Motorola 7 inch tabletop

[dieseljeep]

Originally Posted by timmy

Ok well I cannot find a reason for the hum in the audio so I'm wondering if it's at all possible that the noise I call a hum Could be a buzz , could this maybe coming from the IF and sounding like an ac hum. I had placed an electrolytic cap between the audio amp and output and was able to filter Alittle bit but no matter where a cap is put, audio or power supply the hum or buzz is still there. The ground on the tube sockets are all soldered. Swapping tubes make no difference. All that seems left is the IF or the verticle but I went over the verticle and found nothing.

A hum at minimum volume usually indicates power supply hum or AC ripple getting into the B+. Disconnect the speaker and with an 8 ohm or so dummy load, signal trace the audio circuit to see where the hum originates.

[timmy]

Is it possible one of the 2 silicon diodes I put in place of the seleniums are leaking ? I didn't try subbing them.

[timmy]

Originally Posted by dieseljeep

A hum at minimum volume usually indicates power supply hum or AC ripple getting into the B+. Disconnect the speaker and with an 8 ohm or so dummy load, signal trace the audio circuit to see where the hum originates.

I'm not to sure what your saying by disconnect the speaker and trace. I do have 470m b- to chassis but there has to be another way to track this down. Different tubes nothing works and the chassis grounds at the tubes are super clean and soldered . Should there be voltage from chassis to b- , because I'm getting about 239 Vdc at those points.

[dieseljeep]

Originally Posted by timmy

I'm not to sure what your saying by disconnect the speaker and trace. I do have 470m b- to chassis but there has to be another way to track this down. Different tubes nothing works and the chassis grounds at the tubes are super clean and soldered . Should there be voltage from chassis to b- , because I'm getting about 239 Vdc at those points.

What I really meant was disconnect the speaker so you can hear the hum where it originates through the signal tracer, if you have one!
Are you reading the 239VDC using a digital meter? You might reading some stray voltages.

[timmy]

Yes I'm using a digital meter and getting 239 Vdc b- to chassis. No I don't have a signal tracer.

[dieseljeep]

Originally Posted by timmy

Yes I'm using a digital meter and getting 239 Vdc b- to chassis. No I don't have a signal tracer.

Try connecting the chassis to the B- and see if there's any change.
You can make a form of signal tracer out of a small amplifier. Even an old battery pocket radio.
Kind of running out of ideas, as the job is not in front of me.

[timmy]

Originally Posted by dieseljeep

Try connecting the chassis to the B- and see if there's any change.
You can make a form of signal tracer out of a small amplifier. Even an old battery pocket radio.
Kind of running out of ideas, as the job is not in front of me.

Well ok but if it's a dead short I'll have fireworks all over, lol. I already tried a cap between chassis and b- and b+ and made no change. I have several 9 volt radios but don't have an idea how to use it as a sig tracer.