Cav 50 issue

[walge]

Hi

I have a Cav 50 that needs some check.
I can hear a mechanical "buzz" after few minutes of power on.
If I took out the 6SN7 it stops; with only the EL34 inserted non noise.
Also with only the input tubes.

Very strange.

Any Idea

Thanks

Walter

[admin]

Not that familar with the CAV50 but it may be a bad tube. I would just do a visual inspection of the internal components as well. Make sure there are no burn marks, bulging caps, cold solder joints, etc.

[walge]

Thx for answer
I have some test equip and I will check.
Internal seems to be fine

Walter

[roberto]

Hola Walge and welcome,

If you take out a tube, you might have an open circuit, and the current will not drain. I think that you have the power transformer's sheets or plates loose. There are four screws that secures these plates, just re-tighten the screws holding their nuts, they must be real tighten. At the case of the transformer is where these screws are located. You do not have to remove anything. Just make sure these screws are very thighen.

Also Admin is right. A bad tube could cause too much current drain and make the transformer to vibrate.

Happy listening!

[admin]

I would try Roberto's recommendation first. It's easy and fast to do and will rule that out. Also, I probably don't have to say it, but make sure the unit is disconnected from power and let it sit for a while to make sure all the caps discharge before poking around in the unit.

[roberto]

That's a great advice, the other day I just lost my screwdriver, because I thought that I previously I discharge the power supply, and I was wrong. I made a short-circuit that melted the metal of my screwdriver. It is wise to be sure that everything is discharged...don't do silly things as I did!!! I do not recommend to you my jump, and I grab the ceiling with my nails, like a cat! The wife came and said: Roberto, what are you doing there? Do you want me to bring to you a latter?...oh boy, what a question!

Happy listening!

[walge]

Hi

I have a my lab and I check all voltages and current.
Withn only the 34 inserted and biased at 45mA each one I haven't any noise.
When I put the 6SN7, also only one, the noise appaer.
So maybe is a oscillation, tody I will check with scope.

Walter

[roberto]

Hola Walter,

What are you going to check with the oscilloscope? Or I am not understanding you. You said that it was a mechanical noise, a buzz, this will not appear in the scope. Coming from the speakers, yes! That's a different story. Be careful with the scope probe. Some are not for the voltage that you might find there. The output tubes plates could have circa 400V DC or more. Be careful, this is a lethal voltage.

On the other hand, you might find from where the oscillation is coming from. Always check first the cathode, the the grid and then the plate. If the cathode is clean, and you have the noise at the plate, that tube is not OK.

If the buzz is coming from the power transformer, just re=tight the nuts of the screws securing the metal case of the Xformer.

Happy listening!

[walge]

Hi
Do you think will possible to find the schematic?
Today I will check with scope and new 6sn7

Walter

[AnotherJohnson]

Is it mechanical radiation from the component surfaces? Or is it in the audio signal path, hence present in the speaker output?

If it’s mechanical radiation from the chassis, cover, or transformers, it’s probably failed transformer insulation.

If it’s present in the speaker output, it’s probably a leaking capacitor or degraded diode that is allowing AC line hum to get through where it was supposed to be blocked.

It’s easy to roll tubes. If that doesn’t solve anything, it’s not the tubes. I would be surprised if it were the tubes.

[walge]

Hi

Solved the issue.
In on old thread in this forum there is another guy with same problem.
Are the 4 diode and one cap for filaments. I changed them with 6 A instead 3 A and a cap and everything is ok

Many thx to all

Walter

[roberto]

Thanks Walter,

This is a new problem for me! Thanks for sharing the solution too.
Happy listening!