Heathkit iT-12 signal tracer to much gain??

[tux139m]

Volume set to near minimum gain is high, some hum in volume which increases with volume. I changed the 12AX7 to a 12AT7 but made little difference.
The voltages are high as shown in schematic. This is my first post so I hope it is clear. I checked the tubes and all the resistors and caps plus looked at the wiring all checks as shown.
this is my first post hope it is clear,
thanks for any input to this problem,
thanks
Bruce Tiedeman

 

[JonSnell Electronic]

Your mains is 12% high and so will the voltages be, so they are correct. The gain control should reduce to zero, if not replace it. If you require less gain, try an ECC82 (12AU7).

 

[Keit]

This is a very simple circuit. If the gain control pot is turned fully anti-clockwise, the grid of the first tube is earthed, and can have no signal.

Therfore, either the pot earth terminal is not earthed, or the pot is open circuit at teh eathy end - a not uncommon fault in some brands.

Check that the earth terminal of teh pot is indeed earthed by visual inspection. If it is, unsolder the wire from the pot terminal. Check pot with an multimeter on the ohms range. Turn the pot while the multimeter is connected - you should see the resistance betwen either side terminal and the centre terminal changing. You'll find it open circuit on one side. Replace the pot.

 

[tux139m]

I am sorry, I should have said volume set near minimum the amp is still loud, the control goes from 0 to 100 and at 20 is loud, I set it to 10 on the volume scale when using. I can also reduce the level to 0 and hear a low level hum wich seems not related to the volume control. I would like to have the vlolume set to a little higher or near mid range in use. I checked the pot and it shows 1.4 meg from center, to each side, but still shows around 50 ohms when turned to zero. Just hoping to reduce gain of amp.

thank you Jon and keit

 

[tux139m]

I put in a 12AU7 and it did not make much difference either, lots of hum and still to much gain, that 12CA5 gets really hot but unit is not drawing excessive current. I think it is a poor design. Is there any other changes we can try, at the CA5??
thanks again,
Bruce Tiedeman

 

[tux139m]

No pronounced hum now, satifactory operation. I can't pinpoint what I did to solve this problem, I worked on the volume control and inproved the ground on the large Electrolitc Capacitor Can, dressed some of the leads to the chassis. I do think the 12AU7 made a big difference.
Was going to change the pot but cleaned it improving the ground only.
I purchased this off of ebay originaly working, but was not! It was a mess with multiple wiring problems. Happy to have it working, much more invested than planned.
thanks again for steering me in the right direction,
BruceTiedeman