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Old 11th December 2008, 02:47 PM   #1
nfm is offline nfm  United States
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Default Hi-Fi Stereo went BOOM

Hi guys,
If you have a spare minute, please help a college student. I have been upgrading my aging Hi-Fi stereo with by replacing capacitors. It all went smoothly and I got nice results so I decided to replace all capacitors and the only ones that were left before it went BOOM were the caps on the amp board. The moment I turned on the stereo after upgrading amp caps and for a split second I heard a pop/crack/buzz but I continued to listen, it sounded great for 30 secs and suddenly the there was huge BOOM/bass almost sounded like my speakers exploded. I checked back and noticed wrong polarity on one 22uF cap on the amp board. I corrected the polarity and soldered the brand new one but still the stereo doesn't give off any sound, it's dead silent. I already replaced the STK4231II stereo amp/chip with brand new one and still no sound. I also replaced the AC->DC diode on the amp board, still no sound. What I noticed is that the stereo doesn't make a tick sound like it always did when it was turned on or off. I think relay makes the ticking sound, could it somehow get fried?

Here's the amp board:
Click the image to open in full size.

^ Notice the blue building in the upper middle, that's where the relay (OEG OSA-SS-224DM3M) & big resistor is, and the L R speaker outputs.

General view:
Click the image to open in full size.

Pre-amp and microprocessor:
Click the image to open in full size.

^ There's one bipolar red blackgate capacitor in the middle-left, on top of that is the pre-amp area. In the middle is the NEC microprocessor, apparently it's a DAC.

Now the backsite, the NEC microprocessor is to the left, and NEC pre-amp is to the right, in the middle is a Mitsubishi opamp:
Click the image to open in full size.

I'm no expert, I throughly checked all aluminum caps with a multimeter and they all charge and discharge properly, also there are no blown caps. I'm suspecting there's a problem with the relay because the stereo doesn't tick anymore. Also if you notice, out from the transformer there are two AC power lines, one to the amp board, one to the main board. The stereo turns on fine, It spins a CD, I see the wave animation on the display/LCD screen (that confirms that opamp, pre-amp & dac works), it's seems just like it's muted. The STK4231II stereo amp/chip isn't warm to touch it like it used to be.

Please help, I would like to listen to music again, I don't have much time and knowledge debugging circuits .
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Old 11th December 2008, 03:46 PM   #2
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Default Fuse-resistors

Check the grey-blue small resistors near the big STK IC. These are fuse-resistors and if the STK fails these fail too!
(been there done that)
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Old 12th December 2008, 01:14 AM   #3
nfm is offline nfm  United States
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Hi QSerraTico_Tico,
Thanks for the help, one of the grey-blue resistor was busted, along with 4 other resistors. The IC is still isn't warm to touch and makes no sound, no tick at startup either. The caps, diodes, and resistors seem to be good on the amp board, the only thing I'm not certain are the 2 3-leg transistors/mosfets:
Click the image to open in full size.

Here's a better shot of the relay and protector IC NEC 1237HA:
Click the image to open in full size.

Any idea which part made a tick sound when my stereo used to work? What initiates STK4231II startup?

Thanks.
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Old 12th December 2008, 06:42 AM   #4
jitter is offline jitter  Netherlands
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Quote:
Any idea which part made a tick sound when my stereo used to work?
Most likely the big blue speaker relay. Comes on with a short delay after switching on power.
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Old 12th December 2008, 02:12 PM   #5
AndrewT is online now AndrewT  Scotland
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Default my aging Hi-Fi stereo

or is it No-Fi?
__________________
regards Andrew T.
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Old 12th December 2008, 05:43 PM   #6
Mooly is online now Mooly  United Kingdom
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Default Re: my aging Hi-Fi stereo

Quote:
Originally posted by AndrewT
or is it No-Fi?


At least it's not all glue and sellotape like some.
Deary me !! I have worked on hundreds of units like this. You have to accept that it was working, you got enthusiastic, now it's pancaked.
The relay almost certainly connects the PSU to the amp board.
You need to go back to basics, if you have no circuit get the data sheet for the STK and trace the supplies to it. See what drives the relay coil. See if the relay coil driver transistor is being turned on and off correctly from the micro. It's all basic faultfinding, don't assume anything !! For the output stage to work it needs power. Trace it back !! And remember that you need to find what you did wrong in the first place -- or it will do it again. All good fun. When you check resistors make sure you check them out of circuit. Any residual voltage ( a few mv ) across one is enough to totally confuse a DVM on ohms.

Edit, Any fuses or safety resistors on the upturned PCB on the transformer. I see re reading it all you put a cap in backwards -- good luck, 20 mins tops to fix it then.
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Old 13th December 2008, 06:12 AM   #7
nfm is offline nfm  United States
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jitter, AndrewT, Mooly, thanks for taking your time .
This is a very nice stereo with great columns, if it wasn't I would be fixing it or upgrading caps in the first place.

I'm 100% sure that the fuse and resistors on the transformer PCB board are OK. The datatsheet for the STK blows. It's either the relay or the NEC IC protector that is busted. Any idea where I could find the equivalent replacement part for the OEG OSA-SS-224DM3M (5A 50VAC 3A 125VAC 3A 30VDC) relay since these are not sold anymore?

Thanks guys .
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Old 13th December 2008, 07:44 AM   #8
Mooly is online now Mooly  United Kingdom
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Unless the relay contacts have welded together ..... Check it out ?? We don't guess here Is there voltage across the coil ? If yes, are the contacts closed. If no voltage across coil trace the relay drive back !
Just looking at the picture I may be wrong saying that that relay switches the power to the amp PCB. It location on the PCB looks to near the speaker outlets, but that changes nothing fault finding wise .
What's this NEC IC protector ? IC protectors are 2 legged devices usually marked something like N10 which means -- bizzarely 10 * 40 to give the value in milliamps, so that would be a 400ma protector. It should read short if it's OK.
The power amp PCB. When you switch on is there AC voltage across the bridge rectifier middle pins ? If no, trace the connections back. If yes does the STK have the correct supplies to it. For example -/+ 22 volts or whatever it is.
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Old 13th December 2008, 04:19 PM   #9
bwaslo is offline bwaslo  United States
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I have no experience with this particular box, so I may be wrong about this, but typically those relays are there to protect the speakers should the power amp go bad and put out a big DC voltage (speakers really don't like DC). The relay would be in series with the speaker contacts in that case, so as to disconnect amp from speakers.

If so, t is very unlikely that the problem would be the relay itself (particularly if you were replacing capacitors before it went). It's more likely that, were you to measure with a DVM and with the amp on, you'd find a DC level on the "amp side" of the open relay contacts and that the relay and protect circuit was doing its job of preventing your woofer voice coil from igniting.

It's even possible that the problem could be not in the power amp itself, but in the circuit driving it (if the PA is a type able to pass DC from in to out). But more likely, one of the (non main) power supply voltages that feeds the PA or earlier circuitry was blown out (a transistor or IC regulator probably) by the backwards cap, and that is upsetting things. If you haven't already tried, I'd first ascertain whether there is DC on the amp side of the relay contacts (make sure you are on the contacts, not the actuator coil pins), so you know whether this is what you are trying to debug.

Also, check for voltage (when unit is turned on) across that cap that was previously backwards -- if there is none now, I'd start there, to see where the voltage was supposed to come from. But in either case, keep in mind that you may have more than one problem now, parts failure (or misconnection) can take out more than just one or two things.
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Old 13th December 2008, 07:43 PM   #10
nfm is offline nfm  United States
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Hi,
The bridge rectifier has been replaced before starting this thread, you can see how I bend the legs .
There's ~53.5v across the DC filtering caps, and 5.0v where the backward 22uF was.
The same ~53.5v goes through 2 of the legs of the blue relay. Here's a datasheet for the interesting IC protector: http://pdf1.alldatasheet.com/datashe...C/C1237HA.html
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