kenwood ka-5500 protection relay and voltage problem

marcoeloco

New Member
Hi all, I agreed to have a look at a friend’s Kenwood ka-5500 amp, and now I’m thinking maybe I shouldn't have. He told me it had a fault with the speaker protection on one channel and when the amp is turned on one of the meters went to full scale and the protection relay cut out.

I decide to have a look and recap it fully as it had been sitting for years. I thought I would recap it first and then try and find the problem as it may have been a cap causing the problem. It wasn't.

After firing up the amp the same problem occurred in one channel and then it changed. Sometimes the meter will float up a small way and others it will go up more quickly and cut out the relay.

I check all the voltages from the power supply, control amp, and power amp.
I checked the bias, dc offset and it was all ok
I found up to 5 volts ac and the speaker output depending where the meter was and also the base of the main output transistors. I traced the voltage back a bit further but it was too hard with the board in the amp. I decided to remove the board and remove all the transistors on that channel check them I did not find a problem. I then re soldered all points on the board. I am hesitant to put the board back as I don’t think I have found the problem.
Any ideas as to what could be causing this?
 
So its going into protection. Did you pull and test the outputs?
Have a dim bulb tester to check for shorts?
 
Yes I did remove and check the output transistors and they were ok. The amp does not go into protection mode, it may do that when you first turn it on then if you turn it off and back on the needle will then rise, sometimes very slowly to about half scale on the high scale. I don't have a dim bulb tester and am unsure how to check for shorts.
 
You probably have some silicon in the process of dying/dead. Diode testing of transistors/outputs may find the problem.
Nice graphic from markthefixer, red/black refer to mm probe orientation, either OPEN LOOP or about 650mV expected
Use the diode function on your multimeter

DiodeTest.JPG

Build a dim bulb tester, you will need it. Many plans/how to's on AK, just search.

When the protection circuit detects excessive DC voltage it will release the speaker relay thereby disconnecting/protecting
the speakers. So checking for dc offset at the speaker posts does not show the problem, will try and get some test points.
 
Wait a few seconds after power-ON and measure the dc voltage at pin 7 on the power amp board, black probe connected to chassis.
Repeat for pin 25. These are at the emitters of the output, it will be close enough. Need to confirm it's not a problem with the VU circuit.
upload_2019-2-3_19-50-27.png
 
Thank you for this, at the moment the board is out of the amp I will need to reinstall it before I can do any testing. The only way I could get a decent reading on my digital meter was with it set on ac at the speaker outputs, It appears to me that the output transistors are some how getting a large AC signal to the base from further back causing the meter on the front of the amp to read and go high. I could also get a high ac reading at the base of the output transistors as well.
when this voltage goes up or down so does the meter on the front of the amp
 
AC present would point to failure of the rectifier bridge/diodes. You could try replacement, they are like $2-
I would find another MM
Also, if the relay does not click after power on then speaker post measurements will give garbage results.
 
Thank you.The relay does click in and then depending on how hight the meter at the front of the the amp goes (on high scale) it will click out. more often than not it stays in and the meter will hover at half to three quarter scale and this gives about 5v AC at the speaker output. I will check all the voltages again when I reassemble it. I have another meter to check it with.
 
I decided to replace all the Transistors on the board while I had it removed. When I eventually reassembled it the problem was gone. I Had aslo replaced the output transistors. I could not believe the clarity and punch this little amp has. I demonstrated it to my friend before I gave it back to him and it blew him away as well.
 
Happy you got out of that pickle im sure. Should be good to go for quite some time.
My 5500 is going strong after recap.
 
Since we are on the subject of speaker maybe this is relevant. I am working on a couple of KA-7002's. I disconnected the 224k/50v capacitors in series with 0.47 ohm resistors that connected two pins on the driver board socket to ground so I could rotate the socket and thereby gain access to the board.
It worked great. Fixed the drivers then hooked up dummy loads. Again, worked great. Then hooked up speakers. Not so good. the amp went into overload/oscillation very quickly and at about a 1 Watt load. Lots of head scratching, reading this forum until it dawned on me that the r/c components were the solution.
A couple short clip leads confirmed my suspicions about the networks purpose. (I think.) I will spend some time later today reading the above referenced article just to be sure.

Good job, marcoeloco, you fixed it and did a friend a favor.
 
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