Sony TA-F6B PSU repair

[johnm]

Here's a picture which shows the die-cast part which acts as both a heatsink, plus an anchor to which L604 and T3 is fitted. The insulating silpad fits underneath that.



Note the parts which press Q601 Q613 against the heatsink have been removed in this pic. I'll be using BUT11As here, and for the 4 transistors underneath the PCB. Those 4 transistors press against 4 mini heatsinks, which in turn press against the large silpad, and then the die-cast part through cut-outs underneath in the PCB. You can see the mini heatsinks plus the large silpad towards the top-left of the picture below:



These were all taken last week before I'd removed the caps & bad transistors.

[jaycee]

Ahh now I see. Hm, they only used one big silpad for ease of assembly to be honest. You could have just as easily used 4 TO-220 sized pads, or TO-3P ones if you wanted to be sure. Similarly for Q601 Q613, two pads would work just as well, and they used one for ease of assembly.

Not sure why they bothered to insulate underneath the switching transistors. They could have used the fact that the tab is connected to the Collector in the PCB design. Pity they didn't as that would've improved thermal conductivity too by not needing any insulation. Again personally, I would have used TO-3P pads and just cut them down a bit with a sharp scalpel. Cutting the notches in them doesnt really seem to be neccesary.

[Mooly]

I think using T03P pads like Jaycee and myself wondered about might just be do-able if you had too. It's still not the same seeing a piccy as handing the thing somehow. The bottom line is all the transistors of course have to be insulated.

Looking at your pictures in post#1
If you can fit the two reg transistors as they were in that piccy then we should be able to test the first part of the circuit.

[johnm]

Thanks Jaycee/Mooly - yes looking at it I cannot see why one couldn't use TO-220 insulators under each of the four switching transistors, pressing against the die-cast attachment with the transistor clamp (which I will make this week). This would do away with the large insulating pad, plus the 4 mini heatsinks too.

I'm stumped why Sony didn't do this - it would have been cheaper & easier to produce surely? I'm going to take another look & more pics and see if I can see any reason at all not to go that route. The back of that die-cast block actually is the bottom of the SMPS - it screws into the bottom of the black aluminium box shown in the picture a few pages back. I would have thought as Jaycee says that having the transistors 'closer' coupled to the die-cast block would improve heat dissipation too.

As it stands you have the transistor clamp, the transistor, then some heatsink paste (no insulation incidentally), then the miniheatsink (1mm thick aluminium rectangle), then more paste, the large silpad, then the die-cast block...

Hmmmmmm!

[johnm]

The picture below should give a slightly more obvious look at how it all fits together - the only bits not in the shot are the 4 mini heatsinks, but you can see from the remains of the heatsink paste where they fit on the silpad. I just do not understand the logic of having a mini heatsink, then silpad, then the large die-cast block/heatsink!



And a look at what the topside of the board looks like without the die-cast block fitted (whoops sorry should have rotated this before uploading!):



- John

[jaycee]

Ahh now it is clear what the silver pads on the underside of the PCB are. I thought they were just tinned sections.

They used paste AND silpad ? Hmm... modern silpads do not need paste, and in fact if you use it you hinder performance. I guess the materials have changed since. I'd honestly just use TO-220 or TO-3P pads. As long as everything's still insulated as before, it'll be fine.

[johnm]

Originally Posted by jaycee

Ahh now it is clear what the silver pads on the underside of the PCB are. I thought they were just tinned sections.

They used paste AND silpad ? Hmm... modern silpads do not need paste, and in fact if you use it you hinder performance. I guess the materials have changed since. I'd honestly just use TO-220 or TO-3P pads. As long as everything's still insulated as before, it'll be fine.

Yeah I thought the same, but in the service manual it says to put a little heatsink paste on the visible rectangles of silpad, as well as some on the mini heatsinks. It's almost like they were deliberately decreasing the efficiency of the heatsink process perhaps? Or as you say the silpads weren't very efficient back then, and used more for insulation than heat transferring abilities.

As to the two transistors on the side, the service manual does not mention needing to use any paste. The last guy who worked on this smothered it on (you can see it on an earlier pic of mine - post 111 - behind those two red transistors) to the extent I'm not surprised those two blew!

I think I'll cancel my order for that large silpad (it came to £18 from RS once VAT and delivery were included!!!) and get four small ones instead.

- John

[johnm]

Originally Posted by jaycee

Ahh now it is clear what the silver pads on the underside of the PCB are. I thought they were just tinned sections.

They used paste AND silpad ? Hmm... modern silpads do not need paste, and in fact if you use it you hinder performance. I guess the materials have changed since. I'd honestly just use TO-220 or TO-3P pads. As long as everything's still insulated as before, it'll be fine.

P.S. would you also do away with the four rectangular mini-heatsinks too? I can see no reason for those now. More efficient to couple them (via the TO-220 insulators) directly to the die-cast block, which is itself coupled to that black aluminium box which acts as a heatsink itself.

Thanks,

- John

[jaycee]

Not sure which part you mean? The metal bracket that screws over this, that you said was missing?

[johnm]

No - the four mini-heatsinks which fit into those cutouts on the PCB, on the underside.