Kenwood KA-1100SD. It can be fixed!

tarior

Dirty pool, old man?
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Though maybe not as tidy as I would have liked. At least so far.

This amp has been languishing around my shop for some time. All of the output devices on the right channel were completely shorted, and totally NLA for about the last 12 years at least.
After some careful study, I was able to determine that DAT0612N/P and DAT1018N/P were actually nothing special. Just a couple of somewhat weak-kneed transistors in one package with an inconvenient leadout.

After a whole lot of testing, fretting, worrying, etc. I ended up being fairly confident that the drivers and TA2030 switching chip were still OK.
In fact, I am at a complete loss to determine the cause of failure. The base resistors to the outputs were completely undamaged, and no other faulty components were found.
I replaced the DAT parts, all with Sanken 2SC6011A/A251A as I had them on hand and they are a fairly stout device. If I had it to do over, I might have use 2SC6145A/A2223A for the high power section, as they are LAPT devices with pretty nice gain linearity.
But I think these will do just fine.

In testing, I installed only the "low power" transistors first. Bias and DC dialed right in nicely. The waveform on the scope indicated that the switching chip was starting to do its thing as signal increased. I was able to get about 2vrms output while still in a 100 watt bulb in the DBT. Just watching for oscillation or other oddness.

All seemed good. Time for the leap of faith, and power up on the wall socket.
The amp delivered about 18.6vrms (1KhZ), then the chip would switch, and output would go to just about nothing.
After tinkering with it for a while, I went ahead and wired up the "high power" section, tested, observed, and went to full power.
Now it could pretty easily drive 36.4 vrms across 8 ohms before the wavetops started to flatten. No oscillations or overshoot.
Since I still have no idea while it failed in the first place, I was puckering a little while driving it into clipping at 20K. Clipping behavior was stable, no oscillation or overshoot. It might live!

Now, I guess I have to decide if I will make the other channel match, or leave it be. And I guess it needs to be re-capped and refurbished and whatnot.
I think it will be a pretty sweet amp.
As I mentioned earlier, it's not as tidy as I would have liked. I wanted to buy a couple of boards from DRMAudio, but I think he's defunct.

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This is a great sounding amp. Makes my Rock Monitors and L166s really sing. Though not at the same time.
 
Nice repair. Perhaps this post will help save another one of these amps someday.

Rob
 
Had one of those many moons ago. Agreed, it was great sounding. It was a smoker's unit. I flushed it twice and left it outside for a week. I looked forever for the matching tuner but finally gave up and sold. About a month later, the tuner showed up in my rounds but by then it was too late.
 
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Great ingenuity. Then only thing I could possibly suggest is making the leads shorter IF able for more physical stability.
 
That was about as short as I could make them and still be able to manipulate them into place and solder them.
I should probably design a board and have one of those Chinese board printers make up a few of them.
 
Eventually maybe I'll get around to doing the other channel. It's not a a priority right now.
 
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