Pioneer chip amp problem..Arrgh!!!

[dmx2020]

Hi guys,

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

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.

[Speedskater]

The virtue of having only one bad channel is that you have a known good reference for all your tests and measurements.
The problem with repairing a receiver is so many small parts in a confined chassis.

[LAJ]

My guess is it is the chip. Those STKs are famous for that kind of failure.

[mjf]

hello.
perhaps only a (bad) contact problem.
so i would begin with cleaning all fuses,switches,potentiometers........... and the headphone jack.
greetings

[dmx2020]

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.

[dmx2020]

And btw, i tested it with a pair of headphones last night, and it sounds perfect on both channels. But as soon as the speaker is connected, the distortion begins from the right channel.

[mjf]

yes,try to clean the headphone jack,with contact spray or so........;
are there relays between the speakers and the powerchip? if so perhaps one is bad.

[dmx2020]

I forgot to mention about the headphones, when the speakers are connected BOTH the headphone right ch and sp. rch are distorted. So i doubt its the relay or anything. Got a feeling the problems withthe chip or before the chip.

[dmx2020]

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. Any thoughts are much appreciated. Thanx heaps!

[stratus46]

Originally Posted by dmx2020

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. 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.

G²