Technics SA-424

[jdefonseka]

Hello everyone - A technics SA-424 receiver followed me home today. Unit is in excellent cosmatic condition except for no audio output as described by the seller. Main amp board uses two STK8040 hybrids for left and right channels. When I put a meter to the speaker outputs, it reads o dc volts on both main and remote speakers. Previous owner described the speaker protection relay click which no longer happens. Could a blown STK module cause the protection relay not to click?

This is a little different from my SA-310 which has the new class A technology because it doesn't have relays. So when my stk module was shot, there was 35V dc at the speaker terminals.

I also noticed the PCB around the relay is very dark. I'll post some pictures soon.

What do you guys think?

Thanks

 

[jdefonseka]

Here are the pictures

 

[BinaryMike]

If the relay doesn't pull in, then you have to check DC offset on the power amp output lines instead of the speaker terminals.

The dark PCB is a result of heat from the power resistors, and it looks like the lytic caps nearby are toasted too. Better check *all* of the power supply voltages for correct voltage and low noise.

 

[grateful]

Yes check for voltage at the output coils behind the relay.

 

[jdefonseka]

I cleaned the board out pretty good. I will check the DC offset once I get it installed back again. Thanks for your reply. So, if the stk module is tost, then DC offset should be way off range? But how's that not turning the relay on? I guess if it's producing voltages way off, then the protection kicks in?

Thanks

 

[BinaryMike]

I guess if it's producing voltages way off, then the protection kicks in?

You got it! :yes:

 

[jdefonseka]

Thanks for your help. If I have a faulty STK module, do you know where I can find one? I did a google search but I don't think there was a vendor in the US that was selling it.

 

[jdefonseka]

Here's the scoop. One of the STK8040 modules are burnt. Creating 41 Volts at pin#3 of STK8040 module. Speaker protection circuit picks up this signal way before it gets to the speaker terminals and DOES NOT turn on the relay. Making no music at the speakers :yes: So, I found the faulty STK module and removed it from the amplifier. I got tunes Ordered 4 more STK modules from england cuz there isn't any available here in US. Can't have too many STK modules :thmbsp:

Took apart the STK module as usual :scratch2: RIght top corner looks like it's missing a piece which i couldn't find. Anyway it's burnt

Jeff