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Why pull and redo power transistors on the heatsink

mikey99 said:
If the outputs are soldered in place, it's a lot more work than if they're socketed.
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Not in my mind. Solder wick works well for this. The reason I say this is I've seen cooked solder joints on mistreated amps where the thermal stress has actually cracked the solder joints. If I'm going to go through the trouble of pulling the board to touch up the solder, I might as well pull the transistors, test them, clean and redo the thermal paste and solder them back in with fresh solder for the next 40 years :D I've found working but weakened transistors this way.
 
I found this for Locktite.. the purple is the way to go here
222 - Purple - Low strength thread locker, designed for precision metal fasteners under 3/4". Protect threads from rust and corrosion. Removable with hand tools. Temp range - 65 to 300 degrees F. Cure Speed 20 min. Full 24 hrs.

242 - Blue - Medium strength thread locker for fasteners up to 3/4". Cures reliably even on stainless steel. Tolerant of oil and other contamination. Protects threads from rust and corrosion. Parts can be disassembled with hand tools. Temp range -65 to 300 degrees F. Cure speed 15 min. Full 24 hrs.

262 - Red - Permanent strength thread locker for fasteners up to 3/4". Designed for extreme environmental/chemical conditions. Especially useful for holding tight Grade 5 and 8 fasteners. Protects threads from rust and corrosion. Localized heating and hand tools required for disassembly. Temp range -65 to 300 degrees F. Cure speed 30 min. Full 24 hrs.
 
When performing your test.... Make sure the ambient temp and AC supply voltage is the same during all testing.....assure that it is dead air and just convection flow...unless the unit has a fan....

Also make sure the overall heatsink is clean before you start your test or just make sure you don't disturb any of the dirt/dust....

Measure the mounting screw torque before disassembly....the thickness of, viscosity of and air voids / cracks in the old heat sink compound. The type, quality and condition of the mica insulator or if uses another type or none.

Test the old grease and characterize its age verse the new grease....

Assuming the original goop ... I mean heat sink compound....was installed correctly in the first place....

The purpose of the grease is so that the thermal conductive material ( usually an oxide of type ) is held in suspension and "flows" into the voids in the surfaces of the heat sink, mica and transistor case. As the volatitive compound burns off from heat over time , voids form and overal thermal resistance climbs....

Remember that the co-effecient of expansion of the Aluminum, Mica and steel are all different. There is a shearing motion that occurs during heating and cooling....the transistor tries to wipe itself clean of the oxide and create an air void in the parallel axis to the Mica.... and the Heatsink tries to separate itself from the mica....

So how much degradation occurs....? If I was an ME...I would have done that thermal analysis.... Maybe I'll ask my son the ME.....


Or you can just freshen up that 45 year old heatsink compound if the unit has been used....

A similar problem occurs with head gaskets on cars with Al heads and cast iron blocks....

jk

whoaru99 said:
Bah humbug. LOL.

It ain't that hard to do a relative measurment. Absolute accuracy, yeah, that's a bit more tricky but still ain't that hard.

Back to my question, do you have any source for empirical data that shows the effect of old/dried out heat sink compound vs. new? I've browsed around a lot and found plenty of anecdotes and theories but no hard info.

I have some older amps and may do my own experiment just to see, since I keep thin film thermocouples and other sorts thermocouples and gadgets (accelerometers, pressure transducers, etc.) on hand for work purposes, not unlike what you used to do.

If changing the compound improves the thermal conductivity, one would expect the transistor junction temp and possibly the case temp would come down a bit but the heat sink temp actually may go up a bit, yeah?
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jstang said:
When performing your test.... Make sure the ambient temp and AC supply voltage is the same during all testing.....assure that it is dead air and just convection flow...unless the unit has a fan....

Also make sure the overall heatsink is clean before you start your test or just make sure you don't disturb any of the dirt/dust....

Measure the mounting screw torque before disassembly....the thickness of, viscosity of and air voids / cracks in the old heat sink compound. The type, quality and condition of the mica insulator or if uses another type or none.

Test the old grease and characterize its age verse the new grease....

Assuming the original goop ... I mean heat sink compound....was installed correctly in the first place....

The purpose of the grease is so that the thermal conductive material ( usually an oxide of type ) is held in suspension and "flows" into the voids in the surfaces of the heat sink, mica and transistor case. As the volatitive compound burns off from heat over time , voids form and overal thermal resistance climbs....

Remember that the co-effecient of expansion of the Aluminum, Mica and steel are all different. There is a shearing motion that occurs during heating and cooling....the transistor tries to wipe itself clean of the oxide and create an air void in the parallel axis to the Mica.... and the Heatsink tries to separate itself from the mica....

So how much degradation occurs....? If I was an ME...I would have done that thermal analysis.... Maybe I'll ask my son the ME.....


Or you can just freshen up that 45 year old heatsink compound if the unit has been used....

A similar problem occurs with head gaskets on cars with Al heads and cast iron blocks....

jk
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I think you are trolling a bit, here.
 
Mike, how do you test the transistors, just the standard diode check or do you run them under power and get a graph?

The mounting torque of the removed screws isn't too critical as there is a mounting torque specified for installation of different transistor packages. Surprisingly low 6 inch-pounds according to one Burr-Brown Application Notes: Mounting Considerations for TO-3 Packages.

Maybe someone can use an IR thermometer and measure across a heatsink before and after a goop refresh and let us know what they find. Things to look for are temp evenness across the entire sink (if all the transistors are doing the same thing, output for example) and increase in the temp of the heatsink under identical conditions. This would indicate that the heat is leaving the transistor and going to the sink as designed. Harder to do this last one, but if you crank the same thing through the rig and measure, maybe you will see a difference. Additionally, checking the temp of the transistors themselves, IR on the case where labeled would work, and seeing if the transistor is cooler as the sink gets warmer compared to the before situation.
 
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Sometimes that sort of thing (that fingernail polish-looking stuff) is used for inspection / process marking or tampering indication, not necessarily as thread locker per se.
 
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jstang said:
When performing your test.... Make sure the ambient temp and AC supply voltage is the same during all testing.....assure that it is dead air and just convection flow...unless the unit has a fan....

Also make sure the overall heatsink is clean before you start your test or just make sure you don't disturb any of the dirt/dust....

Measure the mounting screw torque before disassembly....the thickness of, viscosity of and air voids / cracks in the old heat sink compound. The type, quality and condition of the mica insulator or if uses another type or none.

Test the old grease and characterize its age verse the new grease....

Assuming the original goop ... I mean heat sink compound....was installed correctly in the first place....

The purpose of the grease is so that the thermal conductive material ( usually an oxide of type ) is held in suspension and "flows" into the voids in the surfaces of the heat sink, mica and transistor case. As the volatitive compound burns off from heat over time , voids form and overal thermal resistance climbs....

Remember that the co-effecient of expansion of the Aluminum, Mica and steel are all different. There is a shearing motion that occurs during heating and cooling....the transistor tries to wipe itself clean of the oxide and create an air void in the parallel axis to the Mica.... and the Heatsink tries to separate itself from the mica....

So how much degradation occurs....? If I was an ME...I would have done that thermal analysis.... Maybe I'll ask my son the ME.....


Or you can just freshen up that 45 year old heatsink compound if the unit has been used....

A similar problem occurs with head gaskets on cars with Al heads and cast iron blocks....

jk
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Yup, all relatively straighforward stuff to have some reasonable semblance of an apples to apples before/after comparison.
 
jstang said:
Maggys aren't a very difficult load.... actually they are very amp friendly because of the light wight of the diaphragm and mostly a resistive load...
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Yup, you are right. I was just listening to the speakers playing loud and the amp got hot like it should. Big deal
 
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Really? Do you have the ability to test the viscosity and condition of the old "Goop"..... How are you going to know it's condition? Or are you just going by age....?

Age alone it not the determining factor of the degradation of the compound.... Its heat over time... One can not assume the compound that has stayed at room temp to be the same condition as compound that spend the same time under a hot transistor 45 years....

Similar reason way capacitors are life cycle rated at 85* or 105*.....and are give a shelve live at room temp....

Time at temp affects aging....

whoaru99 said:
Yup, all relatively straighforward stuff to have some reasonable semblance of an apples to apples before/after comparison.
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Blue Shadow said:
...

Maybe someone can use an IR thermometer and measure across a heatsink before and after a goop refresh and let us know what they find. Things to look for are temp evenness across the entire sink (if all the transistors are doing the same thing, output for example) and increase in the temp of the heatsink under identical conditions. This would indicate that the heat is leaving the transistor and going to the sink as designed. Harder to do this last one, but if you crank the same thing through the rig and measure, maybe you will see a difference. Additionally, checking the temp of the transistors themselves, IR on the case where labeled would work, and seeing if the transistor is cooler as the sink gets warmer compared to the before situation.
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If I end up doing something I would probably use one of my EV7100 amps for experiment. I can log up to 8 channels of thermocouple simultaneously with my field test rig.

As I recall, the EV7100 uses two TO-3 per channel for outputs. I'd put a thermocouple on each of those, then maybe four thermocouples evenly spaced on the heat sink.
 
jstang said:
Really? Do you have the ability to test the viscosity and condition of the old "Goop"..... How are you going to know it's condition? Or are you just going by age....?

Age alone it not the determining factor of the degradation of the compound.... Its heat over time... One can not assume the compound that has stayed at room temp to be the same condition as compound that spend the same time under a hot transistor 45 years....

Similar reason way capacitors are life cycle rated at 85* or 105*.....and are give a shelve live at room temp....

Time at temp affects aging....
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I have access to a full materials lab if I really wanted to call in a favor. But I would not waste a favor like that just for this.

I would simply load the amp as for an FTC style preconditioning test, record the heat signature, then take apart, clean, and reassemble to spec with new grease. Then, rerun the test under the same/similar conditions.

That would be convincing enough for me to decide if it's worth it or not. I don't really care if anyone else changes their action or opinion about it. I just want to know if there seems to really be some meat here or if it seems largely just make work...with something more than forum anecdotes and theory.
 
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Amplifier Watt meters typically measure voltage....and have a calibrated face that gives an estimate of power.... Clipping is not limiting power.... Clipping is the limiting of peak voltage.... Power is PIE .... Maggys are mostly resistive load.... WHICH IS AMP FRIENDLY....


Blue Shadow said:
Meaning I could dump all the power thing could produce without clipping and be comfortable I'm not going to blow it up. The heat of the sinks indicate the amp was producing a good bit of power. It seems to clip between the 20 and 50 watt meter indications on the original adjustment.
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I don't care about amp friendly, I care about running the amp til it got hot for proof of performance. Sure stuff a watkins woofer on that system and blow up the poor sui, I don't care it isn't mine. I know Magneplanars are an easy load to drive but they can take power, lots of power but not clipping. Had when I was selling gear in the 70s got another set 15 years ago and bought a lot of fuses.

Isn't a resistive load what they use to test amps? What is your point?
 
jstang said:
If there is "goop" being used...get it out of there...and replace it with proper "dielectric heat sink compound". Not computer CPU heat sink compound which tends to be electrically conductive.

"Goop" .... Really?

A thermal couple taped to the heat sink? Where? The heat being generated is at the transistor's semiconductor junction which is located at the transistor case's base between the leads.... Did you use any "goop" to mount the thermal couple to the heat sink?

If your amps are running so 100% cool, check the bias....or try running them without the thermal mass provided by the heat sink....and see what happens to them.

I don't mean to be overly harsh, but this a fundamental requirement to prevent thermal runaway of the output drivers....this really should not warrant any serious debate..... Should it?

Johnk
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Heh, don't take it personally.
No doubt that I'm using the correct (non-metallic, not "PC") goop
It was an informal test, not a big deal. I do know how to measure temps, take my word for that. I can provide references, certs and take a test on proper measurement practices, if you wish.
I didn't say my amps were running cool. As I recall, the outputs were around 100°, heatsink I think was around 90°. Is that cool? (I always check bias, before and after work on the unit).
Debates are fine, I was just giving my own experience as an example.
Note that I did say, despite my personal, informal testing, I do think "best practice" is to re-fresh the goop - and there should be no debate about that.
I do question how much difference it would make in specific circumstances.
 
The whole reason for the test is to compare the difference between USED compound and new.... Yet you have are not determining how USED the USED compound really is .....

Not a valide test without knowing the real condition of the old removed heatsink compound..... I have access to a lab too....and don't need it for this either....

Do know how the amp has been used for the last 45 years.... ?

The grease dries out ( volatile compounds burns off faster with heat ) and more heat over longer time creates voids... those are the cracks we have seen in the dried out heatsink compound....


whoaru99 said:
I have access to a full materials lab if I really wanted to call in a favor. But I would not waste a favor like that just for this.

I would simply load the amp as for an FTC style preconditioning test, record the heat signature, then take apart, clean, and reassemble to spec with new grease.

That would be convincing enough for me to decide if it's worth it or not. I don't really care if anyone else changes their action or opinion about it. I just want to know.
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whoaru99 said:
It was a relative measurement.

As long as it was done the same before and after, getting the same reading before and after means no change regardless of the absolute accuracy of the reading.
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This is correct, and thanks. I didn't want to say anything, but I have access to certified, expensive temp measuring equipment for my job. I am well aware of accuracy and techniques to get relative measurements, as well as absolute measurements.
 
I am not going let the notion of perfection stand in the way of more information. I know sometimes it's hard for Engineers not to get wrapped around the wheel in that regard but more data point are good as long as we have an understanding of what was done.
 
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