Hi
I have measure up my amp after repair of power supply.
However the problem (flat amplifier) is to high voltage,
2sk43-1 q501 q601.the rank is right idss 2.
r502 is ok, the rail is 37v but when i mesaure the 2sk43 the drain voltage is around 21v(7.5v)
I changed the R503 value to 1315k and get 7.5v
Same problem with the other jfets q504 q604.
I'm little lost here any ide what is going on?
Do i miss something?
br Tommy
I have measure up my amp after repair of power supply.
However the problem (flat amplifier) is to high voltage,
2sk43-1 q501 q601.the rank is right idss 2.
r502 is ok, the rail is 37v but when i mesaure the 2sk43 the drain voltage is around 21v(7.5v)
I changed the R503 value to 1315k and get 7.5v
Same problem with the other jfets q504 q604.
I'm little lost here any ide what is going on?
Do i miss something?
br Tommy
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The first hint is when you said--"repair to the power supply " , those small signal fets are low noise - high gain but low voltage working .
Have you checked them to see if they are "blown " which would be open circuit thereby showing a high voltage at the drain ?
What you have done by upping the drain feed resistor is limiting the voltage to the drain that isn't the way to go its designed for the original resistor value put it back to--750K ohms then check out the fets .
Have you checked them to see if they are "blown " which would be open circuit thereby showing a high voltage at the drain ?
What you have done by upping the drain feed resistor is limiting the voltage to the drain that isn't the way to go its designed for the original resistor value put it back to--750K ohms then check out the fets .
Hi!
Someone have mess with preamp before, mixed the rail voltage on one channel to adjust the output signal (phono2), and the jfets on phono1 was gone.
I revert to the original values on the resistors and replaced a pair off the broken jfets with two matched 2sk170gr idss 3.8 on the phono1
The q904 and the d 909 was blown.
I replaced them and tuned in the correct voltage 150v and 37v.
when i put back the original 750K, i got 21v drain and around 0.38v source.
next step check the signal fets 2sc634 on the powersupply.
Okej the amp sounds good, but i want the repair should be done correct 🙂.
Someone have mess with preamp before, mixed the rail voltage on one channel to adjust the output signal (phono2), and the jfets on phono1 was gone.
I revert to the original values on the resistors and replaced a pair off the broken jfets with two matched 2sk170gr idss 3.8 on the phono1
The q904 and the d 909 was blown.
I replaced them and tuned in the correct voltage 150v and 37v.
when i put back the original 750K, i got 21v drain and around 0.38v source.
next step check the signal fets 2sc634 on the powersupply.
Okej the amp sounds good, but i want the repair should be done correct 🙂.
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You say now-quote- "the amp sounds good " if it sounds "good " with a big difference in the fet drain resistor which is not to the original circuit design then just leave it as it is but that would not be my choice .
You either do it right or not at all .
Fact - its the wrong value of resistor therefore the value of some component has changed elsewhere.
You either do it right or not at all .
Fact - its the wrong value of resistor therefore the value of some component has changed elsewhere.
yes, i suspect that some preowner have mix or try to fix something.
the big question is where to start.
I can se that on the powersupply the 2sc634 is replaced with KSC1815 ,
is that a problem?
the big question is where to start.
I can se that on the powersupply the 2sc634 is replaced with KSC1815 ,
is that a problem?
He has uprated it ,I have used those BJT,s in the past for audio but not for a PS .
The problem ?--- he shouldn't have changed it to that audio one , the 2SC634 has a typical gain of 70 Hfe
The 2SC1815 can have a gain up to 700Hfe .
When you start increasing the gain on a power supply designed for much lower gain active devices you are bound to get voltage problems.
Change it back.
The problem ?--- he shouldn't have changed it to that audio one , the 2SC634 has a typical gain of 70 Hfe
The 2SC1815 can have a gain up to 700Hfe .
When you start increasing the gain on a power supply designed for much lower gain active devices you are bound to get voltage problems.
Change it back.