SX-580 Rail on output

[goldswimmerb]

Hey guys, I have recently come into possession of an SX-580 that has issues, at first I had no sound at all, and negligible DC at the terminals, which I traced down to R401 (33ohm) being open in the power supply. After swapping that resistor out I now have a new issue (well, one that was probably why that resistor popped originally).

On my DBT I am seeing the following Voltages at my STK Pack(s):
Pin # Voltage
0: 27.9v
9: 27.2v
8:26.6v
3:26.6v
2:-27.2v
1:25.4v

I'm not used to testing STK amps so I am not sure if the above behavior is normal for a blown pack, It seems unlikely that both packs would be blown, right? I'm used to discrete BJTs where I can pull and test the individual Transistors.

Thanks!

 

[merlynski]

If those voltages are the same on both, IMO both are blown. I would start by pulling both out.
Then, I would start troubleshooting by making sure all the power supplies are good.
Post voltages for all the power supplies, please:
Emitter of Q401:
Emitter of Q401:
Plus side of C409:
Minus side of C416 (same as pcb pin 46):
Minus side of C415:
Plus side of C336 (cathode of D309):

 

[goldswimmerb]

If those voltages are the same on both, IMO both are blown. I would start by pulling both out.
Then, I would start troubleshooting by making sure all the power supplies are good.
Post voltages for all the power supplies, please:
Emitter of Q401:
Emitter of Q401:
Plus side of C409:
Minus side of C416 (same as pcb pin 46):
Minus side of C415:
Plus side of C336 (cathode of D309):

I'll get around to posting those voltages when I get a chance, as well as pulling the STKs to test, they don't test as shorted in the circuit which I found strange, but I guess there could still be an internal fault in each.

 

[merlynski]

Opinion:
Because STKs are integrated circuits, unless you find a short between pins, they are impossible to test in circuit with a meter.
After removal, If the rest of the circuitry is good, then the STK is bad, if the rest of the immediate circuit has bad components then it probably was killed by a bad STK. They were used because they were cheaper, not because they were better than discrete.
over.

STK-0050 replacement for SX-780 and others

 

[EastPoint]

I have gone through at least a half-dozen SX-580s, usually in terrible condition and all with one or both channels railed like that. I just chop them up for parts--the cases are too much of a pain to clean up. If you end up pulling and checking the STKs you can probably get genuine replacements on eBay--there are a few sellers with real ones on there.

 

[goldswimmerb]

If those voltages are the same on both, IMO both are blown. I would start by pulling both out.
Then, I would start troubleshooting by making sure all the power supplies are good.
Post voltages for all the power supplies, please:
Emitter of Q401:
Emitter of Q401:
Plus side of C409:
Minus side of C416 (same as pcb pin 46):
Minus side of C415:
Plus side of C336 (cathode of D309):

Alright, had shelved the unit for a bit, but Q401:
E:13.34
B:14.02
C:31.2
Q402:
E:45.7
B:46.4
C:47.8

C416: -29.4v
C415:5.7v
D309 Cathode: 13.1V

C409 Appears to be ceramic, .047UF

 

[merlynski]

It must have been a late night, massive typos.

Alright, had shelved the unit for a bit, but Q401:
E:13.34
B:14.02
C:31.2
Q402:
E:45.7
B:46.4
C:47.8

C416: -29.4v
C415:5.7v
D309 Cathode: 13.1V

C409 Appears to be ceramic, .047UF

Q401 E: looks ok
Q402 E: looks ok
C416-: looks ok
C415-: should be -26vdc
D309: looks perfect
C409 should have been C403+, and I missed several supplies.

Lets try that again . . .
C403+:
C414-:
C413+

Did you pull the STKs?

 

[goldswimmerb]

It must have been a late night, massive typos.



Q401 E: looks ok
Q402 E: looks ok
C416-: looks ok
C415-: should be -26vdc
D309: looks perfect
C409 should have been C403+, and I missed several supplies.

Lets try that again . . .
C403+:
C414-:
C413+

Did you pull the STKs?

Will get back to this later, but yes I did pull the packs, they test the same on my multimeter as a known good pull I got from elsewhere, so I do not believe they are bad.

What I did notice is that everything seems good for a second or two after turning on, and then the outputs start railing.

 

[merlynski]

Will get back to this later, but yes I did pull the packs, they test the same on my multimeter as a known good pull I got from elsewhere, so I do not believe they are bad.

What I did notice is that everything seems good for a second or two after turning on, and then the outputs start railing.

ok, leave them out if they are not back in already, it is easier to TS the circuit without them in. You will need 4 resistors of the same value between 270Ω to 1kΩ. Looks like there is at least one power supply problem.

 

[goldswimmerb]

ok, leave them out if they are not back in already, it is easier to TS the circuit without them in. You will need 4 resistors of the same value between 270Ω to 1kΩ. Looks like there is at least one power supply problem.

Only one is in the unit at the moment. Definitely agree there is an issue in the PS here. Judging by C415 being wrong would I possibly have a bad R410, or diode? That seems to be in the higher currently/main filter side of the PS.

 

[merlynski]

Only one is in the unit at the moment. Definitely agree there is an issue in the PS here. Judging by C415 being wrong would I possibly have a bad R410, or diode? That seems to be in the higher currently/main filter side of the PS.

I am suspicious of R410. Does it look smoked or measure out of tolerance? The voltage on the - side of C414 will provide a clue as well.

 

[goldswimmerb]

I am suspicious of R410. Does it look smoked or measure out of tolerance? The voltage on the - side of C414 will provide a clue as well.

R410 measures fine. I'll check more voltages when I'm less busy. Thanks!

 

[goldswimmerb]

It must have been a late night, massive typos.



Q401 E: looks ok
Q402 E: looks ok
C416-: looks ok
C415-: should be -26vdc
D309: looks perfect
C409 should have been C403+, and I missed several supplies.

Lets try that again . . .
C403+:
C414-:
C413+

Did you pull the STKs?

Aight, so back at it,
I actually checked the wrong resistor (Mistook another 100ohm for R410). Replaced the fusible resistor there and now the unit is working. The only issue I see right now is the FM tuner is stuck in muting when in stereo mode as the needle only deflects to the right when tuning and has no movement leftwards.

Also, DC Offset is showing to be about 100mv, and I do not see an adjust point.

 

[merlynski]

Aight, so back at it,
I actually checked the wrong resistor (Mistook another 100ohm for R410). Replaced the fusible resistor there and now the unit is working. The only issue I see right now is the FM tuner is stuck in muting when in stereo mode as the needle only deflects to the right when tuning and has no movement leftwards.

Also, DC Offset is showing to be about 100mv, and I do not see an adjust point.

There is no adjustment for dc offset on an SX-580. The usual solution is to replace the 2SA798 dual transistor in the differential amp section of the power amp (Q301, Q302), they are known troublemakers with a history of aging badly. Use a matched pair of modern KSA992F from a reputable source. If that doesn't do it, likely the STK is bad.

I am not a tuner guy so I can't help with that.

 

[TommyW]

Not trying to hijack this thread and considering the problem was resolved, I thought I would seek some guidance as I am exhibiting the exact same issue on a SX-580 with just one of the STKs(Q1). So far, I have pulled the STK in question and tested for shorts but found none. With the STK pulled I am getting 28V on the collector of Q303, when according to the schematic it should be 1.7 volts. I have replaced Q303 with a KSC2383 with the same results. I have tested all of the previously mentioned components with the exception of D309 as I cannot seem to find it anywhere on the board diagram I have. Any thoughts on what else to check. Thanks!