Kenwood KR-A5050 protection mode - Page 1
Forum section: electronics / repair

author: EifX
author_rank: Contributor
timestamp: September 24, 2020, 01:59:07 pm
content: I have on my bench a Kenwood KR-A5050 integrated Amp that I want to repair.The problem is as followed.I have it on a signal generator and a scope to test it on 8ohm/200W resistors.When I switch it on everything good fine. The protection relay switches in and when I turn the volume up a little I see 2 sinewaves on the scope.When I turn the volume up, the sinus wave distorts a little, and the protection kicks in. also the transformer starts making noise when I turn the volume up.When I try to measure the channel that is soundless...at the moment I put the scope lead on the transistor of the channel that has no sound...protection circuitkicks in. I have already checked every transistor, capacitor, and diode, Zener diode in the amp. All of them check out fine or at least they seem ok.Also when I turn the balance to the right channel both signals on the scope go down. When I turn the balance to the left both signals stay the same.Can someone help me out here?

author: bob91343
author_rank: Super Contributor
timestamp: September 24, 2020, 11:00:22 pm
content: Check the idling current and offset voltage on the output stage; compare channels.If the balance control is affecting the wrong channel it may be a miswire at the input jack.I don't remember model numbers but if that is an old model it's likely I have repaired it; for a number of years I ran a Kenwood authorized service station.

author: mzacharias
author_rank: Frequent Contributor
timestamp: September 26, 2020, 03:04:21 pm
content: Flat-pack transistors often develop bad emitter-base junctions. If this unit uses them for drivers or output transistors, that is certainly something I would suspect.

author: bob91343
author_rank: Super Contributor
timestamp: September 26, 2020, 09:42:00 pm
content: I wouldn't waste time trying to isolate the problem. Just replace the output transistors and perhaps the driver transistors. They aren't usually expensive and you may well find that it will work after you do that.

author: EifX
author_rank: Contributor
timestamp: September 27, 2020, 12:19:27 pm
content: Thanks for the suggestions guys. I'll try them out.I suspect the main power caps because when I turn up the volume I hear the transformer making a high pitch noise and then the protection kicks in. Can I replace (for test reasons only) the 2 6800uF caps for 2 10.000uF caps?

author: EifX
author_rank: Contributor
timestamp: September 27, 2020, 12:21:27 pm
content: What do you mean with flat-pack transistors?Are those the rectangular transistors in the amp?If you could upload a picture that would help.

author: EifX
author_rank: Contributor
timestamp: October 13, 2020, 07:11:55 pm
content: Ok, I have got somewhere with this thing.I have replaced all the power transistors and 2 caps (2 X 6800uF 63v).Now the amp is turning on and isn't blowing fuses anymore.Only now I have one channel that has no signal.I have traced is back to a High Voltage Analog Function Switch Array.It is used to select the input sources. Now the right channel works fine: CD-Input on pin 4 send the signal to CD-output on pin 6.But for the left channel CD-Input on 24 doesn't have an output signal on pin 23. So it's very obvious.But I can't seem to find these IC's anywhere.Can anyone point me to them?Thanks in advance.