Troubleshooting Technics SA-310

[DcibeL]

I am trying to repair this 20 year old amplifier for a friend of mine. The amplifier inside this thing is sort of a half IC, half solid state amp (it says "new class A synchro bias" on the front). The story is that my friend's cat tripped over the wire and it started to smoke a little. The left speaker output plays a VERY loud buzz through it, and the headphone jack does not work at all.

There are 2 resistors in series with the headphone jack that are burnt, but still measure as the correct value. There is also burn marks on the voltage regulation section of this unit, but the devices are still fuctional because the right channel plays fine.

The schematic for this unit is almost identical to the one for the SA-410 found here. If anyone has some time to kill and wouldn't mind looking at the schematic and maybe looking for something that may be destroyed if the speaker output was shorted the help would be greatly appreciated. Thanks.

[DcibeL]

Don't everyone answer at once.

[carlmart]

Originally posted by DcibeL


The schematic for this unit is almost identical to the one for the SA-410 found here. If anyone has some time to kill and wouldn't mind looking at the schematic and maybe looking for something that may be destroyed if the speaker output was shorted the help would be greatly appreciated.

Perhaps you should help a little more, because it was not easy to find the circuit. The URL doesn't work as it should and after a manual search I got to a file that is splitted in several parts.

You should join the circuit diagram parts with a graphic program and then put a reference here to a diagram where the whole power amp is and nothing else. The SA-41o is a receiver, so it's not easy to get to the parts we should see.

Then you might get some answers here.


Carlos

[DcibeL]

Hmm...I just clicked the file I linked to and it worked perfectly. However, the link in your quote does not go to the same place.

I will draw up a diagram when I get back from school today, and hopefully that will help a bit. Until then, sa41005.pcx and sa41006.pcx have the amplifier circuit on them. I know it is a bit hard to look at when the circuit is split into 2 pictures, but I will change that soon.

[DcibeL]

I have stuck the two images together, as well as reduces the image size so it's easier to view on a monitor. If you look at the very bottom of the diagram, you will see the voltage regulation section. the pcb is burnt in this section, but the devices are still operational. Up in the top left there is the voltage amp circuit. There is some discoloration on the pcb around the 2 transistors up there, which I assume is burnt pcb. Almost dead center is the power amp module. There isn't any physical damage in this section, but of course there could be internal damage on the power amp IC. In series with the headphone jack are 2 330R resistors which are very burnt, but still measure the correct value. I am planning to replace these just to be safe. The 2 switches to the right of the headphone jack switch the speaker outputs on and off. The SA-310 doesn't seem to have the transistors and relay that are present in that section of the diagram, unless I am blind and just can't find them on the pcb, but I think a relay would be pretty easy to spot.

My plan for tonight is to remove and test the parts in the voltage amp section and see if they are still in good condition. If anyone has any other suggestions, that would be great!

[moamps]

Hi,
have you checked control DC voltage at output power amp IC stk2048 without speakers conected? IMO, this IC is faulty.

Regards
Milan

[DcibeL]

It was real hard to test those pins with the amplifier turned on, due to where they were located, but it would seems as though this big IC has been destroyed. Thank you for your help.