Fender Frontman 212R Volume 80% down problem

[Phil Allison]

Leonardo Capossio wrote:

** 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.

** When in "mute" mode, the power stage is completely shut down and cannot get hot. But does the heatsink get hot when active but not played, even if this take an hour ?

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.

** Figure 7 is the appropriate one.

The best test is with a finger tip, the hole should not feel rough when you rub your finger over it.

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.

** That does not matter, but they do need to be held down and have a smear of thermal grease under them.

What you are describing is called " thermal runaway " and I have seen it with other Fender models that use the same heatsink arrangement.

In those examples, careful de-burring of ALL the holes, re-greasing and making sure bias tracking devices were thermally coupled to the heatsink fixed the problem completely.

BTW:

You are lucky the Frontman 212 merely shuts down, the ones I worked on all blew up as there was no over temp sensing device fitted.


..... Phil

 

[Leonardo Capossio]

On Tuesday, December 1, 2015 at 2:05:16 AM UTC-3, Phil Allison wrote:

Leonardo Capossio wrote:



** 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.

** When in "mute" mode, the power stage is completely shut down and cannot get hot. But does the heatsink get hot when active but not played, even if this take an hour ?

After half an hour, it is mildly hot, don't think it gets worse over a longer period of time.

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.

** Figure 7 is the appropriate one.

The best test is with a finger tip, the hole should not feel rough when you rub your finger over it.



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.

** That does not matter, but they do need to be held down and have a smear of thermal grease under them.

What you are describing is called " thermal runaway " and I have seen it with other Fender models that use the same heatsink arrangement.

In those examples, careful de-burring of ALL the holes, re-greasing and making sure bias tracking devices were thermally coupled to the heatsink fixed the problem completely.

BTW:

You are lucky the Frontman 212 merely shuts down, the ones I worked on all blew up as there was no over temp sensing device fitted.


.... Phil

I'll try to de-burr this holes and do the thermal coupling correctly.