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Old 1st March 2003, 03:13 PM   #1
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Default Heatsink grease - I'm confused ;-) ?

Hi!

Since the heatsinks of the old ripped Marantz amp I refitted with modern parts gets very hot I decided to also replace the old heatsink grease.

I unscrewed the amp from the sink, cleaned everything up, and placed about the same amount like the original grease of Arctic Silver III on the STK080G chips, then screwed everythin back together.

And now, o wonder, the heatsink gets even hotter!

After thinking a little bit about this I begin getting confused...

Since the heat transfer with the new grease from the ICs / transistors is better than with the 24 years old grease, is the heasink supposed to get hotter? Like, say, more heat is being transferred to the sink, so the sink gets hotter while the ICs stay cooler?

And how big is the influence a metal case on the heatsink? The sink originally was touching the metal case of the amp on three sides (sides and bottom), while nowadays it sits on wood... And I think that the heatsink never got so hot when the receiver was in its original state (with internal reveiver and pre-amp connected)...

You cannot really touch the heatsinks for very long - I have no way of measuring the temperature right now, but it's pretty hot. Should I maybe use a bigger heatsink, or are those STK chips not so picky about heating?

Mhmmm... maybe a fan would help... but I don't like that option...


Bye,

Arndt
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Old 1st March 2003, 03:22 PM   #2
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Lightbulb Heat

Hi,
If you transfer more heat to the heatsink it gets hotter.
STK IC's normally don't get hot when on a heatsink except when running at full power.
I am sure the metal case did conduct much heat off the heatsink as the metal is a far more better conductor than air. Or see the metalcase as a extension of the heatsink. I am sure this is the cause for the hotter heatsink now.
Old Crown amplifier D-150 used the whole chassis as the heatsink.
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Old 1st March 2003, 03:30 PM   #3
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Hi, Arndt

http://radvs.boom.ru/ims-unch5.html
has a little data on the STK.

http://www.datadart.com/al/sanyo/Stk075.pdf is a bad datasheet
I think STK's never run hot at idle.
Just warm should be normal.
Check their power supply.
Should be around 27-39V

Hope this can help a bit.

/Hugo - thinks more heat sink is not necessary.
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Old 1st March 2003, 03:38 PM   #4
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As has been noted, better thermal conduction from source to heatsink results in hotter heatsinks and cooler dies. The effect should not be all that pronounced, though ....

Now, if you want better cooling, the cheapest way to do it is to raise the unit above what it sits on so that you get better convection. This is very effective.

Petter
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Old 1st March 2003, 03:50 PM   #5
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Oops, gave you the wrong link to STK080G

http://www.datadart.com/al/sanyo/Stk075g.pdf should
be better.

The recommended power supply here is 27.5V

The quiescent current is 100mA Max.
So the heat sinks can only become warm at idle.

Maybe one of your mods is the cause?
Did they get hot before the mods?
Maybe someone can point out how to measure the idle current?

/Hugo
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Old 1st March 2003, 04:38 PM   #6
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Well, the "mods" I did were as follows:

1. take out the amplfier-board, disconnecting pre-amplifier and receiver part

2. disconnect the VU-meters from the outputs

3. replace the two filtering caps TOWA 6800 uF / 42 V with BHC SlitFoil 8200 uF / 50 V

4. take out the old "wrap-the-wire-around" pins, replacing them with new soldering pins

5. replace ALL electrolytics with Panasonic FC of same value, sometimes higher voltage rating, except for 1uF / 50 V which I could not get as Pansonic FC, so I used Elna Silmic, and 47 uF / 63 V, for which I took BC components elko

6. (just incorporated) replace old heatsink grease with ArcticSilver III


Actually, all those modifications resulted in colder components except for the heasink. There is a resistor mounted on board (R806), which is packaged in a special case (so it does not desolder itself), which got very hot with the old electrolytics (I actually burned my finger once on it), which now stays relatively cool to the touch.

But I have to admit that I did not check voltages yet. I have the service manual here, but I don't know how to measure quiscent current (maybe with 1 ohm resistor in series with supply, measuring voltage across...?), and there is no trimpot at all on the board with which to set the current...

I have to say that you really got me frightened now... there are no STK080Gs available any more, and the only STK that comes close to 080G in the name I can get is a STK086, which costs 44.50 (about 46 $)...

So, again, help to rule out any electronical faults in the circuit would be very appreciated...

Bye,

Arndt
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Old 1st March 2003, 04:49 PM   #7
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Well, I just measured the supply voltage. It is almost exactly 40 V. While this seems to be very high if compared to suggested voltage, it is the voltage that Marantz intended for this amp...

Btw, since the original transformer hums quite a bit, would it be OK to just use one with lower voltage, or would that force me to exchange a lot of other components as well? - And, the amp also need some additional voltages, which I would have to supply with additional toroids, here the orig transformer has additional windings...

Bye,

Arndt
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Old 1st March 2003, 05:02 PM   #8
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Arndt,

If I frightened you, sorry for that!

STK's usually don't have trimpots in there circuits.
Measuring the idle current could be done with a 1ohm resistor
in series with VCC at pin 8 and (or) pin 6.
If you have a scope I would check pin 1 and pin 7 for oscillations.

Careful with the probes, discharge the main caps before measuring !!!

The only strange thing I can find in your mods is the resistor.
Is it a high power resistor? Several watts?
Can you post a piece of schematic?

As to your last post: I wouldn't do any mods to the amp anymore.
First try to tame that heat.

/Hugo - won't respond anymore today, is going out to a nice little restaurant now...
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Old 2nd March 2003, 11:49 AM   #9
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Hi!

I just scanned the schematic. Here is the picture (note that there is a strong line visible which is not a connection but a shadow from where the paper was folded):

Click the image to open in full size.

And I only mentioned the resistor because of it producing a lot of heat before electrolytics replacement, and nearly none at all after the replacement.. The original Marantz BOM has R806 as a 150 ohms / 3 W resistor. If you have access to Farnell pictures (catalogue), it looks like FCB2 resistors from Meggitt Electronic Components, Farnell No 148-736 (for example).

Well, maybe I can hunt down some voltages using the schematic, since the "should be" values are printed at important junctions.

I don't have a scope, though, and underneath the board there are two ceramic caps directly soldered to pin 4 and 5 of the IC, maybe to prevent oscillations... they seem not to be in the schematic. Also two small electrolytics were installed underneath the board, but I don't have access to it right now, so I cannot tell exactly where...

Anyway, so far thanks for the help...

Bye,

Arndt
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Old 2nd March 2003, 12:14 PM   #10
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Neither the 32.8V nor the 14.5V seem to be used for the STK's.

I think they were used for the preamp or the receiver.
Anyone, correct me here if I'm wrong.

So I wouldn't pay much attention to R806.
You can easily measure the idle current by replacing F8(9?)02 and
F803 with 1ohm resistors.
Of course you will measure the idle current of both STK's then.

The ceramic caps will probably be there to prevent oscillation, as you mentioned.

Go ahead and measure the voltages around the IC's!

/Hugo - Think Elso could be right if all measures ok
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