Transistors in a blanket?

[dr0ss]

The following is a daughterboard my Rotel A12 integrated amp:

There are four transistors (I assume) clad in plastic jackets (orange in the photo). The plastic is sticky from time and probably heat; note the little bits of dust and detritus stuck to the plastic. Any ideas why Rotel would have decided to do this? Surely not to protect the transistors from the cold, this amp does not run cool. I'm thinking of cutting the plastic off before it melts completely and causes a gooey mess. (I recently lost a pricey sheetfeed scanner after the rollers dissolved.)

 

[sergeauckland]

It looks to me like each enclosure contains two transistors which need to be thermally coupled, hence the insulating enclosure. Removing these would affect the thermal stability of the amp, so not a good idea, unless you replace them with something equally effective.

The plastic going gooey wouldn't affect the operation, unlike a sheet feeder that relies on correct paper handling.

If there's some other explanation, then I don't know.

S

 

[Doodski]

@sergeauckland has it right. The coupling is for thermal tracking of the circuitry. It's all been calculated by the designer and it needs to stay the way it is with them together. Leave them be.

 

[solderdude]

Yes, this is for thermal stability (looks like thermal paste or silicone in there).
Also possible these were Hfe matched pairs before they were fitted.

 

[dr0ss]

OK, thanks. I guess that would explain the oddball shape. FWIW, when my scanner rollers melted they didn't just become sticky, they literally turned into a fluid and dripped all over the circuit board and the desk it was on. Our tropical climate does funny things to plastic and rubber.

I'll leave these transistors alone for the present, but if they start to decompose further I'll try to find a solution that leaves them coupled together.

 

[Doodski]

OK, thanks. I guess that would explain the oddball shape. FWIW, when my scanner rollers melted they didn't just become sticky, they literally turned into a fluid and dripped all over the circuit board and the desk it was on. Our tropical climate does funny things to plastic and rubber.

I'll leave these transistors alone for the present, but if they start to decompose further I'll try to find a solution that leaves them coupled together.

It's normal to see them become discolored and dark brown from the heat.
I've seen them really really dark brown from being baked from the heat.
There are special parts available for clipping the transistors together, they are made of metal.
They are usually found in expensive amps but you might be able to find them for sale online.

 

[antcollinet]

And I learn another new thing from ASR. First for the day.

 

[dr0ss]

There are special parts available for clipping the transistors together, they are made of metal.
They are usually found in expensive amps but you might be able to find them for sale online.

If they're bound together with metal, for example some kind of strap holding a pair of TO92 packages flat side to flat side, I don't see any reason why I'd ever want to mess with that. The plastic cover is the part that I'm worried about.

 

[Doodski]

If they're bound together with metal, for example some kind of strap holding a pair of TO92 packages flat side to flat side, I don't see any reason why I'd ever want to mess with that. The plastic cover is the part that I'm worried about.

I searched online for those clips that I've seen but I found nothing. They are rare and maybe are custom made. Is the plastic sticky when cold or breaking down?

 

[dr0ss]

It was sticky when I looked at it, which was while cold. I don't know if it gets worse when hot. Deterioration like this is pretty common with plastic and rubber where I live. I don't think it is critical at this point, just something for me to keep an eye on.

If it does get critical I suppose I can try to get help/advice from Rotel.

 

[kchap]

It was sticky when I looked at it, which was while cold. I don't know if it gets worse when hot. Deterioration like this is pretty common with plastic and rubber where I live. I don't think it is critical at this point, just something for me to keep an eye on.

If it does get critical I suppose I can try to get help/advice from Rotel.

I would leave it until something has to be done. If it was necessary, assuming the the transistors are TO-92 or similar, I would cut the plastic sleeves away and clean the transistors with cotton buds and isopropyl alcohol. Apply a little bit of thermal compound to flat surfaces. Align the transistors so the flat surfaces touch and slide a piece of heatshrink tubing over the transistors. I would use a piece of tubing that just fits over both the transistors and wouldn't apply the heat gun for very long.

 

[Doodski]

I would use a piece of tubing that just fits over both the transistors and wouldn't apply the heat gun for very long.

Going off the sleeves like the ones that @dr0ss has in his unit and the ones that I've seen first hand that are just like his I think that is silicon heat shrink tubing. It could be high temp shrink too. If silicon that could explain the rubbery stickiness.

 

[JayGilb]

Interesting that they are laid out in a front to back configuration and thus required thermal paste and a coupling cover.
I have seen back to back sets with a dual hole, single aluminum heat sink that also served as a coupling device.

Maybe a Sziklai pair configuration.

How old is this amp ?

 

[Doodski]

Hmmz. I searched for a service manual and none to be found.

 

[Nitreb]

Is it rust I see there?

 

[DonH56]

Is it rust I see there?
View attachment 166805

Looks like the exposed copper edge of a tinned blade jumper.

 

[Raindog123]

What if it's phase-changing thermal interface material? Like modern CPU-to-heatsink pads - soft-solid while idle and liquid in operation? Ahead of its time?

 

[Raindog123]

I searched online for those clips that I've seen but I found nothing. They are rare and maybe are custom made…

Ha!

Rotel component identification question

Hi all, I have a Rotel RA-12 integrated amp. I opened it to blow out the dust and noticed that 3 of the 4 pictured components on the riser card (marked with red dots) are looking a little cooked (brownish in color): What are these, and is the discoloration typical? Thank you! Updated with a...

www.audiosciencereview.com

 

[solderdude]

I'll try to find a solution that leaves them coupled together.

Heat-shrink tubing works fine.

 

[dr0ss]

How old is this amp ?

I bought it used so don't know (are there date codes in the serial number), the seller claimed 2020 but I have no reason to believe that other than it was in pristine condition (other than these sticky brown sleeves). I think Rotel put this model into production in 2016.

Ha!

Nice find! There aren't many posts about Rotel on ASR, but somehow I missed that one. The RA-12 shown there is the previous model to my A12, but that daughterboard looks very very similar.

I searched for a service manual and none to be found.

Nor a schematic.