Interesting PL700 Heatsink observation

[Gepetto]

As I am rebuilding my lab PL700 mule into a full comp version with the new backplane boards I am noticing that the negative side heat sinks (the quasi comp side) show evidence of much more heat than the upper positive half of the same channel. This discoloration is the purpling effect that the black anodized heat sinks take on when overheated. This amp was pretty much stock before I started the rebuild.

Anyone else notice that on their PL700? It is the 2nd and 4th heat sink counting over from the transformer.

 

[laatsch55]

I've noticed that on several Joe. Didn't sink in that, that was a sign the quasi was experiencing a differential heat problem..

 

[Gepetto]

I've noticed that on several Joe. Didn't sink in that, that was a sign the quasi was experiencing a differential heat problem..

I think it is a sign that the quasi portion was oscillating and producing the extra heat. Just my guess, I did not analyze this amp before tearing into it.

 

[Gepetto]

As I am rebuilding my lab PL700 mule into a full comp version with the new backplane boards I am noticing that the negative side heat sinks (the quasi comp side) show evidence of much more heat than the upper positive half of the same channel. This discoloration is the purpling effect that the black anodized heat sinks take on when overheated. This amp was pretty much stock before I started the rebuild.

Anyone else notice that on their PL700? It is the second and 4 heat sink counting over from the transformer.

Pictures of the discoloration by location.

 

[laatsch55]

That's an extreme case...

 

[Northwinds]

Would this also affect the quasi 400's? I notice one heatsink (left one when viewed from the back) seems to get hotter then the other faster

 

[wattsabundant]

I occasionally use an infrared gun on each output when I repair an amp. I shoot the outputs in the middle of the case from as short of distance as possible. The intent of the test is to make sure no one transistor is running significantly hotter than the others. The last amp I did this to was a 700B. I noticed the inner heatsinks (transistors) were about 10 degrees C hotter than the outer heatsinks. Also, as you would guess the temerature of the bottom output and the driver was about the same. The temp increased 3 or 4 degrees from bottom to top. The amp under test has a stock driver board. I'm certain it wasn't oscillating. These results would seem to conflict with Joe's observations.

The amps I've seen that were clearly oscillating have had roasted zobel networks.

Normally I would prefer to use a themocouple to take temperature readings. However when measuring outputs it takes a long time to get them all with a thermocouple. By the time all are measured the first ones have risen. I stop the test if any transistor exceeds 90 degrees C.

As I look at the pics Joe sent, the area under the transistor did not change color. I would anticipate this would be the hottest area.

 

[Gepetto]

I occasionally use an infrared gun on each output when I repair an amp. I shoot the outputs in the middle of the case from as short of distance as possible. The intent of the test is to make sure no one transistor is running significantly hotter than the others. The last amp I did this to was a 700B. I noticed the inner heatsinks (transistors) were about 10 degrees C hotter than the outer heatsinks. Also, as you would guess the temerature of the bottom output and the driver was about the same. The temp increased 3 or 4 degrees from bottom to top. The amp under test has a stock driver board. I'm certain it wasn't oscillating. These results would seem to conflict with Joe's observations.

The amps I've seen that were clearly oscillating have had roasted zobel networks.

Normally I would prefer to use a themocouple to take temperature readings. However when measuring outputs it takes a long time to get them all with a thermocouple. By the time all are measured the first ones have risen. I stop the test if any transistor exceeds 90 degrees C.

As I look at the pics Joe sent, the area under the transistor did not change color. I would anticipate this would be the hottest area.

Hi Don
That is the area under the transistor is spared from oxidation by the gas tight seal formed by the transistor/insulator pad/thermal compound.

Zoebel network was stock and in cherry condition.

 

[Northwinds]

Would this also affect the quasi 400's? I notice one heatsink (left one when viewed from the back) seems to get hotter then the other faster

My question has no merit then Joe???

 

[laatsch55]

Certainly it has merit. But the answer brings up some portential problems. If an emphatic "yes" then what's your conclusion?

 

[Gepetto]

My question has no merit then Joe???

Sure it can happen on any amp I suppose. I just happened to be dissecting this amp and noticed it and thought I would post. All amps will have some thermal differences depending on the accuracy of the components used and the surrounding heat sources. On this amp, I would not have posted if both center heatsinks had this because those would tend to be the hottest ones due to heat concentration from its neighbors. Since this one went every other, it was worth noting. This amp worked fine.

What research I have done, the anodizer must not have used light resistant dyes when it was black anodized. This pink/purple fading can either be caused by heat or light if non-lightfast dyes are used. Or if the heatsinks were not properly sealed after anodize.

 

[Northwinds]

Thanks for explaining that Joe

 

[wattsabundant]

Sure it can happen on any amp I suppose. I just happened to be dissecting this amp and noticed it and thought I would post. All amps will have some thermal differences depending on the accuracy of the components used and the surrounding heat sources. On this amp, I would not have posted if both center heatsinks had this because those would tend to be the hottest ones due to heat concentration from its neighbors. Since this one went every other, it was worth noting. This amp worked fine.

What research I have done, the anodizer must not have used light resistant dyes when it was black anodized. This pink/purple fading can either be caused by heat or light if non-lightfast dyes are used. Or if the heatsinks were not properly sealed after anodize.

I could believe that the two different heatsinks were different production runs and possibly different production techniques.