Hi Guys, I recently restored a heathkit T-3 signal tracer. I change all capacitors and electrolytics but when I turn it on and the tubes warm up and I turn the gain control knob to 3/4 it makes a loud hum thats dissapear as I touch the chassis with my hand.
¿do some body have an idea what is happening here?

This probably has something to do with how the chassis is grounded to the AC line. I strongly disagree with grounding the chassis to a third ground wire (three wire cord), especially when the tracer is used with AC/DC sets, but you can try this anyway. If used on AC/DC sets, though, you risk blowing fuses. Try sending both sides of the AC line to the chassis through .05 MFD condensers, or just one, and then try reversing the line cord. Try shielding tubes, especially if they have anything to do with an audio injection circuit (allowing you to inject an external audio signal). Check to see how the filament transformer winding is connected to the chassis. Does it use a center tap? If not, is one side grounded to the chassis? If it is not, you may try balancing both sides to the chassis through two 47 ohm resistors, one per side, or through two .05 MFD condensers, one per side.

Experiment. If this tracer uses a shielded probe, how well is the shielding connected to the chassis? What is it connected to? What was it originally supposed to be connected to?

Shielding the tubes will likely help a lot. Putting shielding springs around leads that run above the chassis can also help if conditions are extreme. The coil springs must be suspended in such a way that they will not slide down the wire and touch such things as perhaps a grid cap at the end of the wire. Crimping the spring so that it grabs the wire insulation can help, or wrapping it with friction tap at the end can help (hardware store). One end of the coil spring must be connected to the chassis.

Start with the easiest and get more complicated only if necessary.

Thomas

Milton,
You didn't say grounding the chassis with your hand. If only touching, it could be a physical resonance of the hardware. Loose laminations, case or screws on the power transformer/chokes could vibrate under conditions of increased load. ie increasing the gain. I'm not familar with the T3, but assume its simply a signal source, not a sweep generator. Had an experience years ago with a sweep gen. It used a small speaker type voice coil with attached capacitor plates, creating a electrically variable capacitor and consequently, electronic sweep. Mfr. called it a "Wobulator"?? A variable 60 Hz signal was injected into the voice coil, and as the level was increased (wider sweep), this huge generator would nearly dance off the bench. Had it not been so heavy, would have taken a massage.
marv
:Hi Guys, I recently restored a heathkit T-3 signal tracer. I change all capacitors and electrolytics but when I turn it on and the tubes warm up and I turn the gain control knob to 3/4 it makes a loud hum thats dissapear as I touch the chassis with my hand.
:¿do some body have an idea what is happening here?

Man, youre totally right, thats what i'm feeling its happening because when I put it in action and the sound begin I took it by the handle to see whats wrong and yes I feel that the cabinet sheet was
!vibrating!? yes I touch it and the sound graduatelly stops. I cure it momentarily putting a cloth down the cabinet.

:Milton,
:You didn't say grounding the chassis with your hand. If only touching, it could be a physical resonance of the hardware. Loose laminations, case or screws on the power transformer/chokes could vibrate under conditions of increased load. ie increasing the gain. I'm not familar with the T3, but assume its simply a signal source, not a sweep generator. Had an experience years ago with a sweep gen. It used a small speaker type voice coil with attached capacitor plates, creating a electrically variable capacitor and consequently, electronic sweep. Mfr. called it a "Wobulator"?? A variable 60 Hz signal was injected into the voice coil, and as the level was increased (wider sweep), this huge generator would nearly dance off the bench. Had it not been so heavy, would have taken a massage.
:marv