Hi,
I have problems with my Kenwood Kr-5030
I replaced burned transistors to MJ21193-4 and set the bias and amp measured on scope and everything was fine.
Then i tried to measure voltages to be sure if everything is in region. So i connected the gnd to near gnd on enclosure and start to measure testpoints.
Everything was in ok region until i touched FTP (failed test point) on the picture. Then i heard little HF noise-oscillation and bias drop to 0V (i have connected 2 multimeters to each testpoints so i could see that. I switched amp off and I measured and on outoput transistor and B-C was in conntact Emitter was ok.
I replaced it and turned on and now Bias is very low...and it is unable to set it anymore. On picture below you see that base voltage of output transistor is too low. 0.41v compare to working channel around 0,55..and -0.65 when bias ist set to right point. At non working channel as i wrote 0.41v and -0,55 at minimum and after setting minus goes to -0,7v but 0,41 goes to 0.42 maybe 0.43v.
You see that on Qm9 is 0.95v instead of 1,2V-datashet ....on working channel i measured 1,15V. Multimeter shows no damage on Qm9 and Dm1.
Any hints what to do.
Thank you Davorin
I have problems with my Kenwood Kr-5030
I replaced burned transistors to MJ21193-4 and set the bias and amp measured on scope and everything was fine.
Then i tried to measure voltages to be sure if everything is in region. So i connected the gnd to near gnd on enclosure and start to measure testpoints.
Everything was in ok region until i touched FTP (failed test point) on the picture. Then i heard little HF noise-oscillation and bias drop to 0V (i have connected 2 multimeters to each testpoints so i could see that. I switched amp off and I measured and on outoput transistor and B-C was in conntact Emitter was ok.
I replaced it and turned on and now Bias is very low...and it is unable to set it anymore. On picture below you see that base voltage of output transistor is too low. 0.41v compare to working channel around 0,55..and -0.65 when bias ist set to right point. At non working channel as i wrote 0.41v and -0,55 at minimum and after setting minus goes to -0,7v but 0,41 goes to 0.42 maybe 0.43v.
You see that on Qm9 is 0.95v instead of 1,2V-datashet ....on working channel i measured 1,15V. Multimeter shows no damage on Qm9 and Dm1.
Any hints what to do.
Thank you Davorin
Attachments
FTP is a (-) input (GNFB). The bias depends on the current through Q5.9 and trimmer Vrm1 and diode assembly Dm3.
The voltage you need to measure is the one between the base's of the two driver transistors.
This will vary with the preset and has to reach a value to overcome the four B-E volt drops of the drivers and output pair. So you need to see around 2.4v or higher between these two points.
Low bias alone will not stop the amp working. If it has no or very poor sound you still have another problem.
This will vary with the preset and has to reach a value to overcome the four B-E volt drops of the drivers and output pair. So you need to see around 2.4v or higher between these two points.
Low bias alone will not stop the amp working. If it has no or very poor sound you still have another problem.
Hi,
Thank you for your answers.
Amplifier is playing, dc offset is ok, around 50mv on both channels. Voltage between both bases are on defect around 2,3v and on working channel 2,34V. But on Fault channel it is between 0.95 and -1,35V
and on working channel is between 1,13 and -1,21v.
And no bias is measured across emitter resistors.
What cause this imbalance or where to measure for fault or what to change? Changed Dm1 but no difference.
Thank you for your answers.
Amplifier is playing, dc offset is ok, around 50mv on both channels. Voltage between both bases are on defect around 2,3v and on working channel 2,34V. But on Fault channel it is between 0.95 and -1,35V
and on working channel is between 1,13 and -1,21v.
And no bias is measured across emitter resistors.
What cause this imbalance or where to measure for fault or what to change? Changed Dm1 but no difference.
I would recommend disconnecting the bases of the both output transistors and see could you get similar voltages on 270ohm resistors on emitters of the driver's. After these results you should concentrate on output trans.
If you don't have similar voltages, you have problems in drivers.
If you don't have similar voltages, you have problems in drivers.
So it plays
The DC offset is good
Bias voltage between the base of the two drivers is similar (2.34 versus 2.3)
I'm going to suggest that you have used modern replacements that might differ slightly in their Vbe characteristics. Modern parts produced with modern doping processes can have slightly different forward characteristics.
You could try increasing the value of the 120 ohm that is in parallel with the preset. Try 180 ohm or 220 ohm.
Always turn the preset to MINIMUM resistance before retesting and readjusting.
Just to inform.
After measuring all the transistors, resistors everything was ok...i replaced all ceramic capacitors. Elcos were already replaced before. Now amp is ok again. At this vintage amps bias drifft quite a lot, because of bias diodes that are not so precise as Vbe multiplier.
So if anybody has similar problem with vintage Kenwoods, try with this.
Thank you for suggestions.
After measuring all the transistors, resistors everything was ok...i replaced all ceramic capacitors. Elcos were already replaced before. Now amp is ok again. At this vintage amps bias drifft quite a lot, because of bias diodes that are not so precise as Vbe multiplier.
So if anybody has similar problem with vintage Kenwoods, try with this.
Thank you for suggestions.
Attachments
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