Technics SA-X11WL distorted output

Nikkk

Member
2016-06-15 4:08 pm
Hi,

I have a vintage Technics SA-X11WL receiver. Few months ago, the right channel started giving distorted noise from speakers. I tried headphones, swapped speakers, tried source from turntable, tuner and even traditional cassette. Same problem.
After this, few days ago, even the left channel started giving same problem. Now both channel are giving distorted noise.
I dont see any bad caps (physical inspection).
Please suggest troubleshooting tips.

The Amp IC is STK4131ii
 
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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Ideally you would use an oscilloscope to look at the signal entering the STK module. That would show where the problem was in seconds.

Basic fault finding involves first confirming that ALL the internal power supply voltage are correct. STK modules are notorious for causing problems and if you are sure the voltages supplying the module are correct, and that the preamplifier supplies are correct, then it would be worth taking a chance and replacing the STK.

The real way to fault-find is to use the correct equipment (oscilloscope and signal generator or known good test source)
 

Nikkk

Member
2016-06-15 4:08 pm
Hi,
I used a signal injector to inject sound signal near the and on the STK IC points. On the IC's points also I was getting distorted sound. Though I didnt understand what does it mean? Does it mean the IC is at fault?
I dont have an oscilloscope at the moment, will have to borrow from a friend who has it.

Getting that IC is quite difficult as Google says STK were manufactured by Sanyo. And on alibaba.com, I see various manufacturers / traders located in Shenzhen, China are offering these STK. They are bit expensive (IC cost + shipping charges & custom duties). Some say testing warranty and some say 30 days. I am not sure which one should I go for. Are all the IC of same quality? or are they manufactured by third party companies and given the STK part number for reference?
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
As the modules are obsolete, genuine product would have to be be pulled from scrapped amplifiers with leads straightened and retinned but often these have been sold faulty, perhaps not properly tested at all. Some are new copies (usually with inferior power transistor dies fitted) and others are refurbished and perhaps re-labelled, but usually also using inferior dies.

The risk of receiving bad product is always high with those hybrid modules so many DIYs fit chipamp modules instead since there is no guarantee that the modules will be worth the cost and time to you in buying, shipping, fitting, testing and removal, return freight etc. for any refund. You will always be the loser at any price, with useless warranties like those offered.

It is easy to show pictures of genuine product but supply something that isn't. Don't be fooled by pictures, just look at the deal offered and think about the possibility that genuine new spare parts for 30 year old products could still be around, at any price. No supplier will freely admit it, but more likely they were just made or refurbished yesterday without any quality system in place to guarantee the goods will meet or exceed the original or any other specification.

Take the risk if you like but check your existing modules thoroughly as Mooly suggested before you spend anything. BTW, have the electrolytic capacitors ever been replaced in your amplifier? At 30 years, it's time to consider this or perhaps output relays may be part of the problem. Unfortunately, without a schematic available, it's not possible to know what is actually in there.
 
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Nikkk

Member
2016-06-15 4:08 pm
Thanks for that information. Never thought it could be refurbished. I thought they may be manufacture the same old ones till now as they were offering in 100s as MOQ.
The caps have not been replaced as per my knowledge. Though the unit was repaired in 1990's by Authorized Panasonic Service center here in India (Technics brand never existed in India, my dad bought this unit as brand new from Ireland in 1980's). But when it was repaired by Panasonic, as far as I know they had replaced the volume control twice and once fixed something in graphic equalizer.
With physical inspection, I dont see any blown / leaked caps. I tried searching for service manual on ebay. They show 'for sale' but its always either out of stock or the seller seems not reliable.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
If you look up the STK data sheet it will show the typical applications (full circuits) and its a pretty safe bet that yours follows the recommended designs.

A lot of these STK's are interchangeable as long as you pick a 'higher' rated version in the sequence.

These links should work even though they look a bit screwy.

etc - datasheet pdf

HTTP 301 This page has been moved
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Nikkk,
In case the STK pak is not defective, you may have injected a signal too high in level, or more likely you may have bad speaker relay contacts, or bad speaker switches. I would run a low level tone and while listening to it, tap the speaker really to see if the distortion changes (worse or better - doesn't matter). Any change doing this would indicate the relay contacts are oxidized. To test the speaker switch, using the same low level tone, slowly operate the control. Any changes in sound beyond off and on indicates oxidized switch contacts.

Always look at the more simple things before assuming the worst. Your output chip might be perfectly fine, so leave it alone and do other testing before something slips and you blow it up. You might even have an intermittent solder connection on the STK chip, or on something else in the path.

Always try to keep an open mind until you can prove something is defective.

-Chris
 

Nikkk

Member
2016-06-15 4:08 pm
Hi Nikkk,
Always look at the more simple things before assuming the worst. Your output chip might be perfectly fine, so leave it alone and do other testing before something slips and you blow it up. You might even have an intermittent solder connection on the STK chip, or on something else in the path.
Always try to keep an open mind until you can prove something is defective.

-Chris
Thanks for that suggestion. Yes, I re-soldered all contacts of the STK IC and nearby components. When I plugged in 1 speaker, the sound was loud and clear. No distortion. But when I plugged in the other speaker, both the speaker's output sound went back to distortion. Then I turned of the Unit for some time. Re-plugged in the unit and 1 speaker, the sound was good. I moved the volume control knob a bit, and the sound again became distort.
I am confused, what does that mean?
 

Nikkk

Member
2016-06-15 4:08 pm
Here is a pic on my Unit.
imja55.jpg