A friend of mine asked me to fix his old Philips AH684 FM receiver / amplifier. It had crackling pots and switches. So I took everything apart, disassembled the pots and switches and cleaned them. I also re-capped the tone control board.
After assembly I wanted to check the bias of the 'main amp drive unit'. Although I have several versions of the service manual I was unable to find the testpoints. Due to my lack of knowledge and experience I am also not able to determine the test points by going through the circuit diagrams. I hope that someone can help me out:
After assembly I wanted to check the bias of the 'main amp drive unit'. Although I have several versions of the service manual I was unable to find the testpoints. Due to my lack of knowledge and experience I am also not able to determine the test points by going through the circuit diagrams. I hope that someone can help me out:
The circuit is missing the output stage...
I would expect the bias to be measured across the 'usual' emitter resistors located on the output transistors. Typically these may be in the 0.22 to 0.47 ohm region and the bias typically around 50ma for a commercial amp like this. So calculate the volt drop expected across the resistor to set the bias current.
Point 8 on your diagram will be the main output line and I think points 6 and 11 will be connected to the emitters of the outputs. So I would guess the appropriate bias voltage (whatever is specified) will be measure across these points.
If you have the rest of the circuit it would all make more sense. What value are the emitter resistors ?
I would expect the bias to be measured across the 'usual' emitter resistors located on the output transistors. Typically these may be in the 0.22 to 0.47 ohm region and the bias typically around 50ma for a commercial amp like this. So calculate the volt drop expected across the resistor to set the bias current.
Point 8 on your diagram will be the main output line and I think points 6 and 11 will be connected to the emitters of the outputs. So I would guess the appropriate bias voltage (whatever is specified) will be measure across these points.
If you have the rest of the circuit it would all make more sense. What value are the emitter resistors ?
@Mooly Thank you very much for taking the time to help me 🙏
Sorry I missed to post a picture of the output stage. It looks very simple.. sadly I couldn't find a version of the service manual where the circuit schematic of both the driver stage and output stage are drawn next to each other..
I will check the voltage drop over R651 and calculate the corresponding idle current.
But I thought that the bias setting instruction referred to the driver stage, but as I said I have next to zero experience with (old) amplifiers. I am just too young..
Sorry I missed to post a picture of the output stage. It looks very simple.. sadly I couldn't find a version of the service manual where the circuit schematic of both the driver stage and output stage are drawn next to each other..
I will check the voltage drop over R651 and calculate the corresponding idle current.
But I thought that the bias setting instruction referred to the driver stage, but as I said I have next to zero experience with (old) amplifiers. I am just too young..
0.39 ohm is quite a high value by todays standards... that helps with thermal stability though.
14mv across 0.39 ohm is 35 milliamps. That sounds about right for a commercial design where heatsinking capacity is limited.
When you measure the voltage do it with no speaker attached. This is because any small DC offset will add or subtract from the reading you see. Don't be surprised if the reading wanders as it warms up, just aim for around 14mv with it all warmed up. Either resistor can be used for the measurement.
14mv across 0.39 ohm is 35 milliamps. That sounds about right for a commercial design where heatsinking capacity is limited.
When you measure the voltage do it with no speaker attached. This is because any small DC offset will add or subtract from the reading you see. Don't be surprised if the reading wanders as it warms up, just aim for around 14mv with it all warmed up. Either resistor can be used for the measurement.
I would also suggest just making a note of what the readings are before altering anything because if no semiconductors have been replaced then the bias should still be correct.
OK 👍
The one thing I'm unsure on is whether the 14mv is meant to be across both or just one resistor. Its a difference that would halve or double the current depending how you look at it. That is why I think you should measure first before altering anything. See what it is running at before doing anything.
I'm starting to think it could well be across both.
This output stage is a 'Quasi Complementary' type with two NPN output devices and so the required current is a little open to debate.
The one thing I'm unsure on is whether the 14mv is meant to be across both or just one resistor. Its a difference that would halve or double the current depending how you look at it. That is why I think you should measure first before altering anything. See what it is running at before doing anything.
I'm starting to think it could well be across both.
This output stage is a 'Quasi Complementary' type with two NPN output devices and so the required current is a little open to debate.