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Old 21st July 2007, 10:46 AM   #1
Nordic is offline Nordic  South Africa
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Red face Help me fix this Yamaha YST-SW015 subwoofer

Hi, I took over this sub form a cazy young guy, who brought it to me for repairs a year or so ago... thought I'd keep it, till I'm a bit more handy with these kind of things...

Cause of error.. mains wire connected to car battery, with PSU selector set to 125V range... which needless to say blew the fuses...WHICH HE THEN HARDWIRED...

So far I replaced the opamp on the output which was burned...

On powering up, all the control lights etc, light up... but no sound... which is where I left it... next time I saw the amp, the guys have cut through wires etc, --- too lazy to desolder.. luckily they are colour coded... at which point I took out the speaker, which tested ok, and gave the guy about $20 and took the whole thing...

Now I have learned how to test transistors... I started working my way through the board... and the only dead one I found so far... is in the relay section of the standby circuit.

Not a transistor I can easily find localy...
2SC4688
Datasheet

Can you guys recommend a replacement?

I removed the transformer and it measures nearly 60VAC on the output... do I assume it is damaged, if I look at the numbers on the schematic....?
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Old 22nd July 2007, 11:11 AM   #2
Nordic is offline Nordic  South Africa
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No help yet?

This board has quite a few dead transistors... and checking the circuit diagram, I have to assume the opamps... and other ICs are also dead...

Here is a replacement I made for the inline ne555 chip, the dual opamp next to it has to receive a replacement too....

I think I will just replace that large transistor with 2sc5200, which exceeds al its power specifications.. seeing that I can't find a replacement... the other dead ones seem to be more traceable, like 2sc2240

Also still in doubt over that transformer.... going to bypass it with a smaller transformer... if it works... I'll know the transformer is damaged too...
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Old 22nd July 2007, 11:16 AM   #3
Nordic is offline Nordic  South Africa
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For those who like subs... here is a block diagramme of the amp.
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Old 22nd July 2007, 01:12 PM   #4
jaycee is offline jaycee  United Kingdom
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Transformer sounds like it's ok.

I guess it depends what was toasted to begin with. If he's hardwired it, all bets are off - the whole power amp circuit is probably toasted. You will want to test all parts in that.

Do you have a schematic?
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Old 22nd July 2007, 01:23 PM   #5
Nordic is offline Nordic  South Africa
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Yes I do, but not sure If I can post it... I seem to have a slightly later version than the service manual... and it has the occasionaly incoorectly marked component number... but I think I am getting on top of things....

Luckily the sub has a few relays inside which seemed to have protected the majority of it... It would appear the damage is focused around the auto power on/off circuit, but it did blow the tda7294 on the power amp...

I intend to replace all the opamps, just to be sure...

bridges and diodes seem fine........so far.... still have a board or two to run through....
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Old 22nd July 2007, 04:28 PM   #6
jaya000 is offline jaya000  India
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Better post here or somewhere else so that we can suggest u after study.
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Old 22nd July 2007, 05:28 PM   #7
Nordic is offline Nordic  South Africa
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If you are interested, hit the e-mail button below my post, and I will forward the PDF to you... about 2.6mb
Most transistors test ok...

some dead ones are
Q4 - E7 (LOCATION)
Q12 -D2

someone said the transformer looks ok... but 60V is a bit heavy for the 8V reg that sits after this one... there is a main transformer which should put out about 60V P2P

All transistors in power amp, measure fine useing general diode test method...allthough the chip did burn to a crisp... not the same chip im my amp as schematic... but the chip has been replaced, without fixing anything... obviously power will have a hard time getting there with the relay's switching and senseing transistors damaged...
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Old 22nd July 2007, 07:09 PM   #8
tvi is offline tvi  Australia
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The service manual I have says it uses a STK404-070 amp module.

If you want I can email you a copy?

Regards
James
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Old 22nd July 2007, 08:07 PM   #9
Nordic is offline Nordic  South Africa
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The unit I have actualy has a TDA7294... as I said, my unit is slightly later version, but same model no...
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Old 24th July 2007, 02:08 PM   #10
Nordic is offline Nordic  South Africa
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At first I thought those sil-8 packages for the opamps were daft, but after assembling a test jig, I can certainly see their use from a layout point of view...

Click the image to open in full size.

Replacement opamp tested and inserted into the board shown previously... have to pick up replacement transistors for the dead ones on it tommorrow.

Click the image to open in full size.
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