Pioneer chip amp problem..Arrgh!!!

Hi guys,

I need some help with a pioneer SX-V300 (~1984) receiver i recently aquired :eek:.

On the right channel, it sounds distorted (as if when your radio station is not tuned properly) and it happens on all input sources (CD, tape, phono, etc). The left channel appears to be fine. It is a 45Wx2 amp based on the STK4181II chip. I doubt it is the chip since it still sounds loud even with the distortion.

It would be much apprciated, if someone can shed some light on this issue. And hopeully a schematic if someone has one.
 

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Thanks for the input guys.

yea its hard enough getting into the main pCB with all the wiring, etc. Its one of the smallest amps i have seen.

I was thinking of checking the input pins to the STK with a small speaker and seeing if it is ditorted at the input first. And depending on how it is proceed to checking the contacts, pots, etc.
 
Ok, I have some measurements after testing.

At the speaker output pins of the STK (pin 10 and 13 -- see schematic), the left channel is ok, the right ch is distorted (this rules out any problem with the headphone jack or relay or any connections).

At the input pins of the STK (pin 1 and pin 18), i measured the input with a small speaker and BOTH channels are producing sound as normal (no distortion).

So does this mean the problem is the chip or the some of the components around the chip? I was thinking of swapping pin 1 with 18 to confirm if the problem is within the chip or before. Will post results soon. Btw both pins measure 3.3V DC.

Is it possible for an STK chip to have a distorted output due to a problem within the chip? I have never heard of a chip amp having any problems other than a blown output:confused:. Any thoughts are much appreciated. Thanx heaps!
 

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Ok, I have some measurements after testing.

At the speaker output pins of the STK (pin 10 and 13 -- see schematic), the left channel is ok, the right ch is distorted (this rules out any problem with the headphone jack or relay or any connections).

At the input pins of the STK (pin 1 and pin 18), i measured the input with a small speaker and BOTH channels are producing sound as normal (no distortion).

So does this mean the problem is the chip or the some of the components around the chip? I was thinking of swapping pin 1 with 18 to confirm if the problem is within the chip or before. Will post results soon. Btw both pins measure 3.3V DC.

Is it possible for an STK chip to have a distorted output due to a problem within the chip? I have never heard of a chip amp having any problems other than a blown output:confused:. Any thoughts are much appreciated. Thanx heaps!

Do you have a scope? That would definitely nail it down. What you're describing is what would happen if you lost one of the 2 output devices (or emitter resistors) in the right channel. The amp can deliver normal sound to the very light load of the headphones because the drivers are sufficient for that task. Add the speaker and either the op or bottom of the signal gets mangled. Replace the STK and you'll be good.

 

mjf

Member
2009-01-01 4:53 pm
Austria
hello.
last chance..............there could be a damaged part around the stk,especially the elcos on input 2,2uf and 100uf on feedback...........or so.if you have a dmm you can measure the resistors............if all that does not work the stk should be bad.................
greetings
 
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I swapped the two input pins (pin 1 and 18) to check if the problem is before the STK, and its always the right side output (pin 10) that has the distortion.

So i guess its definitely fault with the STK or some of the resistors/caps around it.
@mjf: i can measure the resistors with my dmm. Is there a way to check the caps?

@stratus46: i dont have a scope with me. Is there any other way to confirm the STK is at fault (may be some resistance mesurement across the output pins 10 and 13...or so)
 
no DC at the terminals, just distortion (like its not getting enough power, or say like a badly tuned radio station)

Mjf...what do u refer to when u say "elcos"? is it the caps?

...Also pin 2 (and 17) gets an input from the 'simulated stereo' switch and other controls of the front panel. This is the (-) input to the opamps of the STK. I do not know what this is supposed to do. Is it some sort of supplementary signal input for the main signal input of pin 1 (and 18)? thanks
 
I measure the input output pins voltage for R/L channels:

Pin 1 (R input) - 297mV
Pin 2 (R ???) - 335mV
Pin 10 (R output) - 29mV

Pin 18 (L input) - 340mV
Pin 17 (L ???) - 380mV
Pin 13 (L output) - 96mV

...the channel with the distortion has only 29mV at the output compared to 96mV of the working ch output. Any clues as to why this might be
 

mjf

Member
2009-01-01 4:53 pm
Austria
i presume you measured dc (mv) without a (speaker) load............
the voltage at the output is the (dc) offset..........ideally 0v.......+-100mv are acceptable,may say within the specs..............ironical: the bad channel has better offset than the working one.
you can compare the working channel with the bad channel........measure the resistors with the dmm..............;
built in electrolytics are not easy to test............i would build in some new ......;
if this does not work the stk is suspicious.
 
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I think i have a clue on what the proble might be...:)

I measured the voltage at all the pins of the STK:

pin 1 - 0.3V (input R)
pin 2 - 0.34V
pin 3 - 0V (gnd)
pin 4 - -14.65V
pin 5 - -0.7V
pin 6 - NC
pin 7 - NC
pin 8 - NC
pin 9 - -45V (-Vcc)
pin 10 - 29mV (Rch out)
pin 11 - +45V (+Vdd)
pin 12 - +44V
pin 13 - 96mV (Lch out)
pin 14 - -45V (-Vcc)
pin 15 - -1.4V
pin 16 - 0v (gnd)
pin 17 - 0.38V
pin 18 - 0.34V (input L)

Notice the difference between the values of pin 15 and 5 (pin 5 is half of pin 15). I measured all the resistor/caps connected to these pins and they seem ok. (see the internal schematic of chip) Any ideas as to why this difference might be.

And also what is pin 4? is 14.65V seem about correct?? someone with some experience with the STK4191 or STK4192 can answer this may be...
 

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