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Manufacturer using part over maximum ratings, is it safe?
Manufacturer using part over maximum ratings, is it safe?
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Old 1st August 2019, 11:39 AM   #1
Salberg is offline Salberg
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Default Manufacturer using part over maximum ratings, is it safe?

Hi, I am working on repairing a Marantz PM17 mk2 and found the fault to be a short in the LC78212 analogue switch. When looking in the datasheet the maximum supply voltage is ± 20V, and on the schematic of the amplifier the power supply voltages is stated to be ± 23V.

So my question is do you think it will be safe to replace the chip with a new from ebay without blowing up? Or do you think I should modify the circuit so the LC78212 gets a lower supply voltages, or maybe you have a good idea of how this could be done?

Thanks for any thoughts.

LC78212 Absolute Maximum Ratings.pngLC78212 Marantz PM17 mk2.png
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Old 1st August 2019, 12:05 PM   #2
starkeyg is offline starkeyg
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Hello,

Personally I would measure what the actual supply voltage is. I have seen differences in what the schematic says and the actual voltages are.
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Old 1st August 2019, 12:14 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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I wouldn't replace with a Fleabay chip, I would source a Sanyo LC chip.
I don't think Marantz would under engineer any of their designs.
LC78212 Sanyo,DIP-30,04+/05+ | IC Chips | UTSOURCE
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Old 1st August 2019, 02:15 PM   #4
Salberg is offline Salberg
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Thanks for replying.

The actual supply voltage is around +24 and -23.5
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Old 1st August 2019, 02:34 PM   #5
Mooly is offline Mooly  United Kingdom
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Manufacturer using part over maximum ratings, is it safe?
I looked at a different data sheet but it to shows 20 volt as a max rating. Curious. I would definitely look at reducing the rails which should be easy in practice.

Swapping these two zeners for a low value will drop the rails. Rail voltage equals the zener voltage less the two Vbe volt drops of the series pass transistors... so 20 volt zeners would give around -/+ 18 volts.
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Old 1st August 2019, 02:50 PM   #6
Salberg is offline Salberg
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But if I change the zeners diodes as you suggest all the the others circuits will also get a lower voltage. This two supply voltages is used in a lot of the other places, so the best way is just to lower the voltage to the LC78212.
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Old 1st August 2019, 05:40 PM   #7
Mooly is offline Mooly  United Kingdom
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Manufacturer using part over maximum ratings, is it safe?
If you want to lower the supply to just this chip then it's a little more involved. There are a couple of options you could try.

1/ Add two series zeners, one for each rail that feeds the chip. Use something like 4v7 Zeners. The current consumption of the chip is very low and so that really suggests that as well as the zeners you should include two resistors of say 8k2 wired from the output of the zeners to ground. That would add an additional known load of a couple of milliamps per rail and ensure the zeners always give a rail of around -/+18 volts. Without the resistors you could find the rails are higer than expected if the chip has a very low current draw under certain conditions.

So dead easy, two zeners and two resistors.

2/ Same number and type of components but different values and arrangement. Add two 18 volt Zeners across each rail of the chip and add series feed resistors. With 24 volt rails and lets say the chip could use 3 milliamps then you would need a resistor of around 820 ohms per rail. That would drop the excess 6 volts per rail and allow a small Zener current to flow all the time.
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Old 1st August 2019, 06:11 PM   #8
ej25awd is offline ej25awd  Canada
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Quote:
Originally Posted by JonSnell Electronic View Post
I wouldn't replace with a Fleabay chip, I would source a Sanyo LC chip.
I don't think Marantz would under engineer any of their designs.
LC78212 Sanyo,DIP-30,04+/05+ | IC Chips | UTSOURCE
UTSOURCE is also an eBay seller that sells counterfeit parts regularly. UTSOURCE is NOT an authorized parts dealer. UTSOURCE is just broker like Littlediode UK. UTSOURCE and Littlediode will gladly sell anything that they can get their hands on.

The older 22xx Marantz series under engineered their electrolytic capacitors. This notion that big names don't make this mistake is just wishful thinking.

Intel screwed up with their Pentium FDIV bug, NASA screwed up with their 1998 Mars orbiter SI units, etc. These are entities that are suppose to be the gold standard in engineering but they still made mistakes despite the rigorous checks they have in their system.

Humans make mistake, we all make mistakes.

Salberg found a Marantz engineering error. Let us not downplay Salberg's findings. Keep an open mind and give Salberg credit for finding this problem for others. These are Salberg's first posts from the last 8 years and I can see why he didn't mention the manufacturer's name on the title.

Kudos Salberg for an informative and quality post!
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Old 1st August 2019, 06:33 PM   #9
simon7000 is offline simon7000  United States
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A manufacturer can get parts tested for a particular specification. That looks like what was done here.

I would not change the zener diodes as that may cause the regulator transistors to overheat.

I would use zener diodes in series with the power input pins to drop the voltage to 18 volts. Even a 4.7 volt 1/2 watt unit will work as the current is under 1 mA.
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Old 1st August 2019, 06:56 PM   #10
ej25awd is offline ej25awd  Canada
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Quote:
Originally Posted by simon7000 View Post
A manufacturer can get parts tested for a particular specification. That looks like what was done here.
Not another anecdote!

Do you have any shred of proof or evidence that supports this unsubstantiated statement of yours?

What is true to one case doesn't necessarily apply to another.
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