Is my STK 2058 IV dead?

[lhaymehr]

Hello.

I have limited knowledge with electronics and virtually no instruments to test things out but I can give you a detailed description of the problem.

I have an old Technics SU-Z400 amp with an STK 2058 IV chip inside and I think I have fried the right channel. The chip is rated 2x50W at 4 ohm and I had a pair of 100W/4ohm speakers connected to it. On the back side of the amp is a switch with two steps:

Step one: Main or Remote 4-6 Ohm / Main and Remote 8 Ohm
Step two: Main or Remote 8-16 Ohm / Main and Remote 16 Ohm

The switch was set to step one initially.

While I was moving the amp (while it was operating ) I accidentally flipped the switch (well moved it actually, it didn't flip completely to second step). I could hear the protection relay click a few times inside and then it went silent.

The wires from this switch lead directly to the power stage of the PCB; Four from the transformer, two to the PCB...

Under normal operation when I turn on the amp it takes a few seconds for the relay to click and then the sound starts. It doesn't start up anymore.

I removed the cap from the protection circuit relay and manually closed the circuit. The left channel works OK but the right channel buzzes loudly, say 60Hz sine and there's a bright spark on the right channel contacts when I close the relay circuit (if the speaker is connected to the right channel).

Is the chip dead or could something else be damaged? I now drive both speakers on the left channel, nothing is connected to the right channel. So far so good, the chip is not even warm.

I'm very fond of this amp, I used it to drive two subwoofers and it had really nice bass .

Any help is highly appreciated.

Thank you,
Vedran

[Netlist]

Originally Posted by lhaymehr

I removed the cap from the protection circuit relay and manually closed the circuit. The left channel works OK but the right channel buzzes loudly, say 60Hz sine and there's a bright spark on the right channel contacts when I close the relay circuit (if the speaker is connected to the right channel).

Vedran, the spark indeed gives an indication that half the chip is probably dead. Or better short circuit.
Only, I wonder where the excessive current is running with the manually(?) closed relay? Something should heat up...

/Hugo

[lhaymehr]

Nope, like I said, the relay only sparks when the speaker is connected to the right channel and then I close the relay, otherwise with speaker connected only to the left channel, no sparks. And I double-checked the heat right now; the heat-sink is cold and the chip very mildly warm. There are very few components on the PCB itself and none of them looks damaged or blown, also no heat there.

So the buzzing on the right channel is a clear indication that the right channel has short-circuited? I tried to find the schematics for STK 2058 IV to find out if it's a dual-mono or a stereo chip but with no luck. If it is actually a dual mono, then I should be fine with just the healthy channel running with no resistance on the burned one, right?

Thanks again.

[Netlist]

The right channel probably carries rail voltage which your speakers don't like, at all. So, if you connect the speaker and got a spark, the current is simply heating up your voice coils. Without schematic, almost definitely a bad STK.

/Hugo