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Kenwood KA-8300 Needs Help ....EchoWars...anyone?

bob adams

bob adams

Well-Known Member
I really screwed up. The other day we had a lightning storm and I unplugged the surge suppressor that my stereo system is plugged into. BUT I forgot that my wife's cordless phone was plugged into the surge suppressor and that phone base was plugged into a phone jack. Lightning struck very close by knocked off the power. It took out the sound card on my computer, another phone in the house, an iron that was plugged in, and my beloved Kenwood KA-8300.
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I have power to the meter lights but nothing else. The protection circiuit never kicks in. The meters don't move. No buzz, no smoke, no smell ...nada, nothing. I looked inside but didn't see any obvious damage on any of the boards. I did notice that three posts on the back of the power on/off switch appeared to have melted solder on them. But the other three posts were ok (total of 6) and two posts have an orange drop resistor wired in parallel with the switch. This is as shown on the electrical schematic. I don't know what the other posts were for as I don't see anything in the schematic showing other wiring out of those posts. (But I'm not good at reading electronic schematics so I can't be sure.)
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Since I do have low voltage power to the meter lights (300mA 7.5V) I know the transformer is at least partial working. According to the schematic I should have about 50V of potential on the secondary side of the transformer. I guess I'll check that.
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EchoWars or anyone else.... based on your knowledge, is the probable problem something that can be fixed? If it's a matter of another switch, that can be replaced from another Kenwood KA unit. I would imagine all the switches (2 position)were pretty much the same.
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I can buy another one of these units for roughly $125 and $30 for shipping on eBay. Still this is a beautiful unit in near mint condition. The round trip shipping for repairs would be roughly $75. No telling the repair cost. Still this has been a great amp. I sure hate to see it sitting in the corner gathering dust. Tell me what you think and thanks.
 
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Did you check all of the internal power supply fuses? Most ss amps with bipolar supplies have two or more besides the mains fuse.

The melted solder is hard to diagnose unless you know it WASN'T there before the... mishap.
 
Mhardy,
I forgot to mention that I did check both fuses. I pulled them and used a meter to check for continuity. They were fine.
 
It is likely there are one or more fried components in the power supply section. Check the main fuse(s) - the power switch may be toast. If the power transformer is open that sucks. Sometimes it takes out the the bridge rectifier and/or main filter cap. If the unit was turned on when hit your power transistors may now be shorted. I like these Kenwood amps - good luck.
 
Also I forgot to mention that I can't be sure the melted solder on the switch wasn't always there. Soomeone in the past had worked on the switch. There are obvious scratch marks on the back of the switch and someone bent and repositioned the metal 'ears' that hold the switch to the switch housing. I was wondering if the extra posts on the back of the switch were spares. Maybe someone had to move the wiring from the three post on one side of the swich to the three posts on the other side of the switch? I dunno......... I wish I was an electronics tech. Schematics are almost Greek to me. I can read them just enough to get me in trouble!
 
I think I may have misread your post. I was thinking the amp was plugged in to the wall outlet somehow. So then the surge got into the amp through the soundcard? If so you would need to take a look at the the pre amp section first.
 
In with the old

I didn't write that very clearly did I?
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The amp was plugged into a surge suppressor that was unplugged from the wall 110V outlet. There was also a cordless phone base plugged into the same surge suppressor. BUT that cordless phone base was also plugged into a landline phone connection. The lightning came in through that phone line, went
through the phone base, back through the phone base 110V cord into the suppressor, and into the amp. Well that's what I think happened.
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Now I know why they say stay off of the phone during thunder storms. Be thankful no one was using the phone.

My Krell cd player once blew a fuse during a lightning storm. Boy, was I relieved to find out it was only a fuse.
 
Take a meter, put it on AC volts and check the secondary windings on the transformer. Generally they are color coded, like 2 red wires will be for the same winding. You should have at least one winding that has a center tap, usually black, going to ground. You should see 1/2 the voltage between the center tap and the other 2 wires for that winding: Green - Green 100V, Green - Black 50V, other Green to black 50V. One of the windings that is center tapped should read a fairly high voltage; 80V across the 2 wires of the same color. This is for the output stage. If you find no voltage like this, you probably have an open transformer, very bad news. You could try to find a replacement from a transformer manufacturer.
 
Thanks Dr*Audio,
Just checked the secondary windings. In this case two reds, two blues and one black come off the backside. I traced the meter lights back to posts attached at the same place as the blues so I figured that was the 7.5V tap. Checked the reds (wiring posts 9 and 11). Each red/black had 36 volts AC. Exactly as the schematic said. Figured I didn't have to check the red/red for a total of 72V. Whew........ No open transformer!
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The power supply board has two pairs of wires leaving where I can check DC voltage. One is +/- 30 volts (wiring posts 4 and 6) and the other is +/- 51 volts (wiring posts 8 and 12). I can kinda figure where the ground is for the 30V pair but I'm not sure about the 51V pair. It looks like it is one or the other of the posts that land the 36 volts of AC I just measured. That can't be can it? Should I just measure it from the original ground (wiring post 10)?
 
Originally posted by bob adams
Thanks Dr*Audio,
Just checked the secondary windings. In this case two reds, two blues and one black come off the backside. I traced the meter lights back to posts attached at the same place as the blues so I figured that was the 7.5V tap. Checked the reds (wiring posts 9 and 11). Each red/black had 36 volts AC. Exactly as the schematic said. Figured I didn't have to check the red/red for a total of 72V. Whew........ No open transformer!
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The power supply board has two pairs of wires leaving where I can check DC voltage. One is +/- 30 volts (wiring posts 4 and 6) and the other is +/- 51 volts (wiring posts 8 and 12). I can kinda figure where the ground is for the 30V pair but I'm not sure about the 51V pair. It looks like it is one or the other of the posts that land the 36 volts of AC I just measured. That can't be can it? Should I just measure it from the original ground (wiring post 10)?
Click to expand...

More than likely most all of the power
supply voltages are going to be sharing
a common ground. You can check by tracing
the ground leads on the schematic and see if
they tie together at some point. If the negative
test points are shown using either one of these
symbols then they are tied together as a common
ground.

Hope this helps
 
Wasn't ignoring the post, but just arrived home after a two-day 1500mi drive.

Looks like the crew is leading you on the right path. If you don't have a manual, get one. If you do, start looking at all the DC voltages as was suggested.
 
You can use the chassis as a ground point. Measure the voltages with respect to the ground. Then if your supplies are ok, locate the output. There will be some white ceramic resistors marked .33 ohm or .47 ohm. One side will go to the output transistor emitter, the other side is the output. You should have 100mV or less at the output, or something is very wrong. You want to see 0V here +/- 5mV. Actually, 100mV is too high also, but it may be possible to adjust it to be 0V.
 
This is a slow going project. I was finally able to borrow a decent DCC (backlit Fluke 87.) from a guy at work to better check the voltages. My meter is one of those things that you hold like a pencil and can't read half the time (POS). So far I know the following:

1) My transformer is fine. I read the correct voltages for both secondary windings. They are approx. 36.5V and 7.5V AC. which are the numbers given in the service manual.

2) The 36.5 V AC is fed to a rectifier on the Power Supply Board.(PSB) and to a group of diodes, resistors, transistors, capacitors and a relay. More about the relay later.

3) The DC voltage leaving the rectifier reads as given in the manual. +/-51V DC.

3) The +/- 51 V DC then takes two of paths on the PSB. One path leaves and goes directly to two Power Amp Boards The voltahe is still +/-51V DC.

4) The other path from the rectifier goes through resistors and is reduced to +/- 30V DC. I read this voltage on the assigned posts leaving the PSB.

Here's my question. When I normally turn the unit on there is a time delay before the aforementioned relay contacts close and the unit "starts" up. This is not happening. Can all the voltages leaving the PSB be correct and I still have a problem on the PSB with the relay? Could one of the diodes, transistors, resistors, capacitors, or the relay be the problem?
 
The relay is quite likely fine and is doing the job it is intended for...which is to prevent the relay from operating if there is DC on the output rails.

You need to check the DC on the amplifier output line (before the relay, of course) and report back. If you aren't sure where to look, locate yon manual as suggested.

The unit also has a switch to connect or disconnect the preamp from the amp...disconnect it and verify that DC isn't coming from the preamp (don't know if this amp uses cap-coupled connection from pre to amp, or if it is DC coupled. Try the switch anyway).
 
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