Mesa Fillmore 50 preamp squeal

[chee16]

I'm working on a 2 month old Fillmore 50 that has developed a squeal on the hi and drive settings when the master is up all the way and the gain up wire high. There is 120hz hum with the master all the way up also, but nothing crazy. Once you start to roll the gain up it gets noisier and then starts to oscillate / squeal.

All tubes have been checked /swapped, twice, haha.

I thought grounding loop. But this is a pcb amp, seems hard for that to develop, not impossible I guess. The preamp grounding is all at the jack (through a lock washer type piece that's mounted to the jack pcb, as the jacks are isolated) while the power amp is grounded separately.

If I pull v1, since the gain pot is in between those triodes, it doesn't act up, but there is still some hum.

If I pull v2, the amp is quiet even with the master up.

All voltages are exactly as the schematic lists. All relays are working. I have a schematic but unfortunately I can't post it. Sorry for that.

Obviously without the schematic, it's harder for you guys, but I'm just looking for a direction to start checking in.

Thank you

[sluckey]

I have a schematic but unfortunately I can't post it. Sorry for that.

Yes, that is unfortunate.

Two month old amp should be under warranty. Pursue that direction.

[chee16]

Sorry guys, I honestly enjoy this forum and mostly read posts to try to learn from the solutions for future reference.

My side job /hobby is amp repair. I'm constantly learning and very careful. Because I'm in Ontario Canada, shopping an amp back to Mesa for warranty work is really expensive. I was asked to do the warranty repair for this amp having repaired ogre mesa amps that were out of warranty with the help of the mesa tech department.

Sorry if this isn't preferable for some, but the owner is aware and has had direct contact with mesa.

I'm going to talk to mesa at some point but was just looking for reasons why high pitch oscillation happens.

I hope this comes of as thankful, in no way snarky or anything.

[sluckey]

Your message is not snarky at all, but without seeing a schematic we're kinda operating blindfolded. Squeal is oscillations. Use your scope to try to locate the source of the oscillations. That's about all the suggestions anyone can make with the info you provided. Best to let Mesa deal with it.

[NewYorker]

Likewise, I'm not trying or meaning to be snarky (snarkastic?), but you might want to consult Mesa's user manual to see if this behavior is a feature and not a problem.

I don't know this particular Mesa model, but I recall reading Mesa documentation that basically said, "Don't do this" or "Do this at your own risk", or something to that effect.

OK, it wasn't worded exactly like that, but the notes would warn that instability might result from certain combinations of settings.

[chee16]

For sure, I appreciate both the good and bad of Mesa. High quality components, high quality board material, etc. Crowded double sided boards with lots of switching to trace.

The owner of the shop has extensive experience with this model, plus the owner used it quite a bit before noticing the issue. I'm not saying you're wrong! Haha. I'm searching for the manual right now in fact. So I appreciate the suggestion.

I might draw out the schematic for just the drive setting and post it. Compared to purge mesa amps it's not as complicated, so wouldn't be that tedious. Plus it would make it easier to read for everyone, haha.

[dwinstonwood]

As pointed out by NewYorker, and I'm not sure if this helps, but from page 5 of the manual:

"NOTE: Avoid setting the TREBLE high (above 2:00) when the GAIN is to be set high as this brings about the tendency for a slightly microphonic tube to ring or squeal."

https://mesa-boogie.imgix.net/media/User%20Manuals/fillmore50-WEB.pdf

[chee16]

Thank you so much! I'm going to check for the settings, I tend to set the tone stack all at 5.

I really appreciate the help btw.

[NewYorker]

And if the problem is microphonics, then swapping tubes might help.

[chee16]

So after checking a bunch of components, solder joints, etc. Finding a broken output primary center tap wire that was soldered poorly to the top of the board, ugh. The amp is working perfectly now.

I'm not fond of working on these and I hate not knowing exactly what was going on.

Now to work on a TC-50.

[NewYorker]

Yikes!  That's an odd one.  A faulty connection between A Voltage and the OT would normally result in NO SOUND at the output.  (Or not?  I have no experience with this kind of failure.  With B Voltage still on the screens and NFB connected could some other odd behavior occur?)  Was it completely broken, or maybe intermittent?

Say what we like about Mesa, it's probably worth letting them know what you found, in case they can prevent this on future units.

Did you replace anything else, or correct any other faulty connections?

[chee16]

Sorry I should have explained better. I noticed the wire /solder joint originally, because of the looks and orientation. You could tell it was done with the board mounted in the chassis, as in the wire wasn't coming through on the under side of the board and there was a large gob of solder on the top. I checked it, it didn't come off at first. Flipping the board must have flexed the last couple strands in the wire that were holding on and it broke. When I removed the old solder, the wire pieces were just sitting in the solder on the top of the board. Obviously with plated through holes on the pcb this can work fine. But it looked like in the process of stripping that wire (just a guess) the exposed threads of wire got flexed enough to break some, leaving the others weak.

Honestly I don't think this was the original issue, just a poor job done waiting to break.

I checked all the cathode components, DC coupling caps, and some filtering components in the preamp. So in checking, any cracked solder was addressed. I also checked all the solder joints on the bottom of the board for visual issues (I know you can't always see the issue) and resoldered some that didn't look ideal.

I also found a 2n6426 whose base pin was cracked, but until I flexed it slight to see the numbers on it, it looked fine.  This transistor controlled a relay that switches a .047 cap in parallel with the .022 for DC coupling off of V1b's plate.

Other then that, I didn't find anything that was the obvious problem contributing to the hum and oscillation.

I was hoping to find the culprit outright, but no such luck.

[Keppy]

So after checking a bunch of components, solder joints, etc. Finding a broken output primary center tap wire that was soldered poorly to the top of the board, ugh.

I just fixed up my 25yo Mesa Subway Rocket and discovered that the safety ground wire was not properly soldered.   Currently, I'm thankful for my health.
BTW, thanks to Hoffman for the schematic collection, which helped me fix the Rocket.


Glad you got the repair done. I hope Mesa paid you well!

[chee16]

Ya the repair was worth my time for sure, haha. I just like to learn something when I can, learning that solder isn't always great from the factory wasn't what I had in mind, haha.

I was lucky enough to get a link to all of Mesa's schematic library. If somebody needed one I'm happy to email it. But can't really post it on here.

It wasn't anything ground breaking anyways, haha. Just lots of EQ shaping components and things being switched in and out.

The TC 50 on my bench now is a different beat. Has a built in overdrive type circuit you can switch in /out before the preamp on any channel. Kind of a buffer type circuit at the end of the clean channel, maybe to boost the highs like a clean boost. Also had midi which makes me nervous, haha.