Damaged power supply... ?

[blight]

Hi!

I am a electronice newbie and i have tried repairing an amp... one of the main ICs was damaged and the output level-meter of that channel was damaged so i bought 2 new such ICs, soldered them in and switched it on... everything was fine until suddenly (i was just turning the volume up a bit) the channel which was damaged before went off and smoke came out of that IC... then also the power supply died for some reason but the transformator was not hot at all and it didn't smell at all!
now when i plug the amp in there's about +/- 120V AC coming out of the transformator, but it's the same signal on both outputs (altough the unit is not powered on! there are some big ceramic (?) capacitors at the on/off switch - could some voltage go through these into and through the transformator?)
wether i set the power switch to on or off - there's about 120V AC like everywhere in the amplifier (GND, signals, ...) and nothing changes when switching the power switch...
Unfortunately i do not have a multimeter to measure the current going into and out of the transformator (if that would help...)

Like i said i am a newbie. Does anyone have any ideas what could be wrong?

Btw: the transformator is inside a metal case and there's glue around it so maybe it burnt through and i didn't notice it? but it didn't get hot... hrm :roll:

Thanks,
blight

 

[Nigel Goodwin]

Well firstly, you say you have 120V AC everywhere, but don't have a multimeter! - so how are you measuring it?.

Secondly, it sounds possible that the new chip may have blown - often there's a reason for it failing in the first place, if you don't repair the reason the new chip will just blow again.

 

[blight]

i have an oscilloscope.
i was hoping that the new chip wouldn't blow up again and that the old one just blew (2 pins were short-circuit) and because of that the output level-meter was damaged...

 

[mozikluv]

hi

try to look for the trouble first in the supply section, disconnect it from the main section. you would need a DMM to check that out. check the primary side fuse, then the filter ceramic cap and the primary winding resistance. then the secondary side, test the winding resistance, if 0 ohms reading, thats shorted, if infinite resistance, its open. check the rectifier section, then the electrolytic cap. if all test are okay check the ac output at the secondary side, if output is same as the rated output your transfo is okay. now check the rectified output, it should be higher by 30% to 40% than the rated output (unrectified). check the secondary fuse. if the secondary output is regulated, check the regulated output, it should read the same as the regulator rating. if okay your power section is alright. the problem is in the driven system. look for burned foil tracks, unusual looking electro-caps, look for shorts.

i believe you have a short somewhere, maybe one of the reason why your transfo did not get hot is becoz you might have a regulator system with a built in short-circuit protection. looking for shorts is not an easy thing to do

'HAPPY NEW YEAR'

 

[Nigel Goodwin]

i have an oscilloscope.
i was hoping that the new chip wouldn't blow up again and that the old one just blew (2 pins were short-circuit) and because of that the output level-meter was damaged...

It's unusual to find someone with an oscilloscope, and not a multi-meter :lol:

I think the first thing you need to do is go out and buy one, you can get very cheap multi-meters which is probably all you need really - should be less than 10 Euros.

What make and model is the amplifier?, and what is the chip you've replaced?.

 

[blight]

yeah i guess it's unusual to have an osci but not a multimeter... it's a Technics SU-8044K and i replaced the 2 main hybrid amplifier ICs (STK0039)... the problem is that even the GND line has some AC voltage on it, wether the amp is switched on or off... but usually when some voltage comes out of the transformator (which is not the same as the input voltage) it is ok? because buying a new such transformator (metal-cased, like 100W or more) might be more expensive than what the amp is worth...

oh yea, i will get a multimeter soon i guess

btw: i can hardly disconnect the power supply section from the main section cause it's the same PCB there are 5 sigals going from the transformator to the PCB... one should be GND and 4 others - 2 connected to the power supply section (going through diodes and to the big capacitors) and 2 other ones connected to the output-level-meter section (which is also on the same PCB)... the 2 lines going to the power supply section carry the same signal (i have a 2 channel-oscilloscope so it's really the same, not one 180° phase-shifted or something) so there could never be a voltage causing current to flow could there? maybe i forgot switching the osci to DC and those 2 signals are AC/DC mixes? i have no idea :cry:

 

[Nigel Goodwin]

Are you sure you've got a ground connection between your oscilloscope and the amplifier? - if you haven't, it will result in massive hum pickup on the scope (which sounds like what you are getting).

 

[blight]

thanks! stupid me :roll:
now everything looks better :lol:

thanks all! :wink:

edit: i borrowed a multimeter and the I/O resistance of all of the pins of the "main ICs" is the same for both channels... does this eliminate any possibilities of what the problem could be? like it's unlikely that the chip blew because one of the power supplies was not working because the resistance from these pins to the big capacitors is like 0.2 Ohms...?

 

[mozikluv]

blown power amp ic



have you tried looking around the power chip? the parts connected to it? better yet get the datasheet of your power chip, is it STK0039? there might be something wrong with your speaker system and also the protection system of your unit. the protection system is directly connected from your power amp output.

 

[blight]

Yes, it is STK0039 and i have a kinda datasheet with equivalent circuits for some STK00XX chips but that doesn't help much... the power supply of the chips seems to be ok

I think this AMP only has fuses (2.5A, fast triggering) to protect the speakers and no DC check on the output.
Could a bad solder point cause i.e. the negative power supply of one of the ICs to go away and thus blow up? (i think this has happened to me with a ne5532 opamp once )
all the other parts look ok :roll:

anyway i have ordered the schematic (didn't find the service manual ) and i hope it helps me in partially understanding the AMP that i am able to repair it
also after the STK0039 blew there was still some sound output on that channel - guess it was 50Hz - wonder where that could come from

 

[Nigel Goodwin]

also after the STK0039 blew there was still some sound output on that channel - guess it was 50Hz - wonder where that could come from

When the output transistors (or devices) fail in an amplifier they will almost always go S/C, this puts a short across the supply rails. This short draws excessively high current from the PSU, causing the ripple voltage on the HT rails to increase - it's this ripple fed through the faulty chip (and the good one!) to the speaker output that makes the buzzing noise.

First sign of output device failure (in older amplifiers) is a loud buzzing noise in the speakers.

I checked on Panasonics website for you, I'm afraid they don't list that model (dealers can download Panasonic manuals for many models).

 

[blight]

when i got the amp it has been at some "repair shop" but the guys said they couldn't repair it (guess they were too lazy too look for STK0039)... the 2 STK0039 were soldered out an with the component tester i found 2 pins of the chip from the damaged channel were short circuit (power supply and something else)
now both of the chips are damaged the same way as the one was when i got the amp i think :cry:

I have ordered the schematic from www.schaltungsdienst.de (schaltungsdienst lange) - i have also ordered the service manual for a damaged yamaha ax-870 which i have
they have the user manual but no service manual for the SU-8044/K but the schematic like i said before
If they don't have a service manual i guess it's hard to find one or there is none? :roll: