistors will often fail (or be the cause of the initial failure) and should always be replaced. All semiconductors should be obtained from reputable suppliers as there are lots of fake and substandard parts around.
Driver transistors will often fail (or be the cause of the initial failure) and should always be replaced. All semiconductors should be obtained from reputable suppliers as there are lots of fake and substandard parts around.
That was fun....
Built a dim bulb tester using an old extension lead and a light fitting with an integrated switch. I fitted a 40W bulb as the Amp is rated to 30W.
Switched it on and the inrush current lit the bulb but it immediately dissipated and the bulb was not visibly glowing.
Still getting hum on one channel, which leads me to think it is not the power supply that is causing the hum as it is a shared circuit. That would imply I have an issue at the pre or power stages. I tested the output of the power supply and it was 72vdc, which is what it says it should be on the diagram.
Was thinking the hum may be excessive gain or an open circuit. Was thinking I will unsolder the transistors and test to see if any have failed. Any advice if there is anything else I should consider?
Built a dim bulb tester using an old extension lead and a light fitting with an integrated switch. I fitted a 40W bulb as the Amp is rated to 30W.
Switched it on and the inrush current lit the bulb but it immediately dissipated and the bulb was not visibly glowing.
Still getting hum on one channel, which leads me to think it is not the power supply that is causing the hum as it is a shared circuit. That would imply I have an issue at the pre or power stages. I tested the output of the power supply and it was 72vdc, which is what it says it should be on the diagram.
Was thinking the hum may be excessive gain or an open circuit. Was thinking I will unsolder the transistors and test to see if any have failed. Any advice if there is anything else I should consider?
The first checks are always the same.
1/ Measure and confirm the rails are reasonably correct (they will be lower than normal due to the bulb). You need to do this for both channels. Confirm the supplies are correct on the output transistors (be careful not to short anything).
2/ Confirm there is no residual DC voltage at the amplifier output (measure before the relay of course). Anything over 0.1 volts indicates a problem.
3/ Conform the bias current is correct/adjustable. If it is, then for test purposes reset it back to the lowest value.
Don't change parts in hope, careful voltage readings should show where the problems are.
1/ Measure and confirm the rails are reasonably correct (they will be lower than normal due to the bulb). You need to do this for both channels. Confirm the supplies are correct on the output transistors (be careful not to short anything).
2/ Confirm there is no residual DC voltage at the amplifier output (measure before the relay of course). Anything over 0.1 volts indicates a problem.
3/ Conform the bias current is correct/adjustable. If it is, then for test purposes reset it back to the lowest value.
Don't change parts in hope, careful voltage readings should show where the problems are.
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