STK 4132II: Distorted output.

[danielsoy]

Hi ppl.

My sony audio box has its audio output distorted like thisattached)

What do u think about this failure?

Thanks .

 

[Nigel Goodwin]

Might help if you gave the model of the unit?, but most likely you need to replace the STK.

 

[danielsoy]

Thanks Nigel for your reply.

Unit is SONY FH-B170 n distortion is more obvious at low volume, at high vol, positive and negative half cicles look pretty symmetrical.

I will try another stk, and then will open the bad one to look inside.

 

[danielsoy]

The stk makes the same effect on this working board: (inside stk.bmp)

I will trace the input signal in its way to the final stage, to see what happens...

 

[Tony Stewart]

This is classic collector emitter leakage resistance across a cascode stage before complementary emitter follower stages. The result is insufficient DC bias for just one polarity .
It would be on the high side PNP transistor that leaks starving the low side of DC bias so they do not overlap at 0V. Diode tests won't work.

Unfortunately without a schematic , I can't give you the Ref Des.
but if you put your finger across the collector emitter of the "right" part with a light touch (100k) you can make the signal worse and thus replace it. There should be a bias pot to null crossover distortion, but that may not work.

 

[Nigel Goodwin]

I've just checked on Sony Assist, and there's no circuit diagram shown in the service manual, it must have only been in the paper versions.

 

[Nigel Goodwin]

If you search for the chip number, you can find examples of it's use though - such as here:

http://www.electronica.ro/audio/STK4132II.shtml

There's not a lot to go wrong externally, but you might try checking the bootstrap circuits on pins 5 and 16.

 

[Les Jones]

Does the distortion occur on left and right channels or just one ? Are the positive and negative supply rails about the same voltage ?

Les.

 

[spec]

STK4132II data sheet, also showing some typical applications:
http://pdf1.alldatasheet.com/datasheet-pdf/view/41586/SANYO/STK4132II.html

 

[Tony Stewart]

Comparing DC levels from left to right ought to isolate the small signal crossover issue, especially pins 5,15. then observe wave there.

 

[danielsoy]

Does the distortion occur on left and right channels or just one ? Are the positive and negative supply rails about the same voltage ?

Les.

Both channels r distorted, +vcc and -vcc are correct.
+vcc ripple is 500 mv peak to peak on pin 11, -vcc ripple is just 200mv on 9-14 pins, looks like +VCC is draining more current...

Heres a video showing oscillograms at pin 1-2-5-15-17- and 18, Look for: bad 4132...
https://drive.google.com/folderview?id=0B8sIJkC01otrZl95T2ZBbFEwSkk&usp=sharing

 

[Tony Stewart]

Inputs are biased negative too much

Pin 3,4 please

 

[spec]

With no input signal can you list the voltage on all pins of the STK4132II module as measured by a voltmeter, with reference to zero volts on pin 16 ?

spec

 

[Tony Stewart]

yes especially one side all pins incl pins 1-10

I have a hunch the mute control is partially to blame

 

[danielsoy]

1: -145mv
2: -145mv
3: 0v
4: -19v
5: -100mv
6: 2.7v
7: -20v
8: -21v
9: -21v
10: 30mv
11: 21v
12: 20v
13: 40mv
14: -21v
15: -80mv
16: ov
17: -120mv
18: -125mv

 

[danielsoy]

STK actually goes muted when I short together collector n emiter of tr11 btw.

I have just removed mute transistor TR11 without improvements.

 

[Tony Stewart]

Check R16 100Ω ( connected to pin 7) but not connected to R13,14 affecting Vdc at pin 5,15 , yet R16 is connected to pin 4

 

[spec]

Can you measure the voltage on TR6 base and TRB base with respect to 0V

I am a bit suspicious of TR8, TR9, or even possibly D2.

After that C11, 47uf bootstrap capacitor, may be faulty; it might be worth changing it anyway.

spec

 

[Nigel Goodwin]

Can you measure the voltage on TR6 base and TRB base with respect to 0V

I am a bit suspicious of TR8, TR9, or even possibly D2.

Are you not aware that those components are in a hybrid IC?, so it's not easily possible to measure voltages inside it - although breaking the casing off 'may' allow you to do so.

I've repaired countless amps using these kinds of hybrids, in almost all cases the hybrid needs replacing.

 

[Les Jones]

In the data sheet link that spec provided in post #9 there is a note on page 6 (External component description) that says about the bootstrap capacitors (C11 & C12) "If these capacitors are made too small, then the total harmonic distortion at low frequencies increases significantly" As there has been no indication of the frequency of the signal used for testing could this be the problem.

Les.