LA-2A hum and oscillation problems - SOLVED !!!

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Purusha

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Jan 14, 2006
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I wanted to use my new LA-2A in vocal recording chain yesterday
and discovered oscillation and hum problems plus I heard German
radio station from time to time :shock: :green:

Otherwise I really like what this unit does to the sound but I guess it
needs a bit of a tweaking before I can really use it.

I followed the grounding instructions from Gregory (drip) like you see on the photo.

LA-2A-gnd.jpg


Could there be a problem with a bad tube? Any suggestions what else to do?
 
The bottom ones are OK as I have the screws with sink in heads which are touching the metal
plates from bellow, but I didn't check the one on the back plate. Will report back in the evening if it's OK.
 
sintech said the following
If using sowter trafos, the Grid Stoppers (1/2 watt 1k resistors) go on just the 12BH7A in series on pin 7 and pin 2.

If I understand it right there in series with the heater lines on those pins listed above. I looked around about it because I didn't know what a grid stopper was.

I got a question.. I have a evil twin DI unit and I took it apart to fix a knob issue. On the tubes inside there was a O-ring placed about 1/2 way between the elements/plates of the tube... what's the idea here?
 
kazper, I'm guessing thats a Ferrite Ring to block RF signals.

That's totally right on the Grid Stoppers, very easy to add to the Point 2 Point version.
 
Is farrite an rubber like material? This thing is just like a O-Ring and rubbery not solid.

I did look them up but I'm thinking you have seen them firsthand.

Is it a snake oil type thing or is it worth it to look into?
 
I ground off a little spot of coating at every joining case screw hole. And then carefully put a #4 internal tooth lock washer between every case part at every screw. Just to be safe. (Between the case parts where ya can't see it.) The washers are very thin so ya can't even tell they are there...
I also used the 2 1k resistor grid stoppers just in case also. Haven't had any problems with noise of any kind. I have the version 2 board. Don't know where they would go on yours.

Look here at the top case screw hole you'll see what I mean:

www.stagefrightonline.com/latop
 
Maybe the back of your case where the star ground is is not connected to the bottom of the case. The coatings on your cases are very robust! I accidentally slipped with my rat tail file on the back and it didn't even penetrate the white silk screen... OOPS
 

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