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Old 13th March 2016, 12:18 AM   #1
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Question SU-Z780 Technics new Class A amplifier lights but no sound no relay click

Hi All,

I have had this amp since new in the 90s.

whilst playing some music it stopped suddenly and the VU lights stopped working along with the sound to the speakers.

I turned the unit off. unplugged every thing except the power cord.

I turned it on again and all the lights came on, however the speaker relay has not clicked like it normally does a few secs after pressing the power button.

I took the cover off the unit and looked and smelled for anything burnt but nothing. The main fuse is of course fine as the power is on. There's a large chip(~50mm) and another 15mm connected to the one mammoth heat sink and a Fan off that. I can see the speaker relay ...well the only one there.

Apparently(from the user instructions) there is a protection circuit that resets itself after a speaker short or overload, but that's not helping as I've got every thing disconnected and I cannot see it anyway.


I tried connecting a CD source to the RCAs but nothing.
I tried the headphones jack, but also nothing.
I've read on this forum that the speaker relay might need replacing.
I've ensured that only one of the 2 speaker circuits is turned on.
I've checked that the VCR switch that diverts the input monitor is off.

there is no schematic on the operating instuctions so I am flying blind here.

are there any internal checks that I can make to help identify the components to replace ? like perhaps bypassing the speaker relay ?

thanks guys, judging from other threads, you have a good community here.

Last edited by armarra1; 13th March 2016 at 04:07 AM. Reason: thumbs down confused with question mark...
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Old 13th March 2016, 04:00 AM   #2
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Question pics for post on technics suz780 amp

a few pics to go with the question...

20160313_114913.jpg

20160313_114413.jpg

20160313_114427.jpg

Last edited by armarra1; 13th March 2016 at 04:07 AM. Reason: thumbs down confused with question mark
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Old 13th March 2016, 04:06 AM   #3
tomchr is offline tomchr  Canada
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It sounds like some speaker protection circuit has kicked in, protected your speaker, and is now preventing the amp from turning on.

Try measuring the output voltage of the chip amp itself. You should be able to access this on the relay pins. You'll probably find DC on the output of the chip amp.

Tom
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Old 13th March 2016, 04:19 AM   #4
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Quote:
Originally Posted by tomchr View Post
It sounds like some speaker protection circuit has kicked in, protected your speaker, and is now preventing the amp from turning on.

Try measuring the output voltage of the chip amp itself. You should be able to access this on the relay pins. You'll probably find DC on the output of the chip amp.

Tom
thanks for the quick response Tom.

is the chip amp the huge one in the centre ?

so basically I'm checking for 24vdc to trigger the relay ?

I'm presuming I get at that from underneath...so removing the board from the casing ?
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Old 13th March 2016, 04:25 AM   #5
Hogwild is offline Hogwild  Canada
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Shouldn't this be in another forum? This is not a chip amp, yes?
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Old 13th March 2016, 04:44 AM   #6
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Quote:
Originally Posted by Hogwild View Post
Shouldn't this be in another forum? This is not a chip amp, yes?
I did a google image search for technics chip amp and a picture of the same chip came up. https://www.google.com.au/imgres?img...AoEQMwgiKAYwBg

I've never heard the term before, but id expect its an amp that has a chip rather than valve or transistors at its heart...but I may well be corrected on that.
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Old 13th March 2016, 05:29 AM   #7
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by Hogwild View Post
Shouldn't this be in another forum? This is not a chip amp, yes?
What would you call it? The SVI3205 power amp module is about as chip amp as it gets.

Quote:
Originally Posted by armarra1 View Post
is the chip amp the huge one in the centre ?
Yep. The black module on the heat sink.

Quote:
Originally Posted by armarra1 View Post
so basically I'm checking for 24vdc to trigger the relay ?
Not really. You'll probably find 0 V at the relay. That's why it doesn't go click. Your task is now to figure out why it doesn't go click. Reasons include:

1) Amp module/chip failure (likely)
2) Over temperature fail (not likely as the amp is stone cold)
3) Failing monitoring circuit or relay driver (not impossible)

Quote:
Originally Posted by armarra1 View Post
I'm presuming I get at that from underneath...so removing the board from the casing ?
I googled "SVI3205 datasheet" and found this schematic (click for larger view). That's probably how your amp is built.

SVI3205.jpg

Note the relay driver signal on pin 18. Measure to see if you get the waveform shown (0 V at power-on and -13 V after a 4-second delay).

Also measure the power supply voltages on Pin 4 and Pin 5. You should see a symmetrical voltage, so, say 60 V as shown above or 45 V. If you see, say, +45 and -0 V, there's a fault in the power supply.

If the power supply looks good, measure the output of each channel on pins 3 and 6. Should be 0 V.

Measure the voltage on Pin 10 (-15 V -- I wonder where this comes from).

Measure the AC voltage on Pin 8.

It looks like a pretty interesting module. Notice the four power transistors on large metal slugs to get rid of the heat.

Click the image to open in full size.

Tom
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Last edited by tomchr; 13th March 2016 at 05:33 AM.
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Old 13th March 2016, 07:40 AM   #8
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Quote:
Originally Posted by tomchr View Post
Not really. You'll probably find 0 V at the relay. That's why it doesn't go click. Your task is now to figure out why it doesn't go click. Reasons include:

1) Amp module/chip failure (likely)
2) Over temperature fail (not likely as the amp is stone cold)
3) Failing monitoring circuit or relay driver (not impossible) -hoping for this Tom.



I googled "SVI3205 datasheet" and found this schematic (click for larger view). That's probably how your amp is built.

Attachment 537101

Note the relay driver signal on pin 18. Measure to see if you get the waveform shown (0 V at power-on and -13 V after a 4-second delay).
pin 18 is 0v always.

Also measure the power supply voltages on Pin 4 and Pin 5. You should see a symmetrical voltage, so, say 60 V as shown above or 45 V. If you see, say, +45 and -0 V, there's a fault in the power supply.
pin 4 is- 65v pin 5 is 65v

If the power supply looks good, measure the output of each channel on pins 3 and 6. Should be 0 V.

so power supply is okay but I seem to have dc voltages at the pins 3 and 6 which is a good reason why I have 0v at pin 18....but why I wonder.

pin 3 is -55v pin 6 is -42v


any thoughts on these.

Measure the voltage on Pin 10 (-15 V -- I wonder where this comes from).
Pin 10 is -14.7

Measure the AC voltage on Pin 8.
pin 8 has 1.1v AC

It looks like a pretty interesting module. Notice the four power transistors on large metal slugs to get rid of the heat.

Click the image to open in full size.
yep the amp is supposed to run in class B over 20watts but a pseudo class A below.

Tom
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Old 13th March 2016, 05:34 PM   #9
tomchr is offline tomchr  Canada
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What's the input DC voltage on pins 13 and 15? If you have DC there, you should have DC on the output if the amplifier module is DC coupled (which I don't know if it is).

You should have 0 V DC on the amp input (pins 13, 15). If you don't, we should figure out why. If you do have 0 V on the inputs and get DC on the output, your amp module is fried.

And yep. The DC on the output is the reason the amp module doesn't assert the relay control signal. The protection circuit is doing its job and protecting your speakers from the DC.

Tom
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Old 14th March 2016, 12:21 AM   #10
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Quote:
Originally Posted by tomchr View Post
What's the input DC voltage on pins 13 and 15? If you have DC there, you should have DC on the output if the amplifier module is DC coupled (which I don't know if it is).

pins 13 and 15 have 0v and -19.6v DC
its not looking good for one of the channels as the zero on 13 shouldn't have DC at its output as you said below

You should have 0 V DC on the amp input (pins 13, 15). If you don't, we should figure out why. If you do have 0 V on the inputs and get DC on the output, your amp module is fried.

And yep. The DC on the output is the reason the amp module doesn't assert the relay control signal. The protection circuit is doing its job and protecting your speakers from the DC. yes I understood this part...but why the DC on one input

pins 12 and 16 are connected to pins 3 and 6 output channels via the RC parallel circuit. so I measured these as well.
so pins 12 and 16 are 7.1v and 31v respectively. I'm kinda hoping at this stage that the DC at one channel's input is affecting how the other channel generates its output from the zero input on its channel, but I'm just being hopeful.

any thoughts on what next Tom ? I'm thinking one channel is fried...but hopeful the DC on the input of the other channel mitigates this


Tom
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