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Kenwood KR-9600 Power Switch Workaround

EchoWars

EchoWars

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Was asked by PM about an issue with a stuck power switch on a 9600. I started to answer simply, but as my response grew and grew in an effort to help out, I realized that the info should be posted here for all to see, since a duff switch on a 9600 isn't too uncommon.

My reply to the PM (until it got moved here):

The last 9600 I rebuilt also had a bad switch, also stuck in the 'on' mode. I saved it by disassembling the switch and getting it back to at least a 'minimal' functionality, and added a 30A 120V relay to do the real switching. In this mode, the stock Kenwood power switch only carries 20mA of current (the coil current for the relay).

I had planned on documenting the mod, but kinda blew it off. But after the stock power switch was 'repaired', the rest wasn't that hard. Had to add the relay, a fuse holder and 0.1A fuse (so the relay coil had it's own fuse), some small wiring from the AC to the switch, and, of course, the relay itself, which mounted right behind the switch and next to the relay board (had to drill two small holes for the sheet metal screws). I also added a nice arc snubber for the new relay contacts, and put a new safety cap on the stock power switch in an attempt to prolong its life.

In case you want to try it:

Snubber: 539-104M06QC100 (Mouser)
Relay: 528-9152-120 (Mouser)
Safety Cap: 490-4217-ND (Digikey)
Fuse Holder: 3526K-ND (Digikey)
5 Fuses (minimum quantity)(0.1A Fast-Blo): F2496-ND (Digikey)

The coil contacts of the relay use oddball .187" male disconnects. The part you need to make a connection is 159-2237 (Mouser). The power contacts of the new relay use standard .250 connectors. 3M part #920044-08-ND (Digikey) will do. These are uninsulated parts and should be crimped and soldered, then shrinkwrapped. The tricky part is incorporating the snubber into the .250 connectors along with the power wires (OK, not that tricky). ;)

The Kenwood has an extra place for a fuseholder right next to the original on a stand-up bracket. All it takes is a screw to hold it in place and the job looks totally pro.

So, besides the parts mentioned, all you need is some small wire to run to the power switch. Hell, 26ga will do, as we're only carrying 20mA of current.

I'm cursing myself for not taking pics and documenting the mod, but I have two other 9600's here that I will eventually do this on, so sooner or later I'll update this thread with pics.
 
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We'll see how long this lasts, but here's an idea for an internal repair of the power switch. I have a Yamaha C-45 preamp that had a switch stuck in the "on" position, the contact pads inside the switch had burned together. In order to try to make it last as long as possible, I sanded the pads back to a very clean and fairly smooth condition, then melted some solder onto the tops of the pads to rebuild the top surface and hopefully make a good connection. It's been several weeks since the repair was done and it's working well so far.

My KR-9400 also had a similar problem, except that the portion of the toggle where the contact pad is located was completely burned off. Fortunately that switch was a DPST switch that only used one side. Moving the wires to the unused side of the switch restored functionality, though I intend to add a relay like this when I rebuild it.
 
Put my own 9600 on the bench this weekend and installed four new 12,000µf 100V CDE caps. Figured that gave me an excuse to add the relay modification. Got a fair number of pics and a lot of cropping and resizing to get it all ready for posting, so it might take a day or so to get this thing documented here.

So, first up, the business end of a 9600. You can see the new CDE caps in the upper right. Just below that is the main 10A line fuse.
 

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Here's the general form of the mod...we'll interrupt the primary to the power transformer with the relay, moving the high-current wires from the power switch to the new Magnacraft relay. Then we'll add a new fuse for the relay coil circuit and wire that to the power switch. When we're done, the power switch will carry a measly 20mA...the current for the relay coil.

We'll also replace the stock safety cap on the power switch, and add a nice QuenchArc snubber to the new relay contacts to keep them in good shape.

The stock power switch is a DPDT switch, and the opposite side of the switch is used for a noise killer circuit. We will leave that alone.

Note that we'll take the AC for the relay coil after the 10A main fuse. Not strictly necessary, but since we have a choice here I'd like to see that if the mains fuse goes, everything is down. Safety first.
 

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There's a spot just big enough for the relay, sitting next to the speaker relay board. With the speaker selector switch & the headphone jack scooted out of the way (won't go far, as there's a few wires running to the top of the unit that restrict movement), you can see where the relay will sit.
 

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Mark the holes (a pencil works). I used a spring loaded punch to divot the metal and drilled the holes for a small sheet metal screw. The tuner varicap and string is on the other side, so drill carefully! The hole next to the tall lip required that I chuck the drill bit just enough to grip the bit so that the chuck cleared. A longer bit would be helpful, but small diameter bits are short...still, it's doable with the standard bit and a 1/4" drill. Get a vacuum and suck up all the metal shavings/

There's a lip at the edge of the relay location that prevents the relay from sitting flat, so I used a washer under the relay and on top to prevent cracking the plastic.
 

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So...we need someplace to mount the fuse. On dedicated 120V and 240V models, there's an extra fuse holder mount right next to the main fuse. Very convenient. On multivoltage models (and I've not ever seen one), I suspect that both fuse holder mounts will be used and you'll have to find somewhere else to mount the fuse holder.

The type of holder I used is designed for this type of mounting. It's an Eagle Plastics #441-EPD201V, which Mouser no longer carries (and says is obsolete). But there's lots of other options. Just need a mounting hole in the middle to attach it to the stock Kenwood bracket.

In the second pic, you can see the mounted fuse and the first bit of wiring. On the mains fuse, the 'line' side of the fuse is closest to the power supply caps. The 'fused' side is where I attached a bare wire to run to the new fuse holder. The wire used is a stripped bit from a Cat 5 cable...probably 28ga or so. Don't need much, as the wire is only carrying 20mA.
 

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Next we cut the primary wires from the stock power switch and desolder the remaining pieces from the switch. Solder wick and a bit of extra flux will help. The stock primary wires are too long, and I clip about 2 1/2" off of them. Then the .250" uninsulated connectors and the QuenchArc device are crimped, soldered, shrinkwrapped, and installed in the relay.
 

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So, the wiring for the 9600 primary is done...that easy. ;) Now we have to get the Magnacraft coil wired so we can actually use the receiver.

First we'll run a wire from the fuse to the relay coil. I had some small 26ga stranded stuff that does the job fine. Connect to the fuse holder (dark green wire, first pic), and run the wire to one side of the relay coil (either side, doesn't matter). The relay connectors are the oddball sized .187". Crimp, solder, shrinkwrap, and push it onto the relay connector.
 

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So we have AC to one side of the relay coil. The other AC leg will run through the switch. First pic shows where we will get the other AC leg. The bottom incoming AC wire runs to the mains fuse and we've already tapped that one to run to our new fuseholder, so we connect our new wire at the top AC leg (the small white one in the first pic). I desoldered the point where the large white wire connects so I could feed my new small wire into the eyelet, then soldered it up again. The wire is then routed to the front of the receiver.

Crimp and solder up another .187 connector to a 4" or so length of wire and push it onto the other relay coil connector. Now we have both wires ready to solder to the power switch.
 

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Attach and solder the wires to the switch, along with the new safety cap. Gets a little busy there, since I decided to re-use the oversized cap insulator. Yeah, that new safety cap is kinda lost in that insulator...

You'll very likely need flux to get the solder to stick to the switch lugs. I found it necessary.

Once the switch is all soldered up, reinstall it and the speaker selector & headphone jack. The speaker wires can simply be routed around the new relay. Oh...and don't forget to install the new 0.1A (100mA) fuse for the relay coil, else not much is gonna happen.

That's it! If you did everything correctly it ought to work like a champ.
 

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Nice .. :thmbsp:

You know now you're going to have to do "Tutorial Videos"

Thanks for the info Glenn..

.John M
 
is this something that could prevent future issues in my KR 9050 as well?
or is the problem isolated to the 9600?
 
The 9600 seems to have a lot of issues with fused power switches, which is why I started this thread. The 9050 has a soft-start circuit that reduces the current through the power switch at start-up, so while it's possible that a switch failure might occur, it's a lot less likely than it is on units like the 9600 that lack such circuitry.
 
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