Author Topic: EEVblog #378 - Dumpster Diving Teardown Repair  (Read 24065 times)

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Offline free_electron

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EEVblog #378 - Dumpster Diving Teardown Repair
« on: October 30, 2012, 07:01:11 pm »
Heatsink tabs ? Those are 'solder thiefs' !

these boards are wave soldered. so they position the tqfp packages under a 45 degree angle in respect to the wave. this causes the solder to jump from pad to pad and the surface tension avoids the shorts. you do need a run-in for the wave so you will find small thief area's on 3 corners. the last corner ( where the wave exits ) has a longer 'thief area' to make sure there is no short remaining between the last few pins as the wave exits. so the board goes through the wave with the short thief area first and exists the wave witht the long thief area.

there is an official IPc standard to design these thieving area's.  the shape is differen for two row as opposed to quad packs.
« Last Edit: October 30, 2012, 07:04:12 pm by free_electron »
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Offline Alana

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #1 on: October 30, 2012, 08:02:38 pm »
At least i know that I'm not a complete moron. I had one of similar HIFI sets to fix with very same problem. Result: i told the owner that it seems like main uP is gone. I wonder if Dave fixes it, and if i can find my mistake from before 2 years [if there was any] :D
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #2 on: October 30, 2012, 08:03:22 pm »
I have a suspicion that's not a switching supply... it looks like the transformer is straight across the mains.
 

Online PA0PBZ

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #3 on: October 30, 2012, 08:43:52 pm »
I have a suspicion that's not a switching supply... it looks like the transformer is straight across the mains.
It is, did you see the clever way they switch the power to the transformer with the bridge and the fet?
Now I also understand why there was no cap behind the bridge.
Keyboard error: Press F1 to continue.
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #4 on: October 30, 2012, 08:57:11 pm »
Yeah, I'm thinking that the reason it's pulsing is they are pulsing the power to the transformer.

That way they can get low standby power (sub 1W) without using a real switching supply which potentially introduces noise (in a good system, e.g. a scope, it -won't-, but the audiophiles would get annoyed otherwise.)
 

Offline Rutger

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #5 on: October 30, 2012, 09:14:08 pm »
How about unplugging the power to the amplifier board and eliminating at least a possible bad output mosfet.
 

Offline ecat

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #6 on: October 30, 2012, 09:26:56 pm »
I fixed a similar Yamaha with a similar turn on problem a few years ago.

Each of the power amps has a protection circuit on the output which combine to drive an 'all is good' relay. When a fault is detected on any of the channels, eg blown output transistor, the relay drops out and the amp appears dead as a dodo. Time to remove the upper board behind the heatsink and get down to some serious audio amp debugging :)

... or do what I did, remove the upper board and start clipping the leads to the output transistors until the problem goes away. I ended up with a nice 6 channel amp instead of a 7 channel, by hay, it was a super cheap fix.

How about unplugging the power to the amplifier board and eliminating at least a possible bad output mosfet.

iirc this doesn't help. No power to the amp boards equals no power to the safety relay so the amp still appears dead.



« Last Edit: October 30, 2012, 09:31:34 pm by ecat »
 

Offline Pentium100

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #7 on: October 30, 2012, 09:36:35 pm »
That was interesting. Maybe the processor checks the output transistors or something and does not start up because it finds a fault?

The yellow wires from the transformer are likely for the filament of the VFD.

Seeing this video made me appreciate my Denon amp even more - in the case that the microcontroller fails I could replace it with a small set of 74HC or 4000 series chips, I would lose the remote control possiblity, but I do not have the remote anyway, so not a big problem (other than the remote, the MCU just switches on the appropriate relays when a button is pushed to select input etc).

tom66 - in a (modern) scope the analog parts are small and can be shielded relatively cheap. The analog part in an audio amp is usually quite big, so you would have to place all those boards in a can.  Besides, scopes usually have 8 bit ADCs, so about 48dB of SNR anyway, compared to 110dB (line inputs) of an audio amp. If the amp has a phono input then the requirements are even higher, since a cartridge outputs only 0.5mV (MC) to 5mV (MM) and my amp has 94dB SNR for the MM input.

PA0PBZ - a very similar circuit is used in a couple of my tape decks to control the speed of the AC reel motors (but with a bipolar transistor and in linear mode).
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #8 on: October 30, 2012, 09:37:52 pm »
Wouldn't such an amp click once so the trafo gets power and then can test the outputs? Or does it do it in standby somehow?
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #9 on: October 30, 2012, 09:40:16 pm »
Wouldn't such an amp click once so the trafo gets power and then can test the outputs? Or does it do it in standby somehow?

Quote
tom66 - in a (modern) scope the analog parts are small and can be shielded relatively cheap. The analog part in an audio amp is usually quite big, so you would have to place all those boards in a can.  Besides, scopes usually have 8 bit ADCs, so about 48dB of SNR anyway, compared to 110dB (line inputs) of an audio amp. If the amp has a phono input then the requirements are even higher, since a cartridge outputs only 0.5mV (MC) to 5mV (MM) and my amp has 94dB SNR for the MM input.

Interesting. It might end up being cheaper for low power analog stuff like this, it is consumer electronics after all...

The new Rigol DS2000 has some 400µV noise floor with a heavily shieled PSU but the older DS1000E still manages around 1mV without (much) shielding.

I noticed some jamicons on the logic board, but they aren't too bad for general purpose caps.
 

Offline Pentium100

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #10 on: October 30, 2012, 10:07:08 pm »
The new Rigol DS2000 has some 400µV noise floor with a heavily shieled PSU but the older DS1000E still manages around 1mV without (much) shielding.
The Rigol has a heavily shielded PSU and shielded analog sections (which most likely include opamps or other devices to amplify the signal (because I doubt that the ADC has 400uV noise floor). Still, those sections are rather small, compared to low power analog sections in an audio amp (volume, tone controls, filters).

400uV noise floor on 0.5mV, 5mV phono inputs and 150mV line input would be at -2dB, -22dB and -51dB respectively.
 

Offline smugtronix

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #11 on: October 30, 2012, 10:16:35 pm »
Are those little brown caps (I'm assuming polypropolene or stacked metal film) Panasonics, or are they some one-hung-low brand? I'd be interested to see what they use for caps in that price range.
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #12 on: October 30, 2012, 10:25:42 pm »
Interesting, some youtuber suggests C405. Found it in the schematic but can't figure out what it might do. Some kind of AC power detection logic?
 

Offline Pentium100

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #13 on: October 30, 2012, 10:43:33 pm »
Interesting, some youtuber suggests C405. Found it in the schematic but can't figure out what it might do. Some kind of AC power detection logic?
Looks like zero crossing detector and power for the 4013 chip.

When the voltage in C411 (the output filter cap) gets too high, it turns on IC403 which pulls the Data pin (pin 5) of the 4013 down. On the next rising edge of the mains input the chip switches off its output and, in turn, the transformer. When the output voltage gets too low, IC403 switches off and the ata pin gets pulled up by R414 and on the next cycle Q404 switches on.

The chip is powered trough the cap (and D406) too.

This is kind of slow PWM, most likely made so that it does not introduce wideband noise to the amp.
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #14 on: October 30, 2012, 10:47:18 pm »
Interesting, some youtuber suggests C405. Found it in the schematic but can't figure out what it might do. Some kind of AC power detection logic?
Looks like zero crossing detector and power for the 4013 chip.

When the voltage in C411 (the output filter cap) gets too high, it turns on IC403 which pulls the Data pin (pin 5) of the 4013 down. On the next rising edge of the mains input the chip switches off its output and, in turn, the transformer. When the output voltage gets too low, IC403 switches off and the ata pin gets pulled up by R414 and on the next cycle Q404 switches on.

The chip is powered trough the cap (and D406) too.

This is kind of slow PWM, most likely made so that it does not introduce wideband noise to the amp.

Yeah, I noticed the output pulsing at around 700mV (for the 10.5V), which is quite high, but it was at a low frequency, so I guess it would be inaudible. Anyway, that is only for the standby supply, to click the relay and maybe a few other digital things -- unlikely to be used in the analog section (I'd take a guess that the 10000u 16V Nichicons have something to do with that?)

I wonder how long this amp would run with no power and the AC detect disabled. It has enough capacitance in it! I modded my cheap computer speakers to have a 6800µF bulk cap because the transformer was dropping out on high peaks (470µF was the original.) Now they last 3-4 seconds with no power and ordinary music.   ::)
 

Offline mzacharias

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #15 on: October 30, 2012, 10:53:35 pm »
I can provide service literature and technical support. I work on these types all the time.
 

Offline mzacharias

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #16 on: October 30, 2012, 11:02:44 pm »


Yamaha Training video on their "green" power supplies.
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #17 on: October 30, 2012, 11:20:26 pm »
Useful video, but his advice about removing the ground on the scope, or isolating the scope is dangerous. The hifi amplifier should always be the isolated part.

I also suspect it doesn't work quite as he says - the data pin should be high most of the time (it essentially equals Vcc when opto off), the clock latches this data in (from the AC), which turns off the FET (through not-Q.) When data falls low, it latches that in, which turns on the FET. It ensures the transformer only ever sees 0V or one AC cycle (full cycle due to half-wave rectifier), quite neat actually...

I have also always wondered if there's a good reason not to use that full wave rectifier with FET to switch low voltage, high current AC -- over using a triac, for example.
« Last Edit: October 30, 2012, 11:22:23 pm by tom66 »
 

Online EEVblog

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #18 on: October 30, 2012, 11:52:23 pm »
Yamaha Training video on their "green" power supplies.

Youtube has a video for everything!, what are odds?

Anyway, followup:
It was C405, a 22nF metalised polyester film cap, 630V.
It read low, about 500pF.
A common fault in these Yamaha's apparently, and a few Youtube commenters pointed it out.

Dave.
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #19 on: October 31, 2012, 12:00:35 am »
Yamaha Training video on their "green" power supplies.

Youtube has a video for everything!, what are odds?

Anyway, followup:
It was C405, a 22nF metalised polyester film cap, 630V.
It read low, about 500pF.
A common fault in these Yamaha's apparently, and a few Youtube commenters pointed it out.

Dave.

Follow up video! Follow up video!
:D
Pweeeeease...

Reminds me of a proud "dumpster" find. My brother told me his garage needed cleaning out, anything I wanted I could keep, got a faulty Xerox 19" monitor (7 bad caps, easy fix), and a nice working surround amp (but for a computer, not as good as the Yamaha.) Along with other bits of useful gear like network switches.
« Last Edit: October 31, 2012, 12:02:53 am by tom66 »
 

Offline mzacharias

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #20 on: October 31, 2012, 12:23:52 am »
Yamaha Training video on their "green" power supplies.

Youtube has a video for everything!, what are odds?

Anyway, followup:
It was C405, a 22nF metalised polyester film cap, 630V.
It read low, about 500pF.
A common fault in these Yamaha's apparently, and a few Youtube commenters pointed it out.

Dave.

Not a Youtube video as such. I had it available as an authorized servicer and made it available only through this link. It's not my policy to release potentially dangerous information to the public.

Mark Z.
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #21 on: October 31, 2012, 12:31:15 am »
I am of the opinion, if someone with no electronic experience is dumb enough to open their product and get electrocuted (which is easy to avoid by following simple precautions), then they have made a mistake in the first instance, and lack of service information won't stop them. In fact, if anything, it might lead to more shocks due to it being harder to determine the high/low voltage parts of the circuit and which parts, if any, are safe to touch.
 

Online EEVblog

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #22 on: October 31, 2012, 12:48:15 am »
Follow up video! Follow up video!
:D
Pweeeeease...

Rendering now...

Dave.
 

Offline tom66

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #23 on: October 31, 2012, 12:51:41 am »
Follow up video! Follow up video!
:D
Pweeeeease...

Rendering now...

Dave.

Dammit, now I won't be able to get up at 10am for lectures!
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Offline Psi

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Re: EEVblog #378 - Dumpster Diving Teardown Repair
« Reply #24 on: October 31, 2012, 01:14:14 am »
A good thing about those sort of set top surround amps is that the amplifier PCB is often separate and only needs power and signal.

It's normally pretty trivial to rip out everything but the transformer and amp board then feed the amp some line level audio.

Then you have a nice simple multichannel high power amp. :)
« Last Edit: October 31, 2012, 01:16:33 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 


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