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Kenwood KR-5400: one channel distortion

M

Martinvb

Member
Hi all, I have been working away on a Kenwood 5400 with one bad (left) channel: there is a variable amount of distortion at higher volumes. Pots and switches were checked and cleaned, power caps were measured and seemed OK (for now). The signal going in the power amp is clean: I measured a 100 Hz test tone going in via Aux at PCB points 1 and 2 (from schematic) [photo #1 and #2]: both channels appear undistorted and stay equal at different volume levels. However, measuring the power amplifier signal going out [photo #3 and #4] there is increasing clipping on the left side that increases with volume. I took out the power transistors and other main transistors and caps in the signal path (marked with green star on photo #3): when not within spec they were replaced. The 2SA627's were both functioning, but their measured hFE's differed quite a bit, so I decided to swap their positions to see if the distortion would change channel–which it did not. The best clue that I have is that there should be still another component in the left signal path that is bad, so I will go over these one by one.
Is this a logica approach? If one of you has another clue or suggestion that would be much appreciated! Have a great day!

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The amp has two 2SC1451 transistors that are known trouble makers. One of them might be the problem or not, but ultimately they should be replaced.
You can try swapping them and see if the clipping moves.

KW5400.jpg

Is there a load on the output of the amplifier while you are testing it?
 
mrk229 said:
The amp has two 2SC1451 transistors that are known trouble makers. One of them might be the problem or not, but ultimately they should be replaced.
You can try swapping them and see if the clipping moves.

View attachment 3424617

Is there a load on the output of the amplifier while you are testing it?
Click to expand...
Thanks for responding and for your excellent suggestion: I will test this tomorrow. Yes, I use 8 Ohm dummy loads when testing. Will report back soon!
 
Martinvb said:
Only with the scope: voltages are the same with low volume, but L < R with high volume.
Click to expand...
You misquoted me. What I wondered was if you used a volt meter to check the various transistors, but you seem satisfied with the scope results.
 
Martinvb said:
Thanks for responding and for your excellent suggestion: I will test this tomorrow. Yes, I use 8 Ohm dummy loads when testing. Will report back soon!
Click to expand...
mrk229 said:
The amp has two 2SC1451 transistors that are known trouble makers. One of them might be the problem or not, but ultimately they should be replaced.
You can try swapping them and see if the clipping moves.

View attachment 3424617

Is there a load on the output of the amplifier while you are testing it?
Click to expand...
Hi, I measured and swapped both the transistors that you marked, but unfortunately the distortion stayed on the left side. Meanwhile I also have checked and replaced all the caps on the power PCB (when >10% out of spec). Now I am working on the rest of the amp board, still hoping to find a faulty component…
 
You might want to check the overdrive/overcurrent protection section of the amp. Perhaps it is triggering early on the positive waveform.
Maybe start with the caps associated with it. I am always a little suspicious of low voltage caps, there are two 6.3 volters in that part of the circuit, and one that appears to be bipolar 10uf at 16 volts.


kw5400.jpg
 
The amp clips on the positive half of the waveform, therefore the problem is in the positive half of the amp. First, measure the emitter resistors in the output stage and driver stage. Ce14 is the bootstrap cap and if it's bad the amp won't drive properly on the positive end of the waveform. Check Re20 and Re22. If all the above are ok, check the Hfe of Qe16 and Qe20 and check Qe12 and Qe10 and Ce34.
 
dr*audio said:
The amp clips on the positive half of the waveform, therefore the problem is in the positive half of the amp. First, measure the emitter resistors in the output stage and driver stage. Ce14 is the bootstrap cap and if it's bad the amp won't drive properly on the positive end of the waveform. Check Re20 and Re22. If all the above are ok, check the Hfe of Qe16 and Qe20 and check Qe12 and Qe10 and Ce34.
Click to expand...
Good analysis; Ce14 was OK, but Re20 was 90% off spec. Replaced it and both channels are fine again. Thanks for all valuable tips and tricks! The Audiokarma community is the best!
BW, Martin
 

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mrk229 said:
You might want to check the overdrive/overcurrent protection section of the amp. Perhaps it is triggering early on the positive waveform.
Maybe start with the caps associated with it. I am always a little suspicious of low voltage caps, there are two 6.3 volters in that part of the circuit, and one that appears to be bipolar 10uf at 16 volts.


View attachment 3425836
Click to expand...
Some caps in this circuit were a bit off, but nothing serious. As mentioned, Re20 turned out to be the cause. Thanks for your help.
 
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