Roland output board DC on speaker

[nosaj]

I'm looking at a Roland KR-105 it has DC on the left speaker about 25v, which has no sound output but the speaker does a click test. It is a STK402-050 stereo packagehttp://audiolabga.com/pdf/STK402-050.pdf

I'm trying to figure out which pins to test to see if one side is shorted out. But I'm having a hard time trying to wrap my head around it.


Thanks,
nosaj

[J M Fahey]

In this order:

1) #8 (+Vcc , some +27V ) and #9 (-Vcc , some -27V)

2) #1 and #15 to check that inputs are at ground potential or not farther than, say, 100mV or so
*IF* quite away, say a few Volts, short them to a nearby known good ground, 56k ground reference resistors or tracks/pads involved might be open.

3) #6 band #7 and #10 and #11 should respectively be near ground, with no signal applied.

Since + and - amp halves are brought out separately (maybe to add individual emitter ballast resistors, although datasheet does not show them) , a brocken track/pad/solder *might* disconnect one half , so the other pulls output towards its power rail, but most likely means a shorted transistor.

In any case you'll need to replace the full IC.

4) as a last test: check DC voltage at NFB pins #2 and #14 ; a broken path might kill NFB and slam output towards one rail.

[nosaj]

In this order:

1) #8 (+Vcc , some +27V ) and #9 (-Vcc , some -27V)

2) #1 and #15 to check that inputs are at ground potential or not farther than, say, 100mV or so
*IF* quite away, say a few Volts, short them to a nearby known good ground, 56k ground reference resistors or tracks/pads involved might be open.

3) #6 band #7 and #10 and #11 should respectively be near ground, with no signal applied.

Since + and - amp halves are brought out separately (maybe to add individual emitter ballast resistors, although datasheet does not show them) , a brocken track/pad/solder *might* disconnect one half , so the other pulls output towards its power rail, but most likely means a shorted transistor.

In any case you'll need to replace the full IC.

4) as a last test: check DC voltage at NFB pins #2 and #14 ; a broken path might kill NFB and slam output towards one rail.

I will check that this afternoon. There is also a relay near the outputs that clicks when turned on. Shouldn't that have stopped DC going to the speaker or the 100v 2.2uf non polar cap? if it is working correctly or do they not work this way?

osa-ss-212dm3 is the relay


Thanks,
nosaj

[nosaj]

In this order:

1) #8 (+Vcc , some +27V ) and #9 (-Vcc , some -27V)

2) #1 and #15 to check that inputs are at ground potential or not farther than, say, 100mV or so
*IF* quite away, say a few Volts, short them to a nearby known good ground, 56k ground reference resistors or tracks/pads involved might be open.

3) #6 band #7 and #10 and #11 should respectively be near ground, with no signal applied.

Since + and - amp halves are brought out separately (maybe to add individual emitter ballast resistors, although datasheet does not show them) , a brocken track/pad/solder *might* disconnect one half , so the other pulls output towards its power rail, but most likely means a shorted transistor.

In any case you'll need to replace the full IC.

4) as a last test: check DC voltage at NFB pins #2 and #14 ; a broken path might kill NFB and slam output towards one rail.

1) good
2) good
3) #6 #7 sit at -25.2
4) #2 -24.2 #14 -0.1 so a broken NFB would that be internal on the power IC?

So dead IC.

Here the data
Pin1 -0.1
pin2 -24.2
pin3 not there
Pin4 26.5
Pin5 -24.5
Pin6 -25.2
Pin7 -25.2
Pin8 27.6
Pin9 -27.6
Pin10 -0.04
Pin11 -0.04
Pin12 -26.2
Pin13 0.00
Pin14 -0.1
Pin15 -0.1

Also the relay voltages included before click and after click voltages on the images.
So I know I need the IC but I think I need to go further back than the relay. Bridge rectifier putting out -28 and +28 respectively.
I'll be further checking, but if someone can let me know what they think I'd appreciate it.











Thanks,
nosaj

[Jazz P Bass]

The second drawing shows the relay engaged.

The Vdc is now going out of the upper relay contact (eek! To the speaker).

If the NFB loop is sound, then the ic is bad.
(from Output Pin #6/7 back to Pin #2)

STK402-050 NFB.pdf

I would throw a new STK ic in.

Note: As far as the output section schematic is concerned, most manufacturers pretty much copy the datsheet.
Sans a few resistor/ capacitor changes.

STK402-050.pdf

What is not shown is the speaker relay.

It may simply be a delay at startup circuit driving the relay.

I would expect to see a protection IC that is monitoring the output for VDC.

Something like this for each output channel.

[nosaj]

The second drawing shows the relay engaged.

The Vdc is now going out of the upper relay contact (eek! To the speaker).

I would throw a new STK ic in.

Yea i had already disconnected the speaker when i started seeing DC on it. Got 2 on the way.
So is the relay a dual relay and the second switch possibly not used?
Thanks,
nosaj

[Jazz P Bass]

Quote:"So is the relay a dual relay and the second switch possibly not used?".

Use your meter and see where the second relay contact goes.

I would expect it to go the second channel if there is only one relay.

[nosaj]

Quote:"So is the relay a dual relay and the second switch possibly not used?".

Use your meter and see where the second relay contact goes.

I would expect it to go the second channel if there is only one relay.

That is correct it goes to the right channel.
Will update when parts get here.
Thanks,
nosaj

[J M Fahey]

The IC is dead because you can think of it like a Big/Power Op Amp and you can't have more than a few mV difference between + IN (1) and -IN (2) , yet you have some 23V DC .

Pull it, confirm that now you have 0V at (empty) pads: 1 - 2 - 6 - 7 which I expect but it does not hurt to double check.

If so, it's confirmed that it's an internal IC problem and a new one will solve the problem.

[nosaj]

The IC is dead because you can think of it like a Big/Power Op Amp and you can't have more than a few mV difference between + IN (1) and -IN (2) , yet you have some 23V DC .

Pull it, confirm that now you have 0V at (empty) pads: 1 - 2 - 6 - 7 which I expect but it does not hurt to double check.

If so, it's confirmed that it's an internal IC problem and a new one will solve the problem.

Will do that tomorrow after work. I really wasn't sure if I could pull the power amp ic and still fire it up with out messing something up. I've already got the IC on the way $8 at audiolabs of GA. Will post voltage tomorrow.

Thanks,
nosaj

[nosaj]

The IC is dead because you can think of it like a Big/Power Op Amp and you can't have more than a few mV difference between + IN (1) and -IN (2) , yet you have some 23V DC .

Pull it, confirm that now you have 0V at (empty) pads: 1 - 2 - 6 - 7 which I expect but it does not hurt to double check.

If so, it's confirmed that it's an internal IC problem and a new one will solve the problem.

12 67 all report 0V. Will update when new Ic comes in.

Thanks,
nosaj

[nosaj]

New Ic, tested no voltage on speaker outs previously was 23 volts dc I belive. Left channel that went out has a raspy sound to it, swapped in a Yamaha organ speaker sounds great now. Previous complain was when you hit keys hard it would have a overdriven sound to it like distortion on top of the notes. All of that is gone now. I believe the Ic may have just been heading south?

Thanks,
nosaj

[nosaj]

now in the datasheet it specifies Rl=6ohms does this mean external speakers should be 6ohms or higher? They did have external speakers connected and i was told that if one of the wires were to short it could take out the IC output.

Thanks,
nosaj