DC is ~36V, (bulb in Dbt 150W )
Kenwood Ka6000
- Thread starter Dart07
- Start date
A bit low but lets check it without the DBT and see how much it moves.
last time I started it without DBT, the transistors in this channel burned out ... now 37.8V still low
I missed that somewhere in the thread. So DON'T power without the DBT for now. Lets do a quick bias check and see where we are at.
Set the meter for DC volts or DC mV if it has that range. Clip the meter leads across R171, polarity does not matter. Again watch the screw and terminal strip.
Turn the amp on and see what the voltage drop across the resistor is and if it is changing over time. It probably will change but it should be gradual. Watch it for a minute or so.
I don't know how fast it gets hot, we don't want to cook things, you have to judge that.
on the meter ~140mV, after a minute it gets warm, not hot, but I didn't test it longer , thank you for Your help
OUCH, that is almost 300mA, way too high. With that static current draw I would expect your bulb to have a bit of a glow.
With the power off locate VR905 on the main amp driver board, note it's position and turn it back and forth from end to end a few times and then place it back where it was.
Put your meter back across R171, turn on the amp with the DBT and slowly adjust VR905 one way or the other and see if the voltage goes down, if it does see if you can get it in the neighborhood of 10mV.
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vr905 is very sensitive and unstable, I set it to 10.2~10.6 because the result fluctuates in this range, after turning it on again I have 12mV but I can't adjust it anymore because it's breaking... after 4 minutes it is not warm
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That is good news and expected bad news. Good news, you can adjust the bias which gives us a good feeling about amplifier health. Expected bad news, the trimmers are crappy and need replaced. I expect they are the open frame carbon style trimmers. Can you grab a picture of the top area of that amplifier board so I can see if that is what you have.
the old trimmers are imprecise but I can replace them later... I connected a meter to the second channel and I got 0.00mV
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That is mostly what I expected to see. That style of trimmer should probably be replaced. You can try to clean them.
I find that the biggest problem area is where the center pin connection and the moving contact slide on top of each other.
The actual resistive element and the wiper are on the other side of the control. You can see it on VR04 in your large picture.
Note the position of them. and gently squirt a little of a deoxit type cleaner on the metal surface and work the trimmer back and forth several times. Return to the previous position. You need a deoxit type cleaner, the no residue quick dry stuff won't work. Obviously you don't do this with the power on.
After the cleaning go back and see if the control behaves better. It will probably still be sensitive but it should smoother and not jump around as much.
Just an FYI, it looks like VR3 may have been replaced at one time?
I find that the biggest problem area is where the center pin connection and the moving contact slide on top of each other.
The actual resistive element and the wiper are on the other side of the control. You can see it on VR04 in your large picture.
Note the position of them. and gently squirt a little of a deoxit type cleaner on the metal surface and work the trimmer back and forth several times. Return to the previous position. You need a deoxit type cleaner, the no residue quick dry stuff won't work. Obviously you don't do this with the power on.
After the cleaning go back and see if the control behaves better. It will probably still be sensitive but it should smoother and not jump around as much.
Just an FYI, it looks like VR3 may have been replaced at one time?
What does "it's breaking" men?
a metaphor, it means that if I want to improve by 5 mV, it changes by 150... it's bad trimmers, I connected a meter to the second channel and I got 0.00mV a bit weird
I'll clean it and there's something else I can adjust for the time being ? I also checked the DC voltage at the speaker outputs, there is a jump to 22v and drops to: Lch ~ 500mV, Rch ~ 300mV, and what about the too low center voltage of 38V instead of 43V?
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I have already set ~43V on both channels, unfortunately without an oscilloscope, only a meter
That is a nice change since it was 38v before. You will find that the bias and center voltage will interact a bit.
Did you try cleaning the bias trimmer pot(s)?
I cleaned them a bit but I'll repeat it again... because the VR906 doesn't work, I move it and the meter readings don't change...
I also ran a little test to check the progress of the repair... I connected the signal and speakers... and it works
, time to restoring...What do you think... will these transistors be suitable? The parameters are slightly better than the 2SC898
BDY56 (Pc): 115 W, |Vcb|: 150 V, |Vce |Vce| 120 V, |Veb|: 7V, |Ic max|: 15 A, (Tj): 200 ?C, (ft): 10 MHz, (hFE), MIN: 20
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