Modified SCT-800 Loud Electrical Hum after paint job

[Maliq]

Wow it's been longer then I thought that i've posted something.  So here we go again.

I couldn't resist to modify my used T-Bone SCT-800, I changed a few caps, stick to the same value and Voltage, and changed the Capsule, then the Tube, first I tryed it with a EH 6072. I had a feeling I lost some db after i changed the tubes so my thought was to try a EH AT7 Gold pin, I had a feeling the sound was better after I tryed a Mullard AT7 tube but this one was allready in use in my go to mic.

Ok, the Microphone functioned perfectly with the new caps, the EH 6072 or the Mullard and the New Capsule.

So i thought about to give it a proffesional paint job. I wanted to have the Main Body and the Headbasket painted first cause the price was higher then expected, and it was just an idea I had so i didn't want to spend much...

and thats where trouble starts..

I've received the parts, ok i was not lucky with the paint job but thats another story. I received the Main body and the Headbasket and main body part got a glossy type silver grey paint. I put the Microphone back together, this time with the Gold pin Tube from EH, fired it up, and I get a loud electrical hum... I can still hear my voice though the mic tho.

I switched back to the old EH 6072 and the Hum was still there.. then I tryed to fire the mic up without the Body and Headbasket..and the Electric Hum was really really Loud.. Still could hear myself but damn what causes this?

So, the Mic worked perfectly and all of sudden out of nowhere I got this crazy electrical Hum sound.. does the new paint job can cause this? something wrong with the power suply? I've not touched it at all to modify. Or did the EH AT7 Gold pin tube causes something to overload?

All I know  is that this sound reminds me of something that is not well shielded.. maybe. I'm not much an electrician. Any ideas?

Regards

 

[kooma]

since it's painted; are the groundings ok?
did you scrape the paint of from ground points etc?

 

[Khron]

That's exactly my thought.

Make sure the (internal) bottom of the headbasket, the ends of the body tube and the top/outside edge of the bottom cap are all sanded off / free of paint. If the metal around the capsule & circuitry isn't all connected to ground, it's almost like it wasn't there at all => HUMMMMMM

since it's painted; are the groundings ok?
did you scrape the paint of from ground points etc?

 

[ln76d]

Multimeter with "beeper" is your best  friend in case like this

BTW. the stock capsule is pretty ok. - 32mm Alctron k67.
I didn't liked it for long time, but i found that properly used sounds really good
There's a lot of other tweaks which you can make in this microphone.
Mods like capacitors swapping for the same values still change not much, all the gearslutz recipies like "use dale resistors" or micandmod "upgrade" kits doesn't change a lot. Microphone still sounds sh...y

If you need some info, PM me.

 

[Maliq]

Thanks, yes, they painted the whole parts inside and outside, top and bottom..and pretty thick too. So it will take some time to remove it with sandpaper, espacially on the bottom inside the headgrill. I'll try that until the mid of next week when I get back home.

I got a Multimeter but honestly, I really don't know how it works, yes i read alot about all this and I'm familiar with all the safety procedures but i'm still kinda scared to meassure a live mic.

The K67 might be ok but since i've installed a 47 type Capsule its a different league right now, the sound is way more 3D, no more harsh top end, no more boomy lows. and way more mids..for my taste alil too much around 800hz but thats ok so far. Also removing the thin small mesh inside the Grill made a difference too.  Thanks, I'll pm you In76d

I'll get back when i've removed the paint

Regards

 

[Khron]

If the paint's particularly thick, i'd "go medieval on it" and have at it with a small flathead screwdriver, since it's in a well-hidden / an internal area ;D

True, the B+ in tube mics is indeed usually in the 105-120v region in most tube mics, so there's a chance it'll make a spark and a pop, if you short that to something you shouldn't But on the other hand, that voltage is usually heavily RC-filtered, so that discharge would "only" come from the last reservoir cap.

There might be other brands too, but i know for a fact that Fluke make "shroudable" probes, which should make it more difficult to accidentally short stuff together See here.

But on the other hand, the "beeper" (ie. continuity test) and/or the ohms range, is supposed to be used when the tested device is powered OFF, so there shouldn't be anything to be scared of But just to be on the safe side, use the V range to make sure the 400v caps are discharged, to avoid surprises They can hold a charge for quite some time.