Fender FM212R Problems

[gmphadte]

You never replied to one of my question. I will ask again. Is that C40 a tantalum. Post the picture if u don't know. This will help others.
Enjoy

Gajanan Phadte

Originally Posted by badraven

Gajanan Phadte

I don't have a way of testing caps other than to ohm them for a short. I just happened to have two .47uf 50v caps on hand so I replaced C40 and instant WONDERFUL!!!

No more buzzing, the outputs are running about 87 degrees after about 15 minutes of playing on all three channels. And it sounds wonderful! Did I mention that yet? lol Of course I still have the 4.6k resistors at R95 and 96. Is that going to be a problem? I ask because I really like the sound of the amp like this. It seems like it has a fuller sound. Although that could very well be because this amp has always sounds WAY better than my other amp. Maybe I should put the 2.2k resistors back in and see what it sounds like. Hmmm

Anyway I ohmed the old cap and its shorted. So I guess its no wonder the outputs were running hot.

Hopefully this is the end of problems with this amp for a while.
Of course I wish to thank Gajanan for his advice here because it led me to the immediate solution. But I wish to thank all of you for your tireless help. Sorry this went so long. I didn't give up though and I have learned so much from you guys and all the research I have had to do. Thank you thank you thank you! You guys are wonderful!

BR

Time for a beer!

[Enzo]

Aluminum electrolytic, 50v 20%, 0.47uf radial leads p/n 0033606003

[JMFahey]

No, it's not a tantalum electrolytic but a common aluminum one.

Ha ha!! Simulposting!!

Although, of course, Enzo's answer is incredibly accurate

[badraven]

Its already posted but no, it is not tantalum.

What would that mean to you if it were?

[gmphadte]

Tantalum would surely short.

Gajanan Phadte

[badraven]

Just an update: Its been 5 weeks since we found the problem with the amp. I gotta tell you all, this amp plays better than it did when it was new. Running the bias colder has made a tremendous difference in the tone and playability. It has always sounded like it was on the edge of quitting on me or something. It has at times overheated on me in summer months. Now there is no more crackling, popping etc. It sounds solid, smooth, powerful and best of all Good!

Since correcting the amp I have rewired my guitar and corrected a problem I didn't know I created when I wired it several years ago. I even added a mod to keep the fidelity when I roll the volume off as well.
This amp sounds so good these days I have given up on my idea of purchasing a tube amp (for now).

This whole experience has been very rewarding in that it forced me to spend a lot of time learning about amp electronics, I learned to be much better at soldering etc. and it caused me to learn to get better sound from my Spider Jam which I pretty much hated before. And this has carried over to my Fender amp. And most of all, I got to meet some pretty neat people on this forum. Just want to say thanks for all the help and for sticking through my mistakes, stupid quiestions and learning curve. I know I would have given up without you guys.

Jeff

[nigelwright7557]

I used to just take the bias setting recommended for any amps I had.

It was only when I designed my own amp did I give the bias setting any more thought.

I have read lots of threads where people prefer high bias currents.

I found myself that it was surprising how little bias current I needed to get rid of crossover distortion. I found I could get away with as little as 5mA.
I found out later that Peavey are quite famous for using low bias currents too.

[JMFahey]

Same here.
Once I can't see it in the scope, it ceases to exist for me.

[badraven]

Well I know lowering the bias sure made a difference with my amp because I changed it back to stock and did not like it at all. I wonder how much difference changing the opamps made? I changed them all to RC4558's I guess the next time I open the amp I can try some different ones to see what happens.