Most of the time the grease is metal based. The mica washer provides the isolation. You only need to watch that the grease does not bridge the front of the washer to the back, but you should use it on both sides.
It is pretty simple. Put a light bulb in series to the mains cable ~60 W to start with. When you switch the amplifier on it will become bright for a short moment. Then it should turn dim, if the amp is okay. If the light remains bright, that means the amplifier draws too much current. The light bulb will however limit the current so that nothing gets destroyed and you can do some measurements. Careful not to produce shorts with the tips of your meter.Good point! I never heard about it (and you knew that) i will do it , immediatly, can you explain me how to use it??
You should absolutely test some caps to inaugurate that meter.
With music playing the bulb must become brighter, because the amplifier draws more current. Increasing the crossover frequency also increases the current demand, because the subwoofer has to reproduce more music. It is a good sign that the light bulb stays dim, when no music is playing.
The point is to use the light bulb tester with no signal and check whether all voltages are correct. Especially compare the voltages across R45 and R46. They should be the same with opposite polarity or at least similar enough that differences can be explained by component and measurement tolerances. If they are not within, let's say 20-25 % the same, check the components before Q13 and Q14, especially the transistors. If the voltages across R45 and R46 are the same, the heatsinking is probably the culprit.
Switch the amp on and leave it on without signal for half an hour. Then compare the temperatures of Q13 and Q14. Careful, they can become very hot, if something is wrong. Is Q13 hotter than Q14? Add thermal grease.
When everything works fine, check with music. The light bulb will light up and limit the power. When everything still works fine, try a light bulb with more Watts, so the power will be limited less. After a while recheck the voltages across R45 and R46 and compare the temperatures of Q13 and Q14 again. Still everything fine? Try without the bulb tester.
The point is to use the light bulb tester with no signal and check whether all voltages are correct. Especially compare the voltages across R45 and R46. They should be the same with opposite polarity or at least similar enough that differences can be explained by component and measurement tolerances. If they are not within, let's say 20-25 % the same, check the components before Q13 and Q14, especially the transistors. If the voltages across R45 and R46 are the same, the heatsinking is probably the culprit.
Switch the amp on and leave it on without signal for half an hour. Then compare the temperatures of Q13 and Q14. Careful, they can become very hot, if something is wrong. Is Q13 hotter than Q14? Add thermal grease.
When everything works fine, check with music. The light bulb will light up and limit the power. When everything still works fine, try a light bulb with more Watts, so the power will be limited less. After a while recheck the voltages across R45 and R46 and compare the temperatures of Q13 and Q14 again. Still everything fine? Try without the bulb tester.
tomorrow, i will make all the test you said.
Now i can say that i noticed that today, q13 and q14 did not warm up (quite nothing) during the bulb test.
Yesterday, when i was testing (and before the burnout) the same trannies get very hot and the heatsink warmed up too.
don't take this as a culprit, but today, i noticed that the the heatsink was not perfectly attched to the frame where the trannies were mounted (it was bended (you remember, all of this should be caused by strokes from the courrier)
So today before changing q13 and q14 i check the whole heatsink assembly and now i am sure that everything is ok for that side.
I used sylicon thermal grease:
between heatsink and frame
between frame and mica sylicon sheet
between mica and transistors.
tomorrow i will post you the values.
The only thing i am afraid of, is that when the crossover go over a defined value, the music get distorted, and it's when the bulb light up more but this is not a natural behaviour believe me....
Now i can say that i noticed that today, q13 and q14 did not warm up (quite nothing) during the bulb test.
Yesterday, when i was testing (and before the burnout) the same trannies get very hot and the heatsink warmed up too.
don't take this as a culprit, but today, i noticed that the the heatsink was not perfectly attched to the frame where the trannies were mounted (it was bended (you remember, all of this should be caused by strokes from the courrier)
So today before changing q13 and q14 i check the whole heatsink assembly and now i am sure that everything is ok for that side.
I used sylicon thermal grease:
between heatsink and frame
between frame and mica sylicon sheet
between mica and transistors.
tomorrow i will post you the values.
The only thing i am afraid of, is that when the crossover go over a defined value, the music get distorted, and it's when the bulb light up more but this is not a natural behaviour believe me....
i have doubt, have to check DC across R45 and R46, with input connected and running? i mean playing music?
----EDIT---
i tested in both way, but i'm not sure i put the multimeter joints in the right point.
do i have to put them like if i was reading the resistance (but with the volt setting in the insturment?)
If yes, it reads with or W/o music input few mV i think that is due to noise i the air.
i don't know how to measure!
i tested in both way, but i'm not sure i put the multimeter joints in the right point.
do i have to put them like if i was reading the resistance (but with the volt setting in the insturment?)
If yes, it reads with or W/o music input few mV i think that is due to noise i the air.
i don't know how to measure!
No signal should be present. The best thing is to terminate the input with a jumper or a resistor.
Exactly. It should show a few mV as a result of the bias current through the resistors.
ok i am triyng to understand, what i am doing is so called BIAS probe isnt'it?
I checked, i disconnected the input, powered up the amp, the lght glow up and then dimmed until it switched off.
Bulb is 25w.
I set the multimeter in volt position, DC. It autoscaled to mV, but it went nuts showing numbers even if it is not connected (is this behaviour correct??)
Then i put the tester joints (don't know the correct english word) in the same position i did to test resistance, but the multimeter (that before was going nuts) it stabilized to 0000 for both resistances.
What am i doing terribly wrong?
sorry for my newbiness....
I checked, i disconnected the input, powered up the amp, the lght glow up and then dimmed until it switched off.
Bulb is 25w.
I set the multimeter in volt position, DC. It autoscaled to mV, but it went nuts showing numbers even if it is not connected (is this behaviour correct??)
Then i put the tester joints (don't know the correct english word) in the same position i did to test resistance, but the multimeter (that before was going nuts) it stabilized to 0000 for both resistances.
What am i doing terribly wrong?
sorry for my newbiness....
Yes. The probe leads act as antennas and pick up whatever electromagnetic signals are around.
Probably nothing. It could simply be that the meter is not sensitive enough for the small voltage that is present. Or that autoscaling puts it to a too high measuring range.
Each mA of bias current across a 0,22r resistor corresponds to 220 µV. Even several mA would only be a few mV and might be below the resolution (and measuring tolerance). You could try to replace the resistors with bigger ones for the measurement. Or desolder one leg of the resistor and make a direct current measurement.
Both shouldn't matter.
With the meter set to Amperes you put it in series. Desolder one leg, connect one probe to that leg and the other to the pad that lag was soldered to before.
Just to be clear, everytime i probe before, i was using the multimeter in Volt position (the new multimeter)
Now if you want me to measure amperers, i will use the old multimeter, it has a scale from uA to 200ma.
I will desolder one leg of each 0,22 resistor and check amperes from the desolder leg and the pad where that leg was soldered to.
i think it's right what i understood, i'll let you know.
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