Ts4d Motorola

[jr_tech]

Do you still measure a high voltage between the chassis and b- ?

jr

[timmy]

Originally Posted by jr_tech

Do you still measure a high voltage between the chassis and b- ?

jr

No hv neg to chassis I obviously went from b+ to chassis. My mistake.

[jr_tech]

What do you hear if you disconnect the grid (pin 2) of the 25L6 audio output tube.

jr

[timmy]

Originally Posted by jr_tech

What do you hear if you disconnect the grid (pin 2) of the 25L6 audio output tube.

jr

I didn't disconnect anything else but pin 2 of the 6sq7, still humming.

[jr_tech]

Originally Posted by timmy

I didn't disconnect anything else but pin 2 of the 6sq7, still humming.

So now i'm asking what happens if you repeat the experiment on the next (last) audio stage.

jr

[old_tv_nut]

Are the filaments in series, or is it possible to pull the audio amp tube and see what happens to the hum? Do you have a tester to check the audio output tube for heater/cathode short, or a spare output tube?

[timmy]

Well I could take off pin 2 but nothing will happen, heater. Or you meant grid I think ?

[timmy]

Originally Posted by old_tv_nut

Are the filaments in series, or is it possible to pull the audio amp tube and see what happens to the hum? Do you have a tester to check the audio output tube for heater/cathode short, or a spare output tube?

I tried 2 other tubes both of them and my tester does check for shorts. It is series.

[jr_tech]

Originally Posted by timmy

Well I could take off pin 2 but nothing will happen, heater. Or you meant grid I think ?

Yes, grid, pin 5, sorry for the confusion.

jr

[timmy]

Pin 5 off hum even louder with no hiss as if there was no ant.

[jr_tech]

Interesting.

What happens if you connect pin 5 of the 25L6 to either end of the cathode resistor (330 ohm R-38) ? Try both ends.

jr

[timmy]

Originally Posted by jr_tech

Interesting.

What happens if you connect pin 5 of the 25L6 to either end of the cathode resistor (330 ohm R-38) ? Try both ends.

jr

After the grid wire was put back on?

[timmy]

Ok, either side of the 330 resistor it blanked out the audio all together.

[timmy]

Well the 330ohm resistor checks at .336k so I have a new 330ohm resistor that checks just that so I'll change that resistor tomorrow. One end of that resistor goes the cap at the amp . maybe it cannot filter because of that resistor being high.

[jr_tech]

Seems like it coud be leakage from the grid to the heater pins.

Is the socket dirty or perhaps charred? Bad insulation on wire to the grid?

jr

Edit add: Why change a resistor that is only 6 ohms off? That is less than 2% off.