Metropoulos Forum

My MetroAmp JTM50 Black Flag has developed a mechanical rattle. I would like advice on the best fix for this. I am hoping that someone with some MacGyver skills will share a tip.

It started when I was testing out an Electro-Voice Force 12 that had no markings indicating its impedance. I still don't know the impedance, but it's DC resistance measured around 6. So, I guessed 8 and set my amp to that. The rattle may have had nothing to do with this, but I mention it in any case.

This is a mechanical rattle and NOT the tubes. The sound doesn't show up in the signal, but it is loud enough where it NEEDS a fix!!! The rattle is especially loud on low notes... E through G especially.

Taking the amp out of the enclosure I poked around, tightening screws. Some allowed for further tightening, but I didn't find any I would call loose. My ears already had me zeroing in on the OT, and so I made sure to tighten the screws that compress the laminates.

I am pretty sure I am in the right place, as I grabbed the transformer and with the same motion you use when twisting a motorcycle throttle, I discovered a clicking sound with each twist.

Hoping for the best after tightening, but with knowledge of the clicking sound, I fired up the amp again. It remained Buzzy and Rattly as ever. Not only that. After I was convinced the problem isn't going away and after powering down, I noticed this rattle when I tap on the amp. The sound of the rattles when I tap it reminds me of the sound of a kazoo. If I reach in and apply downward pressure on the OT while tapping on the case, the rattle stops... so I am pretty sure I found the source... so now, I ask for advice on how to fix this!

I have seen a dis-assembled vintage JTM50 where it looks like maybe there was some kind of goop was put under the transformer. Or, maybe that was just the traces of rust. Could I put some gasket material under the transformer? High temp silicone?

For the record, the output transformer is a Drake 784-128.

Thanks for any and all advice!

Tony

Does it rattle out of the head box? if it doesn't rattle
out of the box then OT might not be the source.

Check the logo / head box screws / shield plate / shield plate vibrating against
the chassis edge / back panel grill / top vent / front & back plexi / metal panels

Pushing down on the OT might be flexing the chassis downward.
That would stop the rattling of the chassis against or on the shielding plate.
Push down behind the OT and see if it stops or changes.

If the OT is loose inside the bells you could electrocute yourself or fry your amp.
It would be pretty tough to damage a transformer like that, not many parts
to come loose, but...anything is possible.

I hate rattles...

Thanks for your good advise. It inspired me to take it out of the box and tap on it to be sure.
I am not sure what you mean by "Bell" I am not sure if this link will work, but if it doesn't, I just put a video on YouTube that can be found with the search terms "Buzzing Drake Output Transformer" or found in my listings as user YardBirdFan.

[youtube]http://www.youtube.com/watch?v=N1Ffsaxf ... er&list=UL[/youtube]

http://www.youtube.com/watch?v=N1Ffsaxf ... er&list=UL

I now know know what "bells" are! I took them off my transformer and cannot see anything that might cause a rattle. I fully expected to see a nut or something inside. Nope.

By the way, to anyone who can see I am not an expert at this, who might be concerned. I know just enough to bleed the caps before I start messing with things.

I have a theory to test and another video coming...

See these photos...

This might be a totally un-related rattle, but I might want to replace my rectifier. Addressing the rectifier rattle might be picking at nits. What is a nit anyway...?

I have a theory on the transformer noise. I will discuss my theory after I get this back together and fired up again.

Anyway, here is a video of my rectifier tube rattle. It was the only tube misbehaving in this fashion.

[youtube]http://www.youtube.com/watch?v=rKMDqbf079o[/youtube]

Fixed!!!! Buzz is GONE!!!

Here is my theory and the fix...

I didn't think to do this before I pulled the bells off, but I suspect that if I would have taken a feeler gauge to measure the distance between the laminates and the chassis, I would have found a small gap. The fact that it sort of flexed when I was wiggling it seems proof enough though. I think that it was just barely touching, and when in operation or when tapped it was giving the kazoo like buzz. (see link in the third posting above to YouTube video named "Buzzing Drake Output Transformer")

I made sure that when I bolted the bells back on that the laminates were touching the chassis and the mounting tabs were elevated off the chassis by about what must have been about the thickness of a credit card. I used a couple folded over leaf bag twist ties. That way, when bolted down, the tabs would have to bend or flex to touch the chassis, and the laminates would have strong pressure to the chassis.

Just seen this - very interesting! Nice to see you figured it out and fixed it yourself!

Great! Glad you got it.

The video really shows the problem.
Glad it was easy and cheap to fix.
I think you just coined a new amp repair phrase...
"Kazoo Transformer" ha...I will remember that one!