Transistor mounting to avoid heat issues, help

[Blue Shadow]

The Luxman L-580 has a couple transistors on each channel of the amp board, 2SB648 and 2SD668 on one channel in my unit and 2SB649 and 2SD668 on the other channel. Odd but what the heck that is what is in the unit and it worked for 20+ years. I did buy it used so maybe...

The amp board of this unit is mounted with the foil side up. This puts the transistor sticking down and the heat they produce rising directly up to the board. That board is now fairly well darkened.

I want to relocate these transistors to the top of the board and would like to heat sink them some what. I think a brace goes across the board that would need to be cut (an opening for the transistors to fit through) and maybe I can even use this brace as a heat sink.

The transistors are TO-126 so I would need some proper micas, plastic hardware...maybe, this might melt and possibly some extension to the legs of the transistors.

Any issues seen if I:
1) observe correct mounting of the transistors when moving them.
2) use wire to reach up to the transistor legs from the board mounting location
3) mount the transistors on the brace, properly insulated from that brace with mica and non-conductive hardware using a properly small amount of heat sink compound.
4) since these did not have heat sinks, will mounting them to a piece of sheet metal chassis brace and not something more like a heat sink be enough or should TO-126 heat sinks be used?

Since the amp was working for years with the 648/668 and 648/649 pairs is there any reason to hunt down a replacement for these transistors (649 specifically but probably should do all 4 if any are changed).

 

[4seatPilot]

I'm mounting a couple hot ones in Yamaha receivers to the chassis with insulator kits. Wire to connect, works great. They stay much cooler now.

sent from the galaxy 2 with tapatalk

 

[elnaldo]

changing the transistors will need a new bias current adjustment. But you could choose a better matched pair if you can measure their Hfe.

Just thinking loud, you could drill some holes in the PCB to exhaust the hot air, instead of modifying the design.

 

[SolderIron]

Moving power supply transistors usually won't have performance problem and solve the heat problem. Moving signal transistors may cost you in stability (oscillation) and cross talk, but it is almost always ok to do. So give it a shot and keep the leads as short as possible..

 

[sKiZo]

Not familiar with the unit, but can you just move the transistors to the other side of the board so they face up?

I'd also add some heat sinks - assUming of course there's room.

Most any electronic supply (mouser, farnell, etc) will have them specifically for that case style.

 

[Blue Shadow]

I'm in the process of moving one pair. Unfortunately they are so close together that heat sinking them individually would be difficult. I've got some TO-220 micas that will work and I can mount them on that removable brace a few inches apart.

The board is crowded so not much opportunity especially in that area as there are a couple of 1.8kΩ resistors that have some trace issues. Glued those back down and soldered an extended lead to the trace from the hole out a bit.

Hopefully, the board is not scorched so much as to make it conductive. This unit worked very hard when I had it in the system driving the 5Ω DCM TimeWindows to the last LED flashing.

The transistors are circled and the cross brace marked. Tight quarters. I have some leads for one channel and will leave the other channel as is. This pic is not my amp. If that is all the overheating my amp had suffered there would be no need to do this work. I would just add a fan. This is the bottom of the amp shown.

 

[SolderIron]

I would mount them on the brace+ micra insulator.

 

[elnaldo]

As far as I can see, the original transistors don't have any heat-sink.

I'd keep it simple, just mount them in the other side of the PCB if there is place x that, and add an small heat-sink as showed in previous post. You can bend the leads 90º so the transistor lies horizontally and the heat sink on top.

Or, more simple, just add the heat-sink where the transistors are now.

Here is a pict of heat-sinks I've installed in similar sized transistors (the orange ones):

http://dl.dropbox.com/u/1640933/AUDINAC/P1090764_resize.jpg

 

[Blue Shadow]

I like those orange heat sinks. I'll do something like that for the two I am not going to move (the traces are still adequate) but the other two will be moved to the brace, requiring an inch or two of lead already installed on the board. This section is pretty burned up and I'm not ready to rebuild the board the way I did on the MB-3045 amp that needed it. http://audiokarma.org/forums/showpost.php?p=1716866&postcount=13

The brace is very close to the board and probably would not allow the transistor to have any room at all. Although they are on the other side of the board, the connections are under the brace. So not much room.

Thanks for all the recommendations so far.