Hi,
I'm currently restoring a Kenwood KA-6100. Most of the caps are replaced and so far it's sound quite nice.
But I'm not able to get the BIAS current to the specified 20mV. Left: ~50mV, right: ~60mV. The BIAS potentiometers are already on the lowest position...
Any suggestions?😕
merci
I'm currently restoring a Kenwood KA-6100. Most of the caps are replaced and so far it's sound quite nice.
But I'm not able to get the BIAS current to the specified 20mV. Left: ~50mV, right: ~60mV. The BIAS potentiometers are already on the lowest position...
Any suggestions?😕
merci
Both channels? Does the idle current adjust at all? Will it adjust higher? If THm1/2 is open idle current will be high, what are the chances of both channels though? Boot strap capacitors Cm53/54 installed correctly?
Basically your problem is that the voltage on the base of Qm17/18 is too negative. Sometimes the pots are shot, try removing and measuring. The wiper of VRm5/6 should go to zero Ohms at both ends. Full cold would be wiper at the VRm5/6 and Rm87/88 junction, least negative position.
Craig
Basically your problem is that the voltage on the base of Qm17/18 is too negative. Sometimes the pots are shot, try removing and measuring. The wiper of VRm5/6 should go to zero Ohms at both ends. Full cold would be wiper at the VRm5/6 and Rm87/88 junction, least negative position.
Craig
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Yes both channels, with ~10mV difference. Both are higher adjustable. When the amp is cold it's 35mV left and 40mV right. Cm53/54 are replaced and should be fine.
sorry, but I don't get the last sentences..
sorry, but I don't get the last sentences..
Another issue I recognised is that the heat sink gets a lot hotter on the right side than on the left..
Do you have the schematic? Can you read the schematic? With the adjust pots at full cold position measure the Ohms from the wiper to one end and then the other end. One end should be zero Ohms, if not the pots are bad.
Craig
Craig
Thanks.
So they show 60 millamps (so that is 20mv across the 0.33 ohm)
(Are you measuring this with no speakers attached ? Any DC offset will either add or subtract from that current depending on the offset polarity)
Have any semiconductors been substituted or replaced in the output stage or drivers ?
So they show 60 millamps (so that is 20mv across the 0.33 ohm)
(Are you measuring this with no speakers attached ? Any DC offset will either add or subtract from that current depending on the offset polarity)
Have any semiconductors been substituted or replaced in the output stage or drivers ?
OK. This is something you need to be sure on (if the unit is an acquisition and could have some history to it) because these type of designs are super critical with regard to the semiconductor type and characteristics.
It seems odd that both channels have the same problem too.
To bring the bias lower, you would have to increase the value of the 4.3k resistor that goes to the bias preset. That would allow the bias transistor to turn on more and so pull the bias voltage down.
The question remains though... why is it wrong to begin with. Is the mains voltage settings OK and are the rails correct ? Not too high ?
It seems odd that both channels have the same problem too.
To bring the bias lower, you would have to increase the value of the 4.3k resistor that goes to the bias preset. That would allow the bias transistor to turn on more and so pull the bias voltage down.
The question remains though... why is it wrong to begin with. Is the mains voltage settings OK and are the rails correct ? Not too high ?
I checked R89, R90, they are both 3.7k instead of 4.3k. Is this normal?
sorry for the noob question: where do I have to check the mains voltage and the rails?
sorry for the noob question: where do I have to check the mains voltage and the rails?
Can we can assume the bias was roughly correct (~20mV) before replacing any parts, so the heatsinks would have been approximately the same temperature?
As already mentioned, for both channels to now have a greater bias current, the error must be systematic one - something you have done inadvertently. Perhaps a wrong multiplier value for the 1k replacement pots or (if you have substituted multi-turn pots) different pin connection layout to the original parts? What are the voltages measured at bases of Qm17,18?
As already mentioned, for both channels to now have a greater bias current, the error must be systematic one - something you have done inadvertently. Perhaps a wrong multiplier value for the 1k replacement pots or (if you have substituted multi-turn pots) different pin connection layout to the original parts? What are the voltages measured at bases of Qm17,18?
They both "read" 3.7K or they "are" 3.7k? 3.7K is not a standard value, in carbon film at least.
Mains voltage is the voltage out of the wall, is the amp set up for your voltage? Germany, 230 or 240?
The rail voltage can be checked at the big filter capacitors, positive and negative should be same.
Craig
Mains voltage is the voltage out of the wall, is the amp set up for your voltage? Germany, 230 or 240?
The rail voltage can be checked at the big filter capacitors, positive and negative should be same.
Craig
@llwhtt
they read 3.7k.
The switch at the back (120V/220V) is at AC 220V. We have 230V in Germany.
@Ian
I didn't replaced the pots..
R89, R90 are missing in the service manual part list. coincidence?
I'll measure later.
they read 3.7k.
The switch at the back (120V/220V) is at AC 220V. We have 230V in Germany.
@Ian
I didn't replaced the pots..
R89, R90 are missing in the service manual part list. coincidence?
I'll measure later.
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OK, the 3.7K is in circuit, try reversing your meter leads to remove the PN junction of Qm17/18 and see if it reads 4.3k.
Your voltages will be about 5% higher than the schematic.
Craig
Your voltages will be about 5% higher than the schematic.
Craig
You can't measure resistors in circuit, for various reasons you will get totally inaccurate results but I would be amazed if there were a problem with any of those small ones.
What DC voltage do you actually measure from chassis to those 0.33 ohms. Is it around 43 volts as shown or is it much higher ? Lets put some numbers in to all this.
Do the output transistors and drivers, that's Qm 19, 21, 23 and 25 match the types shown in the manual.
R89 and 90. Unsolder one end of one of them and measure it. What value is it ?
Okay, it's 47V at the 0.33Ohms. The base of Qm17 is 0.073V and Qm18 is 0.08V.
R89 and R90 are unsoldered 3.8k
R89 and R90 are unsoldered 3.8k
That's a wee bit high (he effect of a 220 volt unit on mains that could be over 230).
3.8k is a totally non standard value so it looks as though this area of the circuitry was open to modification by the manufacturer.
I would fit a 4k7 in their place and see if it adjusts OK
3.8k is a totally non standard value so it looks as though this area of the circuitry was open to modification by the manufacturer.
I would fit a 4k7 in their place and see if it adjusts OK
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