Marantz sr320 smokes when treble knob 100%

[Gusss]

I am trying to recover this marantz sr320…all is now working, except for when the treble is 100%, it sounds like voltage interference going through, and one resistor next to big capacitors R721 gets really hot and starts smoking. But if I bring treble to 80% all is good.. any ideas?
Thanks

 

[Gepetto]

Yes, provide a schematic, the board artwork will take too long to reverse engineer.

 

[Gusss]

Please let me know if it can be read, thanks for the help

 

[Gepetto]

R721 is in the zoebel network for Q701, the headphone amp. I would suspect that for this to be getting hot, that Q701 has a high frequency oscillation going on when you crank up the treble.

 

[Gusss]

The Q701 which is STK4843 is new, and the preamp QK01 is also new….
But these could have suffered after installed. One of the caps c720 was swollen…

when you suspect that the q701 has a high frequency oscillation, that means it has to be replaced?

 

[Gepetto]

Why was the original replaced?

 

[Gepetto]

Did you look at the output signal with your oscilloscope to observe the waveform at the output of Q701?

 

[Gusss]

Why was the original replaced?

The guy that sold it to me said that the preamp was the only problem….
when I replaced the preamp, and cranked the treble, I suppose the skt stopped working….

 

[Gusss]

Did you look at the output signal with your oscilloscope to observe the waveform at the output of Q701?

I don’t have an oscilloscope….☺️

 

[Gepetto]

The guy that sold it to me said that the preamp was the only problem….
when I replaced the preamp, and cranked the treble, I suppose the skt stopped working….

That circuit that you pointed to the smoking resistor is the headphone amplifier...

 

[Gusss]

So I’m going to isolate the headphone board, its only connected by a 3 wire ribbon… maybe the jack is shorted

 

[Gusss]

I reversed engineered this….
The already amplified sound goes through the 3 wire ribbon(w701) to the HeadPhone board (PW00). If the headphone plug is plugged in, the sound goes through 2 330ohm resistors (rw01 & rw02).
If there is no HP plugged, it then goes to the PS51 board which is the speaker a/b selector switch.
Then it goes to the PW51 (speaker terminal)

So that part looks good.

 

[Gusss]

I went ahead and tested all of the caps from the tone controls to the amp, and even switched from left to right, and the problem persists. The last click of the trevle knob is what puts that resistor R721 to smoke.

 

[Gusss]

All the capacitors tested very close to the value, and the esr values below the ones from the chart

 

[Gusss]

I also found that resistors R723 & R721 are getting very hot, (with out cranking the treble) . It is working like this, but I believe this has to do with the same problem. R721 is the one that smokes when treble is full

 

[Gusss]

The R727 resistor that smokes is goes directly from the stk amp L output channel… the last click of the treble creates a huummm in the left speaker.
but…

If I turn the balance all to the Left, no humm and no smoke.
If I turn the balance all to the Right, no humm and no smoke.
The problem persists when the balance is centered.

so there must be a L to R unwanted bridge somewhere…

 

[mlucitt]

Could be a bad pot, sending ground to the circuit. If someone turned the pot past the stops, it could be shorting to the case. Pots are pretty easy to check and change out. I would be curious what the wiper to ground sweep looks like.

 

[Gusss]

I’ll be happy to to show you what

the wiper to ground sweep looks like.

If I could only understand… sorry, english is not my first language…

 

[mlucitt]

"the wiper to ground sweep looks like."

Sorry, I get carried away.
With the amplifier off, set your meter to cover the value of the Treble potentiometer in Ohms. So for a 100K potentiometer, set your meter to 100K Ohms. Or, if your meter is auto-ranging, it will do this automatically.
Then place one lead on a good audio ground for the Treble circuit, and the other lead to the Wiper (center terminal) of the pot.
Then sweep the pot slowly through the entire rotation.
It should show a mostly linear increase or decrease (depending on the rotation direction) in resistance but not a short to ground or an open circuit.
A problem at either extreme end of rotation could cause the resistor R721 to get hot.

 

[Gusss]

Yeah, thats fine. Linear increase of balance pot is just fine, to both sides. And the volume pot is also working fine.