Repair: At-home or Technician?

[EricssonB]

Unwanted Preamp Fuzz - YouTube

Fender Bassman 200 amp's been having this issue for a few weeks, and now I have time to confront it. It should be a clean signal, but is weak and distorted.

Eliminating variables, I believe't I've isolated the problem to the preamp section. My best guess is that a capacitor is bleeding energy and can't push a full signal. I figure this because later in the video, I switch over to my guitar that has hot pickups, and the signal is weaker than it is with the bass.

Video shows instrument direct to amp. I deox'd all connections a few times. This is the third cable that I've tried, all yielding similar results.

Can you estimate what the problem might be? Is this something that my decent soldering skills can remedy, or do I need a professional?

Oh look, schematics!

[EricssonB]

So I was walking around and accidentally bumped into this thread.

[Passinwind]

Originally Posted by EricssonB

Unwanted Preamp Fuzz - YouTube

Fender Bassman 200 amp's been having this issue for a few weeks, and now I have time to confront it. It should be a clean signal, but is weak and distorted.

Eliminating variables, I believe't I've isolated the problem to the preamp section.

Meaning you've run a different preamp into the power amp section and the problem goes away, correct?

Exactly what diagnostic and repair tools do you have access to? How many amps with similar problems have you actually fixed?

[EricssonB]

Done some replacement of components before, but not on amps. I work in Comm and they have electronic test equipment that I have access to.

Process of elimination, really.

No matter how the knobs are set, which instrument or cable I use, the signal seems to be weak in the internal preamp. The signal indicator light shows a weak signal, even if I throw in a pedal before the amp and boost it.

[KramerBassFan]

I'd imagine that Fender is mostly tiny PCB stuff.

If you can find the problem, then fix it. But with an amp like that - good luck.

[RickenBoogie]

I'm also highly skilled with electronics, but take my amps in to an amp tech, every time. It's a specialized field, and requires a specialist to do the job properly. Don't take chances.

[EricssonB]

Originally Posted by KramerBassFan

I'd imagine that Fender is mostly tiny PCB stuff.

If you can find the problem, then fix it. But with an amp like that - good luck.

Originally Posted by RickenBoogie

I'm also highly skilled with electronics, but take my amps in to an amp tech, every time. It's a specialized field, and requires a specialist to do the job properly. Don't take chances.

These two points are right on the money.

Oh hey, look, my Federal tax return is on its way back -- whenever they feel like delivering it.

[seamonkey]

You just want to work on the pre-amp?
Stay away from the power amp and AC?

Remove the pre-amp and plug it into a bench power supply.

Signal generator, and a scope.
scope it at each of the little ovals that are shown on the schematic and be sure it matches what the ovals say.

If you burn something up and end up taking it to a tech - tell them your brother did it.

[will33]

Originally Posted by Passinwind

Meaning you've run a different preamp into the power amp section and the problem goes away, correct?

Exactly what diagnostic and repair tools do you have access to? How many amps with similar problems have you actually fixed?

Do this first to make sure you have the problem isolated.


Stuff I do at home---

General cleaning
Soldering bad connections/frayed wires
Replace obviously bad parts like broken jacks, resistors that are burned black, caps with obvious physical damage.


Or general overhaul, meaning just replacing old parts with new ones of the same type and value, even if you don't know what they do. This falls in the category of messing with some amp you don't need and won't miss, just to fool with it and hopefully learn something.


Everything else-real tech
Amp you depend on for the gig-real tech.

[fdeck]

I've done a lot of my own amp repairs, but I'm qualified to do so. I don't know what I'd do if an amp of mine developed a problem in the power amp circuit, but that's never happened to me. Most problems seem to be mechanical in nature, i.e., a bad connection or broken control somewhere.

Passinwind makes a good suggestion, to separate the preamp and power amp at the effects loop and test each separately. Also, try bypassing the effects loop with a cable. Preamp repair is a lot more straightforward, but can be a hassle if you have to disassemble the entire front panel in order to get at both sides of the circuit board. But no repair is safe if you don't know how to work inside an amp safely.

[EricssonB]

Well, something isn't right.

Replaced two transistors with a pair of the exact same. Simple enough job.

http://www.youtube.com/watch?v=8ZBt_...e_gdata_player

[EricssonB]

Alright, I'm not going to blame Fender and their just-less-than-crystal clear schematics, but, I do. I should have noticed this. The 4 and 9 look just about identical on their .pdf so I bought two of the same.

?_?
Two old are on the right, new on the left:

[fdeck]

Why did you think that those transistors were bad?

[T-Bird]

Hi.

#1 SAFETY FIRST. ALWAYS.

Fixing amps isn't hard.

Fixing 'em succesfully with as little effort as possible requires quite a bit of skill though.

You can "fix" stuff by just randomly swapping parts, but it'll get expensive. Fast.
And there's still less than 50% chance that the procedure will be a succesful one.



The schematic Fender is kind enough to share is IME way above average in MI field. The TP readings will get You more than halfway there, and after a reading that doesn't match, the repair is easy.

BTW, IME fixing DIY repairs make more money to real tech's pockets than if he/she can plow on a virgin soil. So to speak .

Regards
Sam

[B-string]

More money to fix a DIY's mess yes, but not very fun or productive. The added work load is not really welcomed.

[EricssonB]

Originally Posted by fdeck

Why did you think that those transistors were bad?

The PCB around the two that I R^2'd were discolored from heat, so it seemed like a quick $2 fix.

I actually took it to a tech a few weeks ago. His results came back inconclusive, but I'm not entirely sure that he looked at it. Would have charged me more than the amp was worth.

Oh yeah, that's why I'm not taking it to a tech -- not hardly worth it if I can tinker with it and do it myself. I've got another amp out in California (when I figure out how to get it to Colorado) that will power this one after its conversion to an extension cab, should this repair lose its momentum/purpose.

[BassmanPaul]

As a question: do you even know how to test a transistor??

[EricssonB]

Originally Posted by BassmanPaul

As a question: do you even know how to test a transistor??

I feel like you're challenging my credibility.

Regretfully, I don't have the means to properly test components.