Sansui R-303, Distorted audio......

[Bobhoskin]

Hi everyone,
Sansui R-303 with a big problem, the audio output it's distorted and if I turn the Volume knob, I can hear something from 10%, than it's all the same, even at 100%, very low volume.

I've tryed every input, tape, Fm etc.. same problem.
The tranformer seems ok, good voltages.

All Caps seems ok (by eye), I've just tryed change the 4 ones that goes to the "Stereo Amplifier STK 4803", nothing changed.

Can be the STK 4803 with that big designed dissipator?
http://i.imgur.com/DRcAvop.jpg
http://i.imgur.com/gzspcae.jpg

I'm not an expert, please give me advise.
(I can make many other photos if needed of each section.)

[Bobhoskin]

Re: Sansui R-303, Distorted audio......

Help

[redwire]

Re: Sansui R-303, Distorted audio......

One thing to try with STK's is to tap them with a screwdriver. The IC bonding wires sometimes fail and you will hear it.
Otherwise, compare voltages on R, L channels on the chip's pins.

[newbiesat]

Re: Sansui R-303, Distorted audio......

Checking the resistor value from the output pin 5 and pin 12 by using the digital multi-meter (DMM). Usually it is 4.7 ohm. The bad one never give the correct value. Must power-off the unit before measuring the value of resistor. Good luck.

[Bobhoskin]

Re: Sansui R-303, Distorted audio......

One thing to try with STK's is to tap them with a screwdriver. The IC bonding wires sometimes fail and you will hear it.
Otherwise, compare voltages on R, L channels on the chip's pins.

Sorry guys for late reply, now I can make tests.
I've tryed moving the STK, and I remade solder on all 16 pins, same problem ....I will try to tap (you mean like give knocks with the screwdriver??)

Checking the resistor value from the output pin 5 and pin 12 by using the digital multi-meter (DMM). Usually it is 4.7 ohm. The bad one never give the correct value. Must power-off the unit before measuring the value of resistor. Good luck.

I've tryed following the 2 resistors that are directly connected to pin 5 and 12, those are named like r8l & r8r, and they measure 10.1 ohm and 10.2 ohm, not really 4.7 ohm (with unit OFF but I didn't disconnect the resistors from the circuit board, bad idea?).

[Bobhoskin]

Re: Sansui R-303, Distorted audio......

Here the photos of the STK Board... on the top view you see the 2 resistors on the left, R8r and R8l, both measure about 10 ohm..... please, give me advise:

Top side:
http://i.imgur.com/cASXAil.jpg

Bottom side:
http://i.imgur.com/RdjaatF.jpg

Sansui R-303
http://i.imgur.com/z2kyQxH.jpg

[budm]

Re: Sansui R-303, Distorted audio......

The STK4803 is a stereo Power amplifier module, pin 5 is an Output for Channel1, pin 12 is an output for Channel 2, so reading between the 2 outputs will not really help.
If you look at the pinout of the IC and the circuits inside the IC, what DC voltages do you get at pin 9 (+Vcc) and the chassis, Pin 7 (-Vcc) and the chassis?
Make sure all the circuit board mounting screws are tight since the boards are grounded through the mounting screws.
Since you are getting bad distorted sounds from both channel, chances are for both amplifiers to be bad is not likely, it has to be something common to both amplifiers.
Does this unit has Main line In and Pre-amp out RCA jacks on the back? Did you activate the Tape monitor switches and the Muting a bunch of times to clean the contacts?

[Bobhoskin]

Re: Sansui R-303, Distorted audio......

Thank you budm
By now I've tried what you suggested on both pin 9 and 7 gives 33volt each, those match with the two big caps, but the value of the caps its 42 volt.
I have clean all buttons like tape monitor...

[budm]

Re: Sansui R-303, Distorted audio......

Do you have the pictures of the rear panel of this unit? I will see if I have the service manual for it or not.

[Bobhoskin]

Re: Sansui R-303, Distorted audio......

Here the photos of the front and rear panel:
http://i.imgur.com/B05EKer.jpg
http://i.imgur.com/CnoEGb6.jpg

Precisely the 2 big Caps are 42V 3300uf
If can help, I can make a video and post on youtub, to listen how bad it works.

[Bobhoskin]

Re: Sansui R-303, Distorted audio......

Do you have the pictures of the rear panel of this unit? I will see if I have the service manual for it or not.

Like you see on the rear panel there is some oxidation due to water of the previous owner that give it to me.
I hope it's fixable.

[budm]

Re: Sansui R-303, Distorted audio......

Do you have powered PC speakers that you can connect to the TAPE output to see if the sounds are OK at that point? Your +/- voltages are OK. We can also turn the powered speaker into signal tracer to find out where the distorted signals start.

[Elysarian]

Re: Sansui R-303, Distorted audio......

Probably a silly question, but have you checked the volume pot itself? (I'm assuming it's a pot looking at the vintage of the unit)

They do wear out and a break in the track on there can lead to the kinds of symptoms described.