Questions Regarding Pioneer SX-680 Problems: It all started with buzzing

[RogerALB]

Well, I checked the 13.5VDC at the point indicated and it matched nicely. While I was at it I checked for the 30VDC at the red point and it was good, too. Both were point to ground.

I did change C405 the other day, as a matter of fact, so it is fresh as a daisy. I checked it again just now and it is the right one in the right place put in the right way.

RogerALB

Never fear over time things will slowly start to make sense, or your brain will explode and then it will all magically fall into place.

Let's have a look at the FM right now. The AM and FM section is powered by the +13 volt supply.

Check the blue point for +13.6 volts if it is not there step back to the red point for +30 and if still missing check the far right side (on the diagram) of R402.
If the voltage is there then check R402 for open. If open ask why would it open? Resistors usually open up because they have too much current pulled through them.

If C405 shorts then you have the full 34.5 volts going through it to ground, poof. There could be a short or low resistance in the circuits on the emitter side of Q401 drawing too much current, or some other reason TBD.

View attachment 3613559



Ohms law for fun if C405 shorted out R407 would have the full +34.5 volts going through it back to ground (shorted cap). I=V/R
That would draw 734mA causing the resistor to dissipate 25 Watts of power, won't last long like that.

P=V*I

View attachment 3613585

OK, well, this is a cross between annoying and embarrassing frosted with bad news.

I decided to check various voltages, etc. in part because I was suspicious. The scoop is that when I test the speaker terminals I get 33VDC from the pin to the opposite pin, i.e. purple to black or blue to black. Black goes to ground.

Tracing the wires along I get 33VDC at the speaker on/off switch pins to ground. Also at the speaker protection fuses to ground.

The schematic indicates 12.v at 8ohm at the speaker terminals.

My question is, does this indicate blown STK modules? It correlates to the weird STK voltages mentioned previously.

Finally, this appears to have blown my left side speaker so I have to either track down junk speakers or dummy loads.

Advice and comments are appreciated.

 

[mrk229]

OK, well, this is a cross between annoying and embarrassing frosted with bad news.

I decided to check various voltages, etc. in part because I was suspicious. The scoop is that when I test the speaker terminals I get 33VDC from the pin to the opposite pin, i.e. purple to black or blue to black. Black goes to ground.

Tracing the wires along I get 33VDC at the speaker on/off switch pins to ground. Also at the speaker protection fuses to ground.

The schematic indicates 12.v at 8ohm at the speaker terminals.

My question is, does this indicate blown STK modules? It correlates to the weird STK voltages mentioned previously.

Finally, this appears to have blown my left side speaker so I have to either track down junk speakers or dummy loads.

Advice and comments are appreciated.

Blown modules? Probably.

Blown speaker, that sucks.


We talked earlier about designating voltages, well shame on Pioneer, that voltage at the speaker terminals is the AC RMS voltage across 8 ohms at full power.
They do tell you something about the voltages on the schematic and they imply AC by the terminology of the boxed V.





In a properly working system you should NEVER have DC at the speaker terminals that is over 1 volt, should be much much less.

Typically when you check/set DC offset it is to be as near zero as possible. We are talking 10's of mV DC or less.
Note: Not all equipment allows you to adjust offset. The 680 does not have that adjustment, still it should exhibit very low offset voltages.

I repeat you should NEVER have DC of any significant value at the speaker terminals.

The SX-680 does not have a speaker protection relay like many other receivers.

The protection circuit is monitoring the speaker outputs for DC above X threshold and will not close the relay on power up or will turn it off during use if it detects DC levels hazardous to your speakers.

You have learned a lesson, burn it into your forehead. Never use good speakers for testing. Especially on gear with no protection relay.

If the equipment does not have a protection relay always check for DC at the speaker terminals with with the meter after working on the power amp circuit.

 

[RogerALB]

Blown modules? Probably.

Blown speaker, that sucks.


We talked earlier about designating voltages, well shame on Pioneer, that voltage at the speaker terminals is the AC RMS voltage across 8 ohms at full power.
They do tell you something about the voltages on the schematic and they imply AC by the terminology of the boxed V.



View attachment 3614182

In a properly working system you should NEVER have DC at the speaker terminals that is over 1 volt, should be much much less.

Typically when you check/set DC offset it is to be as near zero as possible. We are talking 10's of mV DC or less.
Note: Not all equipment allows you to adjust offset. The 680 does not have that adjustment, still it should exhibit very low offset voltages.

I repeat you should NEVER have DC of any significant value at the speaker terminals.

The SX-680 does not have a speaker protection relay like many other receivers.

The protection circuit is monitoring the speaker outputs for DC above X threshold and will not close the relay on power up or will turn it off during use if it detects DC levels hazardous to your speakers.

You have learned a lesson, burn it into your forehead. Never use good speakers for testing. Especially on gear with no protection relay.

If the equipment does not have a protection relay always check for DC at the speaker terminals with with the meter after working on the power amp circuit.

Oh, yes, you are soooo right about the speakers. The good news is that I got them for free and they are Minimus 7 so there is no real loss. That is why I was using them. Besides, the repair will keep me happily occupied until I get new STK modules.

Checked the VAC at the terminals without the speakers, obviously, and well, let's just say it was high.

Once again, thanks for the tips and the time to forward them. It is very much appreciated.

RogerALB

 

[mcdoug3721]

OK, well, this is a cross between annoying and embarrassing frosted with bad news.

I decided to check various voltages, etc. in part because I was suspicious. The scoop is that when I test the speaker terminals I get 33VDC from the pin to the opposite pin, i.e. purple to black or blue to black. Black goes to ground.

Tracing the wires along I get 33VDC at the speaker on/off switch pins to ground. Also at the speaker protection fuses to ground.

The schematic indicates 12.v at 8ohm at the speaker terminals.

My question is, does this indicate blown STK modules? It correlates to the weird STK voltages mentioned previously.

Finally, this appears to have blown my left side speaker so I have to either track down junk speakers or dummy loads.

Advice and comments are appreciated.

The schematic does NOT indicate 12.6 V DC. It says 12.6V AC at 8 Ohms. 33V DC at speaker indicates shorted STK0039.

STK pins should be:

Pin 0 +1.4V
Pin 1 -1.4V
Pin 2 -30V
Pin 3 < 60mv
Pin 8 < 60mv
Pin 9 +30V

Shorted STK will have ~-30V on pin 3 and ~30V on pin 8.

 

[RogerALB]

The schematic does NOT indicate 12.6 V DC. It says 12.6V AC at 8 Ohms. 33V DC at speaker indicates shorted STK0039.

STK pins should be:

Pin 0 +1.4V
Pin 1 -1.4V
Pin 2 -30V
Pin 3 < 60mv
Pin 8 < 60mv
Pin 9 +30V

Shorted STK will have ~-30V on pin 3 and ~30V on pin 8.

Excellent info! It confirms the data I have. Your pin information matches the chart I have been using.

Thanks!!

 

[RogerALB]

Well, I have put the repair of the SX-680 on permanent hold. Clearly I have failed and I do not think it is advisable to put any more money and time into it.

I put in a new pair of STK0039 modules and they worked for a few seconds and then their voltages went through the roof. All the bad voltages, etc. returned and that was the end of that experiment.

Whatever happened to this machine before I got it must have been pretty bad.

Many thanks to everyone who commented and offered advice.