Smoking resistors on a Pioneer SX-770

[Ethan Pierce]

Hello,
I was doing a simple recap of my beloved sx-770, everything was going well until I got to the main amp board. Just to reiterate, all the other boards were recapped and tested functional. When I replaced the caps on the main amp the left channel had no signal so started trouble shooting and seemingly made things worse. There was smoke coming from the driver transistor 2SC904. So I ordered all new transistors just for fun, I replaced them all on the left channel and now the white 0.5 ohm 2w resistors are burning up and there is a LOT of smoke.

Here's the transistors I used as replacements: KSA1220AYS, KSC1815YTA, KSC1815FTA, KSC2960AYS, TIP41A.

Anyone have any ideas? Also, where to get replacement wire wound 0.5ohm 2w resistors. Thank you!

 

[merlynski]

Mouser carries 0.5Ω WW power resistors.
If it is just the left channel smoking check that R25 (220Ω) and R27 (22Ω) are still good. Did you replace the bias adjustment pot, VR3, and set it to 0Ω in circuit? An open in the bias spreader circuit (VR3, D1) can cause what you are seeing. Also test D1, STV-3. It cannot be tested with a diode range on a DMM, but this test jig should show about 1.8v across it:


One 9v battery and a 1kΩ resistor should work fine for testing an STV-3.

 

[Ethan Pierce]

Mouser carries 0.5Ω WW power resistors.
If it is just the left channel smoking check that R25 (220Ω) and R27 (22Ω) are still good. Did you replace the bias adjustment pot, VR3, and set it to 0Ω in circuit? An open in the bias spreader circuit (VR3, D1) can cause what you are seeing. Also test D1, STV-3. It cannot be tested with a diode range on a DMM, but this test jig should show about 1.8v across it:

View attachment 2221377
One 9v battery and a 1kΩ resistor should work fine for testing an STV-3.

I will double check those resistors, I did check them before but I've changed a few transistors since then. What wattage or type of resistors do I need to use to eplace the resistors on this board?

I did not replace any VR but I'll double check the outputs once this thing stops smoking.

I will check the STV-3 as well, I'm not sure how to check it with a 9v and a resistor but I'll start googling around and see if I can't figure out what you mean. Thank you for your time!

 

[merlynski]

Build the circuit I posted (you can just use clip leads) and connect it across the STV, then measure the DCV with your meter as shown.
What I meant by 'a 9v and a 1k resistor' was that one nine volt battery (instead of two) and a single 1kΩ resistor would be good enough to build that test jig to test an STV-3. The test jig shows an STV-4 but that is how you connect the STV-3 instead, to test it.
The STV-3 has 3 diode junctions inside it, so the maximum dc voltage across it would be 1.8v. It will probably measure about 1.2 to 1.5vdc. If you connect the jig 'backwards' your measurement will show the battery voltage, because the STV only conducts on one polarity.

 

[markthefixer]

If that trimpot / variable resistor idle current circuit opens up, the output stage self destructs.
It is the single most self destructive fault in a power amp.
test EVERY resistor with one leg disconnected. LOOK for burned traces / wires.

You started the repair yet did not correct the fault, which cost you more damage.

is the amp a W15-085 or a W23-005 ?


the 0.5 ohm, 5 watt resistor? this one : (mouser) 71-CP5J-0.5

There will be some exact procedures to power on a newly worked on amp, to avoid burning it out.
The initial settings for (VR1 / VR2 50K ) center point voltage ,
and (VR3 / VR4 50 ohms) idle current are critical.
VR3 / VR4 MUST start out at ZERO ohms, which is minimum idle current
VR1 / VR2 is less critical, it should be set for midpoint of about 25,000 ohms of 50,000 ohms.
Then it should be adjusted so that the ground referenced voltage at the collector (case) of Q11 should be half of the ground referenced voltage at the collector (case) of Q9
when that is stable, VR3 / VR4 should be carefully and slowly be adjusted so the R31 voltage ( measured at each end of the resistor at the same time) is 0.015 volts.
That will be 30 mA of idle current for the output stage.

 

[Ethan Pierce]

If that trimpot / variable resistor idle current circuit opens up, the output stage self destructs.
It is the single most self destructive fault in a power amp.
test EVERY resistor with one leg disconnected. LOOK for burned traces / wires.

You started the repair yet did not correct the fault, which cost you more damage.

is the amp a W15-085 or a W23-005 ?


the 0.5 ohm, 5 watt resistor? this one : (mouser) 71-CP5J-0.5

There will be some exact procedures to power on a newly worked on amp, to avoid burning it out.
The initial settings for (VR1 / VR2 50K ) center point voltage ,
and (VR3 / VR4 50 ohms) idle current are critical.
VR3 / VR4 MUST start out at ZERO ohms, which is minimum idle current
VR1 / VR2 is less critical, it should be set for midpoint of about 25,000 ohms of 50,000 ohms.
Then it should be adjusted so that the ground referenced voltage at the collector (case) of Q11 should be half of the ground referenced voltage at the collector (case) of Q9
when that is stable, VR3 / VR4 should be carefully and slowly be adjusted so the R31 voltage ( measured at each end of the resistor at the same time) is 0.015 volts.
That will be 30 mA of idle current for the output stage.

Ok, I appreciate the detailed explanation. I searched Mouser quite a few times and did not find that particular wire wound resistor so thank you for that as well.
Do you know what how many Watts the replacement resistors will be? I dont see it listed in the schematics or service manual at all. I will check ALL the resistors this weekend. I had checked most resistors in proximity to the wire wound and then the output transistor next to it, but im guessing that each time I turned it on I did more damaged and so the issues are migrating around the circuit

 

[Ethan Pierce]

Ok, I appreciate the detailed explanation. I searched Mouser quite a few times and did not find that particular wire wound resistor so thank you for that as well.
Do you know what how many Watts the replacement resistors will be? I dont see it listed in the schematics or service manual at all. I will check ALL the resistors this weekend. I had checked most resistors in proximity to the wire wound and then the output transistor next to it, but im guessing that each time I turned it on I did more damaged and so the issues are migrating around the circuit

Also I have the W15-085 board.

 

[markthefixer]

Default resistor wattage is 1/4 watt.

What / how are you testing each transistor?

 

[Ethan Pierce]

Default resistor wattage is 1/4 watt.

What / how are you testing each transistor?

ok, I ordered the resistors and I am jumping back into this unit. I am testing the transistors by seeing if there is any current coming through the collector and ultimately comparing the working side channel to the non working channel. very scientific, i know