Yamaha YST-SW216 Problem

[TLS Guy]

According to the schematic there are several fuses, the one that might have blown is a 125 V 1.6A fuse, did you check it?

Also look like you can buy parts from Sears, their parts list show only 1 fuse, referred to as fuse 6A, I am not sure that's an error, because the 125 V 1.6A one makes sense to me, and it is right from the schematics.

https://elektrotanya.com/yamaha_yst-sw216.pdf/download.html

https://www.searspartsdirect.com/model-number/ystsw216/1217/0311400.html

Peng,thanks for digging up the service manual and circuit.

There is only 1 fuse, 1.6 amp 125 volt. It is not blown because the indicator light is on.

Interestingly for a sub, this amp is not class D, but an A/B biased amp with an IC chip power amp, driven from +15/-15 volts. So that could produce 100 watts.

So there is an input circuit followed by the crossover ICs. Then IC1 is the power amp, there is a limiter circuit to protect against overload. This is discrete transistor. IC6 provides he servo feedback function.

Now the output of IC 1 goes to a relay RY1 and then to the woofer. The relay is controlled by a protection circuit that is also analog and designed to open the relay from failure of IC 1 from DC offset. This would be he normal mode of failure of a power IC, and could cause a fire if it went to the woofer.

Now there could be failure anywhere all the way from input, the crossover, to the power IC.

However common things commonly happen. 100 watt power amp ICs are not known for longevity. The chip only costs $29 and currently is unavailable. This is how such a low unit cost was achieved with this sub.

So I suspect that relay RY1 is open, because at least one of the power transistors in IC have broken down and letting the DC rail voltage pass to the protection circuit which is doing its job and opening RY1 and not letting DC pass to the woofer. It also I'm sure, acted faster than the 1.6 amp fuse and prevented the DC going to ground through the woofer and preventing the 1.6 amp fuse blowing.

So actually this unit may not be so difficult to service.

I would see if there is a voltage to RY1 keeping it open. DO NOT jump RY1. If there is voltage keeping RY1 open, then you can be certain that IC 1 is blown. Then it could be replaced with enough skill and the amp tested. IC 1 seems to be a proprietary chip of Yamaha's I think. The track record of chips becoming available after they disappear is not good. They should however be available for 7 years from last manufacture.

I do note that failure of these subs seem pretty common, which given the low cost design is not surprising.

 

[M Code]

The power block has been used in certain budget products including receivers & music centers, it was a cost-effective way for a power amplifier circuit. Everything including outputs, drivers, protection circuits are built inside the block...
But if severely overdriven and/or exposed to high temperatures the block will self-destruct internally... Only solution is to replacement, also the power block is an A-B amplifier design, not digital.

Just my $0.02..

 

[21balu]

Can you tell me how to check STK-404 ic, I'm suspecting may be STK ic damaged.

 

[M Code]

Can you tell me how to check STK-404 ic, I'm suspecting may be STK ic damaged.

EZ to test...
Double check the power supply voltages for input and output. If the power supply is feeding voltages into the SK-404 but no output, then the power block is bad. It is not serviceable U need to replace it. Here are a couple of helpful links..

1. Technical data pack (PDF) that has the pin outs/ins including reference voltages.
http://www.onsemi.com/pub/Collateral/ENA2105-D.PDF

2. Replacement part is available for $15 from Amazon.
https://www.amazon.com/STK404-140-STK404140-STK404-140S-1pc/dp/B00YAP2EE6

Just my $0.02...

 

[TLS Guy]

Can you tell me how to check STK-404 ic, I'm suspecting may be STK ic damaged.

I suspect it is damaged.

As I said above I think there is a down and dirty way to check that IC STK-404. When the transistors in that IC fail they will pass the DC line voltage. This will activate protection, and there will be a voltage across the coils of relay RY1 as I explained before. So if there is a voltage across the coil AND the relay is open then the proves that IC1 needs replacement.

If this has to be soldered in then you need little clip heat sinks to put on the pins as you solder them.

 

[21balu]

I checked a tick sound coming from Relay and I checked across relay coil input showing 24v.

Can u tell me one thing, how to check sub whether working or not without any amplifier or receiver.

 

[TLS Guy]

I checked a tick sound coming from Relay and I checked across relay coil input showing 24v.

Can u tell me one thing, how to check sub whether working or not without any amplifier or receiver.

So the power amp chip IC 1 is blown with shorted outputs, and must be replaced.

You can check the woofer by disconnecting it at the speaker terminals and measuring the DC resistance. It is a 6 ohm woofer, so the DC resistance should be around 5 ohms or so.

Next connect wires to the speaker terminals, and then connect them intermittently to a 1.5 volt battery. Every time you make the connection in the correct polarity, then the cone should fly forward with a pop and return when you break the connection. If it does all that everything is OK.

 

[21balu]

I disconnected sub speaker and applied 1.5v to the speaker terminals speaker is working fine, but how can I justify that IC STK 404 is damaged.

 

[TLS Guy]

I disconnected sub speaker and applied 1.5v to the speaker terminals speaker is working fine, but how can I justify that IC STK 404 is damaged.

To absolutely prove it you need a signal generator and o-scope. However the the fact that there is voltage to the relay keeping it open means that the power amp IC is blown. What you do not know is if other components are blown.

So the proper test procedure is to put a 60 Hz signal to the input and follow the signal on the o-scope after each stage, input, crossover power amp etc. Then over drive it and make sure the limiter works to limit the input voltage to the power amp.

However you don't have the requisite test gear so you will have to take a reasonable gamble and replace that IC power amp chip. It only costs $15 for the miserable POS.

Actually the manufacturer, Yamaha have lied in the spec. That miserable POS is actually only a 40 watt sub, and not a hundred watt sub. At 60 watts THD is already 10% and off the clock at 100 watts. It can only be driven to 40 watts with acceptable distortion. So that sub is worth little trouble or expense. Actually it is really a total and cynical waste of resources to produce such a device.

 

[21balu]

Than I'll change that IC but my question is that, can I check with multimeter whether STK IC is working or not. Any reference voltages across each pin. Example No.1 pin 14v, No.5 pin 33v like that.

 

[TLS Guy]

Than I'll change that IC but my question is that, can I check with multimeter whether STK IC is working or not. Any reference voltages across each pin. Example No.1 pin 14v, No.5 pin 33v like that.

No, you can't test function with a cheap multimeter. Actually you should not take a cheap multimeter anywhere near solid state circuits as the input impedance is too low. This can cause serious damage. When testing circuits like that you need an amplified high impedance meter, with input impedance values in the megohm range so you don't do damage and get false readings by loading the circuit. So what you really need is FET VOM to test voltages. These meters use a high impedance MOSFET amp as the input and then the amplified signal is sent to the meter.

This meter has a 50 megohm input impedance for instance.

 

[21balu]

I have these testing instruments

 

[21balu]

Also how to check sound bar sub output ok or not.

 

[21balu]

Today I checked STK 404-070, as below results
Pin 1=0v
2=0v
3=0v
4=-32v
5=-14.27v
6=31.95v
7=-32v
8=-32v
9=32v
10=0v

 

[TLS Guy]

Today I checked STK 404-070, as below results
Pin 1=0v
2=0v
3=0v
4=-32v
5=-14.27v
6=31.95v
7=-32v
8=-32v
9=32v
10=0v

Well you have a good high impedance multimeter. I think that is about a first for me fielding these sort of posts.

The output stage of STK 404-070 is broken down. The collector voltages of TR 7 and 8, at pins 8 and 9, are the same as the base voltages at pins 6 and 7, whereas the base voltages at pins 6 and 7 should be 1.1 volts.

This is all on the circuit in the service manual that Peng found. The correct pin voltages are shown.

So probably the rest of the board is OK, but as I said the only way to know for sure, is to actually test function by providing a signal with a generator and looking at outputs sequentially with an o-scope.

As I have said the basics of a test set up for doing any of this sort of work is a good high impedance meter, a variable voltage Variac for bring up voltages gradually to equipment under test, a signal generator and a dual channel o-scope. That really is the basic requirement and there is no sugar coating it.

My test equipment.

 

[21balu]

Ok than I'll order STK 404-070
Presently I have STK 407-070
Is it suitable to my sub?
Can u tell me, one thing presently how can I check my sound bar output is ok or not.

 

[TLS Guy]

Ok than I'll order STK 404-070
Presently I have STK 407-070
Is it suitable to my sub?
Can u tell me, one thing presently how can I check my sound bar output is ok or not.

No you need a 404 not 407.

Well you need to listen to it. Have you got something to plug the output into? You really have to test that output dynamically. If you play some loud bass heavy music and connect your meter across the output on the AC setting you might see wavering numbers. If you can borrow a sub that would be the best bet, short of the signal generator and scope.

 

[21balu]

Ok thanks I'll try
Presently going to lab, so maximum ill test whether my sound bar subwoofer output is ok or not.
Tell me one thing if I replace STK 404 everything will ok or not?

 

[TLS Guy]

Ok thanks I'll try
Presently going to lab, so maximum ill test whether my sound bar subwoofer output is ok or not.
Tell me one thing if I replace STK 404 everything will ok or not?

I can not tell you for sure it will work. You do not have the equipment to test the whole board from input to output, as I explained. I would say there is a high probability it will work. That as all I can tell you.

 

[21balu]

I taken to lab but presently oscilloscope not available, so one of professor checked output to the speaker and found 1.34v so STK is damaged.
Presently in my place I checked all shops not available
Can u suggest me any other equivalent ic.