DIYbanter - View Single Post - Fender Frontman 212R Volume 80% down problem

M Philbrook

In article ,
says...

El viernes, 16 de octubre de 2015, 1:54:16 (UTC-3), Phil Allison escribió:
Leonardo Capossio wrote:

Well, in the end the problem were those resistors,
now it works like it should. The only problem is that
at high volumes the amp turns the sound off after a while,
I guess because of thermal shutdown of the PT100 on the
power dissipator. But this can be solved by cleaning and
re-applying thermal paste, and in extreme situation
adding a cooler.

** 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.

If it's getting hot like that with the amp on and not generating any
sound, you have other problems.

Put a volt meter on the output in DC mode first, see if you are getting
any DC above a few mVs

After that, put the meter in AC mode and see if you are getting AC
ripple of a significant level, which you should not be.

Do this in ideal mode, no sound.

Me thinks you have big ISSUES with biasing.

Jamie.