Ts4d Motorola

[robert1]

This is where the method of *Process Of Elimination* comes in handy. What i would suggest is to disconnect what is connected to pin 2 on V7 (6SQ7) & then power up the set & see if you still have hum. there should be silence. if the hum stops, You can eliminate the audio amp & output stage as being the problem. the problem then, would be somewhere upstream in the I.F. or in the audio detector stages. If the hum is still there, there is a problem with the source voltage from the B++ rail that supplies the audio amp stage.

Are you sure that the hum is a 60 cycle ripple, & not a buzz caused by sync pulses?

[timmy]

Originally Posted by robert1

This is where the method of *Process Of Elimination* comes in handy. What i would suggest is to disconnect what is connected to pin 2 on V7 (6SQ7) & then power up the set & see if you still have hum. there should be silence. if the hum stops, You can eliminate the audio amp & output stage as being the problem. the problem then, would be somewhere upstream in the I.F. or in the audio detector stages. If the hum is still there, there is a problem with the source voltage from the B++ rail that supplies the audio amp stage.

Are you sure that the hum is a 60 cycle ripple, & not a buzz caused by sync pulses?

Well it sure does sound like 60 hz it's sure loud enough and don't sound like a buzz but rather 60 hz .

[robert1]

Originally Posted by timmy

Well it sure does sound like 60 hz it's sure loud enough and don't sound like a buzz but rather 60 hz .

Ok, have you tried to remove the leads on pin 2 of the 6SQ7?
the only things i can think of is there is a filament to cathode leakage on ether of the 6SQ7 or the audio output tube, or high ripple on ether the B+ or B++ sources. it also could be originating at the volume control itself (broken ground connection).
if you had a scope, you could trace the audio signal path to the point where the AC ripple is coming from.

[timmy]

Originally Posted by robert1

Ok, have you tried to remove the leads on pin 2 of the 6SQ7?
the only things i can think of is there is a filament to cathode leakage on ether of the 6SQ7 or the audio output tube, or high ripple on ether the B+ or B++ sources. it also could be originating at the volume control itself (broken ground connection).
if you had a scope, you could trace the audio signal path to the point where the AC ripple is coming from.

Ok clipped all on pin 2 of the 6sq7 and still got the hum, so I guess it's off to the b+ or b++ . Ground at the volume is intact.

[robert1]

Originally Posted by timmy

Ok clipped all on pin 2 of the 6sq7 and still got the hum, so I guess it's off to the b+ or b++ . Ground at the volume is intact.

Ok. If you replaced that 20 mfd capacitor off of pin 8 of the 25L6, did you connect the negative end of it to the same tie point as the original was?, or did you attach to the closest chassis ground?
the reason why i ask this is because the grounding for the entire audio is isolated from the chassis & connected to the B- rail. If you connected the negative of that capacitor to chassis, that could be the problem.
regarding the volume control, if the ground connection on the wafer in the control is open, it can cause the same problem. you need to disconnect the leads off the volume pot & check for end to end continuity, as well as continuity between the wiper contact as it is moved from one end to the other. i have seem many bad pots cause this problem.

[timmy]

Originally Posted by robert1

Ok. If you replaced that 20 mfd capacitor off of pin 8 of the 25L6, did you connect the negative end of it to the same tie point as the original was?, or did you attach to the closest chassis ground?
the reason why i ask this is because the grounding for the entire audio is isolated from the chassis & connected to the B- rail. If you connected the negative of that capacitor to chassis, that could be the problem.
regarding the volume control, if the ground connection on the wafer in the control is open, it can cause the same problem. you need to disconnect the leads off the volume pot & check for end to end continuity, as well as continuity between the wiper contact as it is moved from one end to the other. i have seem many bad pots cause this problem.

That 20mf ground is ok it's not on the chassis, I know there are the 2 grounds, chassis and b- . I'll check the volume ground, via the wafer. Thanks

[timmy]

Ok the volume control checks out, wiper to end does vary the way it supposed to. So now still at dead end, I even tried other diodes thinking one may be leaky but no luck there and I also paralled every cap still no luck still hums. The Motorola Manuel showed reasons for a hum and I checked that out still no luck. When I first got the set I had to tweek the IF for a clear pic and sound and the Manuel says if the oscillator is adjusted to high it's considered to be overloading and the audio would get a 60 hz hum carried along the verticle circuit, strange, hmmm. Checked around the verticle to no avail. What I also tried was I turned the 3 adjustments L17 ,L19 , L27 4 turns cw then 4 cww to see if that made a difference in the hum if that were the reason but I never did turn them a lot in the beginning.