Fender Bassman 400 Solid state Head

[Jay D]

Hello Forum:

Has anyone worked on a Fender Bassman 400 solid-state head that when the bass player cranks up the amp, the amp goes into a mute state, and after a while, will come back to playing normally?
I am borrowing a schematic of the amp, and find two (2) temp controlled circuits. One is the cooling fan - has it's own power supply and control, and the other one is thermal cutoff to the differential amplifier's current source circuit. In either case, I need to monitor the circuits, and see which one goes on for the mute.
I have some work cut out, but no harm in asking first.

Thanks,

Jay D.

also, new e-mail address dursonix.llc@gmail.com

[g1]

First thing is to determine whether it is overheating. 4 ohm minimum load? Speaker cable good? Fan is going to high speed when hot?
If the amp is overheating, then the temp. mute circuit is correctly doing it's job.

[Jazz P Bass]

Also, you should apply the 'Enzo Whack' to see if it is a loose connection.

[Enzo]

Be careful with your terms. There is a difference between going to mute and just losing output. Mute is a specific thing, and it MAY be responsible for loss of sound or it may be something else entirely.

Not saying you are wrong, just want you to look at the big picture first.

If the amp is thermalling, the heatsink will be VERY hot.

RT2 runs the fan, I see no signal from it going elsewhere. RT3 is the thermal shutdown device, and you can watch it at TP11 to see if it is involved. Rt2 and RT3 are both on the heatsink, so if one is hot, so should the other be. And that means if RT3 is shutting down the amp, RT2 should be speeding up the fan at the same time. Is that happening?

Describe the symptom. WHat I see in your post: the amp works normal at moderate volume, yes? When turned up loud, well more precisely, when played loud, the sound goes away? And how long does it stay that way? Half a second, two seconds. two minutes? Half an hour? And does it stop like a switch was turned, or does it gradually diminish, and when it returns, same question.

Is there any visual indication, like lights going out, or substantially dimming?

Don't overlook the speaker jack itself.

Isolate the problem. When the amp cuts out, can you still get output from the FX send? If you drive the power amp hard from teh FX return, will it still do it?

[Jazz P Bass]

Wow.
Awesome response, Enzo.
Thanks.

[Jay D]

Hello Forum:
I have to first say Enzo - Excellent explanation on the reply. I have to get the correct wording in order to explain the problem fully. This is why I ask questions, and still learning from the experiences.
Anyway, The problem wound up to be a lead from one of the thermistors touching a metal standoff when the amp was vibrating from the bass cabinet, and I also found the filter caps solder joints cracked and making an intermittent contact. The problem is solved, and I use more hot glue around and between the filter caps to secure to the pc board better, and clipped the long lead touching the metal standoff.

Thank You all for your input and help,

Jay D.

[Mattster54u]

I have the same issue Gus good fan working

what could it be?

[Mattster54u]

Same exact issue what was it fuse good fan working