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Chip Amps Amplifiers based on integrated circuits

Technics SUV 550 bit of smoke after shorting bias adjust pins
Technics SUV 550 bit of smoke after shorting bias adjust pins
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Old 19th August 2018, 12:24 PM   #1
ssi is offline ssi  United Kingdom
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Default Technics SUV 550 bit of smoke after shorting bias adjust pins

Bit of an electronics Darwin incident.

Picked up a Technics SUV550 from a charity shop as part of a system bundle. Excellent condition. Cleaned up inside and adjusting bias (to 80mv!) The bias adjust pins are about 4mm apart. Used hooking probes to avoid disaster but I got distracted for a second. The pins may have shorted - noticed a small whiff of magic smoke so shut down instantly. The smoke seemed to be coming from near the power IC.

All seems good. Amp turns on ok and DC still around 3.5mv per channel at speaker terminals and sounds fine.

Would it have been a resistor? I feel stupid so looking for reassurance! Have I got away with it? These amps do not have removeable backs like the older technics amps which are easy to work on.

Looking forward to your opinions.
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Old 19th August 2018, 02:13 PM   #2
ssi is offline ssi  United Kingdom
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Update: There is a large 820 ohm 2w resistor (R525) near the Power IC SVI3205 which could have caused the magic smoke. It reads 825 ohms in circuit so should be OK but runs warm (probably normal I hope!). I have run the amp for a few hours and it seems to be functioing well. The amp itself runs reasonbly cool but it does have tons of heatsinks for the power IC (on the IC itself as well as the heat pipe cooling system).


Not going to mess with it anymore as it does not look easy to work on. Will check on the resistor in a few days time.
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Old 19th August 2018, 09:06 PM   #3
SVI2004A is offline SVI2004A  New Zealand
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Technics SUV 550 bit of smoke after shorting bias adjust pins
Bias is fixed in power IC (though R528 820K influences this)

R525 is output relay supply feed... fact relay is operating- this will be fine.

I’d have suspected issues in V amp area (around 4 transistors on small heatsink)

You got lucky by the sounds of it.....

Best policy is to hook up connections, making sure no shorts, meter set right, then power ON
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Old 20th August 2018, 06:00 AM   #4
ssi is offline ssi  United Kingdom
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Hi SVI2004A. I hoped you wuld reply as your knowledge on Technics is legendary. You helped me with an SUV4x about a year ago and I have 4 now!


I was of course adjsting the ICQ for the voltage amp and not bias. The amp has been working for 5 hours and all is good.



What do you think I shorted? Would it have been the pins for attaching the dvm? Also what was smoking? There are no burn marks. I was measuring the right ICQ of the right channel when it happened. My fear was the I may have damaged the SVI3205 chip package.



The amp sounds really good and very powerful. In my opinion better than the later SUV560 which also has a SVI3205 but not as good as an SUV4x.
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Old 20th August 2018, 09:05 AM   #5
SVI2004A is offline SVI2004A  New Zealand
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Technics SUV 550 bit of smoke after shorting bias adjust pins
I do agree the new class A sound was in a league of its own - the later class AA always seemed a bit constrained by comparo

I know the V4X simply has some kind of magic... I have had some bad speakers that made an all discrete amp sound horrid... the V4X sure showed them who was boss...

I think the Synchro bias circuit has it ready for what comes its way.

The way it is applied in the V4X is more simplistic and direct than the V6X and V8X

V2X is on par, but the V4X has more aplomb.
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Old 20th August 2018, 09:53 AM   #6
SVI2004A is offline SVI2004A  New Zealand
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Technics SUV 550 bit of smoke after shorting bias adjust pins
As for short.... it it was the ICQ (V Amp bias) it was already strapped together under 1 ohm (Emitter resistors) maybe a resistor did momentarily heat up (possibly burning dust off it)

But only momentarily and from what you’re saying has come through without issues....

The power IC circuit is largely self contained- with exception of the class AA bridge having influence on its NFB loop
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