Fender Bassman SS Combo blowing mains fuse - Need help.

[komitet]

Hi.

So I've actually come to this forum because I found another thread here that mentioned what seemed to be a similar topic from years ago. I've tried my best to use the information there and now I want to formally reach out for help.

What I have is a Fender Bassman 200 Solid State Combo amp that a friend owns. I told him I would look at it because it wouldn't turn on. I found the mains fuse blown and replaced it and it came on and worked. I played for 15-20 minutes and then, all of a sudden, a high-pitched squeal came from the amp and it powered itself off. Checked the fuse and it is blown again.

So, I started googling and found the thread here. Quick (lazy) troubleshooting and I have tried to determine if the power transistors are bad. I did not remove them from the circuit (is that necessary?) while taking readings but I think they are fine. I can post my readings if you like. After that, I built a light bulb limiter and powered the amp on with everything as normal and the light bulb is super bright (short in the amp, right?).

I've read a lot about these boards being poorly made and that the solder traces could be poor. How do I go about finding bad traces? Continuity I assume. I've attempted to attach the schematic. Hope that worked...

I don't have an o-scope but I might be able to get my hands on one. But if that could be precluded with someone's wisdom, please throw it my way. Any help would be extremely appreciated.

fender_bassman_200.pdf

[komitet]

Also, what would the tolerance on some of these readings be? I'm used to 10% but I feel like I've found 2 readings that are further out of whack.

TP5 is supposed to -45mVDC but I read -59mVDC

TP7 is supposed to be -40.2VDC but it comes up as -49.3VDC

[komitet]

and now I'm confused because the amp blew the mains fuse again even though it is plugged in to the light bulb limiter...

I thought that the point of the limiter is to prevent that.

[J M Fahey]

and now I'm confused because the amp blew the mains fuse again even though it is plugged in to the light bulb limiter...

I thought that the point of the limiter is to prevent that.

It does, so you probably plugged or wired it wrong.

Please post an actual schematic or clear pictures and state which bulb did you useo.
Also fuse rating.

[R.G.]

[...] I found the mains fuse blown and replaced it and it came on and worked. I played for 15-20 minutes and then, all of a sudden, a high-pitched squeal came from the amp and it powered itself off. Checked the fuse and it is blown again.

The amp is dying because it gets into self-oscillation (that high pitched squeal) and runs itself into destruction. That's problem 1. Problem 2 is that it seems like the output transistors and the rest can live with the self-oscillation for a while ( long enough for the fuse to blow ) and then ...? maybe cool off ?... while the fuse replaced, and work for while, so maybe the fuse is either the wrong or just the right value, depending on how you look at it, to keep it from actually destroying semiconductors.

Both those conditions have to get fixed. What's needed is to find what's been destroyed from the self-oscillation and fix that long enough to find and fix the self-oscillation. Then the fuse question can be handled.

[komitet]

One picture shows my limiter. Wired from the wall to the light socket and then from the socket to the outlet. The amp is plugged into the outlet. Under the socket, I have the hot leads connected to the same terminal and the neutral leads connected to another terminal. I have the ground from both cables connected underneath the light socket and then connected to the ground terminal on the limiter outlet. The bulb is a "100W" eco-incandescent bulb that is supposed to be as bright as 100W but only uses 72W. Closest normal (as in traditional socket) incandescent bulb I could get to 100W. All others are LED's at the store and I've read that those don't work the same.

edit: don't mock me and my "workspace"...haha.

As for the other picture, I read that as a Fast-blow 5 amp 125V fuse...which is what I had in there twice and which blew twice. When I originally opened the amp after I got it from my friend, it had a 5A 250V fuse which I suspect may have caused the issue...I haven't asked him if he put that in or if it came that way. I've read on some forums that an amp should use a slow-blow (timed) fuse to allow for initial heating/charging of caps blah blah blah but I decided that the manufacturer's value is probably best.

Whether or not the wrong fuse has been implemented, R.G.'s comments sound like I'll be easter egging the issue. Which leads me to think that this may not be worth the trouble. In order to find what has been destroyed, I'm guessing I am just testing components ad nauseam until I locate/replace them all. Is there a way to half-step this power amp to better deduce the problem areas?

I could see just scrapping the amp section of this combo, swapping in a better amp head (the space is certainly ample), and hooking it up to the speaker. Thoughts??

[g1]

Your lamp limiter does not sound correct. With no bulb in it there should be no power getting through to the unit under test.
Check with something simple like a toaster or something plugged into it.
See below, switch is not required.