Clarion apa 4400g output issue

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I've got a Clarion apa 4400g with powerguard. Cool amp, rated at 100w x 4 @ 4 ohm.

I repaired the power supply section, it had a broken trace at the remote terminal, and voila, 3 of the 4 channels sprang to life. But... The channel 3,. That one, decided not to spring back to life.

Signal goes from the input to the op amp boards that have the gain and filters, and mono stereo selector. From there I can follow the signal around to the last op amp on the main board, and them to the output section. I can trace both left and right signals to the stk350 voltage Amplifier, but channel 3 doesn't get there. Just before going into pin 14, it gets all messed up at the resistor marked with an arrow. Color bands indicated a 520 ohm 5% tolerance resistor. It's resistance was off, read 870ohm or so. So I pulled it. And I replaced it with a 475 and 33ohm resistor in series. The signal is distorting and not coming out of the replacement resistor. Why would the signal get stuck there


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If the signal is clean on the preamp side of the resistor, the volatage amp could be defective.

Compare the resistance between the input pin and the other pins for that channel of the STK to what you read on the other 3 channels of the STK ICs.

MANY of the STKs on ebay are counterfeit and far from original. It's possible that you could get one there that works but the odds are against it.
The circuit is near identical to the Mac version, I have both. The STK's on these do fail, they never put any sort of heat sinking on them so they run very hot when pushed hard which the full range amps often are since power guard will protect the speakers. I got lucky with my MC431 and replaced one of the STK's and it now works again, got it from ebay since that was the only place I could find it. YMMV.

Why would the signal get stuck there?

Because like mine, your STK has likely shorted and needs to be replaced. If you're able to confirm that (either by process of elimination or by carefully desoldering them and swapping to see if the fault follows the suspect part), you should add a small piece of aluminum when you put a new one in. I did that to the 431 and Clarion version amps, just a short piece of angle aluminum held on by spring steel clips. They run a whole lot cooler this way so I feel better that they will live in the future.
Good luck with hoping Clarion will have parts for a 25 year old amp, I didn't even try. Pacparts has no STK's in that part number. I went through this already, it's ebay or bust. Thankfully the ones I got while certainly not genuine, did work. All you can do is slap a heatsink on it and hope it lives. That or design your own voltage amplifier with discrete parts, which is certainly possible but not for the faint of heart. Pinouts for STK's are available, if you make some be sure and sell a few here lol.
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With respect, why bother? I already found one that works, so the point is sort of moot. And if it worked for me, it'll likely work for him too. I get that we should source original parts whenever possible, but when we can't I'd say having even a counterfeit one that works is preferable to a non-working amplifier. But that's me.
It wasn't very long ago, maybe a year or so? Anyway many of these amp chips have been out of production for a long time now, it's not unreasonable to expect a chinese replacement if that's all you can get. Occasionally you can find genuine ones on ebay, but they will most likely be pulls of dubious operational status. So, it's a crap shoot.
So I tried to find one that I was confident was legit, but ended up just grabbing one from eBay that said it was legit. So, we'll see. I popped it in tonight and the amp seems to work fine now. So fingers crossed.

Thanks for your help on this one!
She is all back together makin tunes again.


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