On Monday, November 30, 2015 at 12:16:15 AM UTC-3, Phil Allison wrote:
Leonardo Capossio wrote:


** That Fender model and similar ones uses a thick, alloy bar to couple heat from the output devices to the bottom of the case - right ?

Often there are burrs left around bolt holes in the coupler that prevent the metal surfaces mating properly.

Those holes need to be de-burred in a drill press.

Once done and regreased, output devices run cooler and there are no more shut downs.


... Phil

Right.

I'm starting to think this is the problem. I checked the trasistors and they are fine, they are the same as in the schematic.

Y removed them from the heatsink, cleaned everything with alcohol (even tried not to touch it with my fingers) and put fresh thermal paste (with isolation) between the metal plate that holds the transitors, between the plate and the heatsink, and between the heatsink and the case, but still the metal plate is hot as hell (can't even touch it) and the heatsink is very hot, but not so much.

I wonder if there is a video on how to do it properly.



** Two things are not clear:

1. Does the heatsink get very hot whenever the amp is on or do you have to PLAY something ?


You have to play for a while at low volumes (10min tops), or play for a minute at the highest volume. In idle, with a plug connected (otherwise the amp mutes), it does not shut down.

2. Have you found and fixed any burs on the holes in the heatsink coupler and or the chassis ?


If by burr you mean what is described in this page: "http://www.nmri.go.jp/eng/khirata/metalwork/basic/bari/index_e.html", then I don't see any worrysome burr on the heatsink.

BTW:

That amp has an odd, non adjustable bias arrangement.

Transistors Q14 and Q 15 should be in thermal contact with the heatsink.



They are on the heatsink, but their tab is completely plastic.