http://antiqueradios.com/forums/viewtopic.php?t=92995
I was bragging all about how deep rich and full it sounds especially when placed back inside the cabinet. That's when I noticed how "rich" the sound seemed to be.
http://www.nostalgiaair.org/PagesByModel/527/M0018527.pdf
Well that's all very true ...but as I was getting ready to put it on the shelf and prep it for an EBAY sale and return it to the wilds...
I just happened to notice an irritating little ac hum most noticeable only when the volume control was way down at minimum. At the point of no modulated of program audio... just ac hum.
As I brought up the volume the program audio masked it pretty well so I was debating whether or not to just let it go and write it off to "normal".
But I couldn't do it.
I cracked it open and looked at ripple with my scope ( almost nothing) and tried adding more filter capacitance... all to no avail.
Now the cabinet was off and when the chassis sits alone on the bench like that the hum is hardly noticible... at least not irritating.
But as I slipped it back in the cabinet... whatever acoustics that cabinet adds... also seemed to enhance that ac hum! ...lol
So I pulled it out again and studied the schematic. I thought maybe I'd try adding my own 3rd filter stage in the power supply.... but further tests proved that meaningless... esp since the total existing ripple was only about 500 mv p-p.
So I tried tracing the signal by grounding the grid at each of the THREE audio stages in this model. ( Yes that 6th tube .. a 12sk7...is used as a 2nd audio)
Starting with the 35L6 output amp I grounded that grid and the hum was totally gone.
So I went back one stage to the grid of the 2nd audio and when I grounded that the hum disappeared too.
Then I went back to the grid of the 12sq7 1st audio and grounding that also silenced the hum.
Going backwards to the IF stages didn't improve anything... so I stuck with the audio section.
I noticed that as I lightly squeezed my two fingers over the body of C8 the .01 coupling cap to the grid of the final audio amp the hum got louder a tad.
So it seemed the location of that cap was picking up some AC.
I decided to try shielding that cap by making a grounded coil of wire over the cap's body from end to end. Well that seemed to make a nice dent in the hum level so I was encouraged.
I did the same thing ... touching the body of C5 the input to the 2nd audio... and that acted similarly so I also used a grounded coil of wire as a shield on that cap and again had some improvement.
When I tried it with C4 the coupling cap from the volume control there seemed to be an improvement again... but I wasn't completely satisfied.
So I started poking around the volume control and checking the layout.
I noticed that the ground wire on the low end of the pot was a little long... so I removed it and instead ran a very short little jumper right over to the ground side of the AC power switch.
I also found a mystery 47k resistor going between the top or high side of the pot and the junction of C3 and R4 where they meet the IF lead.
It was probably a factory production line change part but I pulled it out anyway and made a direct connection from the high side of the pot to the junction of the IF lead and C3-R4.... as it's shown on the print.
Then I tested everything again and found that about 80% of the hum was now gone.
As my hand or fingers came anywhere near the body of the coupling cap C4 from the volume control, it seemed to increase the hum level again.
So this time I re-wound another nice coil of insulated wire completely along the cap's body and a little down each lead and soldered BOTH ends of the coil wire shield to ground (B-).
WELL... that finally did it... SUCCESS !!
Virtually no hum now at all... not on the bench or even back in the cabinet.
I feel so GOOD now because these are the most irritating kinds of problems and we rarely if ever REALLY get them solved.
And this one is [b]solved[/b] ...beautifully.
But it wasn't [b]just one[/b] single simple thing.. it was a combination of ALL those solutions/fixes.
... but at least I have the consolation of knowing that it wasn't an unknown strange kind of "magic" cure.
Nope.. this one was fixed... step by step... but fixed for sure.
Now [b]why[/b] was this a problem in the 1st place I ask myself?
Myself answered ...and me thinks that the extra stage of audio amplification probably makes it all more susceptible to the slightest noise... since it's designed to really amplify more than a typical AA5 set.
(I think.. that's why)
Ahh.. let's see.. hummm 5:17 am?... ok
Time to go to bed... lol
Location of a ground connection can be very important especially around the 1st audio amplifier. More important on a radio with good low frequency sound.
A radio with good low frequency response will amplify 60 cycles more than other radios.
Even though a chassis may be grounded there is a slight voltage difference between points. Sometimes one side of tube filaments connects to chassis. This can put a fraction of a volt across a chassis.
Norm
:I recently posted pictures both here and over on ARF in the [i]Antique Radio Discussions[/i] thread and some glowing performance info on a Sonora RDU-209 six-tube AM set that I just finished.
:
:http://antiqueradios.com/forums/viewtopic.php?t=92995
:
:I was bragging all about how deep rich and full it sounds especially when placed back inside the cabinet. That's when I noticed how "rich" the sound seemed to be.
:
:http://www.nostalgiaair.org/PagesByModel/527/M0018527.pdf
:
:Well that's all very true ...but as I was getting ready to put it on the shelf and prep it for an EBAY sale and return it to the wilds...
:
:I just happened to notice an irritating little ac hum most noticeable only when the volume control was way down at minimum. At the point of no modulated of program audio... just ac hum.
:
:As I brought up the volume the program audio masked it pretty well so I was debating whether or not to just let it go and write it off to "normal".
:
:But I couldn't do it.
:
:I cracked it open and looked at ripple with my scope ( almost nothing) and tried adding more filter capacitance... all to no avail.
:
:Now the cabinet was off and when the chassis sits alone on the bench like that the hum is hardly noticible... at least not irritating.
:But as I slipped it back in the cabinet... whatever acoustics that cabinet adds... also seemed to enhance that ac hum! ...lol
:
:So I pulled it out again and studied the schematic. I thought maybe I'd try adding my own 3rd filter stage in the power supply.... but further tests proved that meaningless... esp since the total existing ripple was only about 500 mv p-p.
:
:So I tried tracing the signal by grounding the grid at each of the THREE audio stages in this model. ( Yes that 6th tube .. a 12sk7...is used as a 2nd audio)
:
:Starting with the 35L6 output amp I grounded that grid and the hum was totally gone.
:So I went back one stage to the grid of the 2nd audio and when I grounded that the hum disappeared too.
:
:Then I went back to the grid of the 12sq7 1st audio and grounding that also silenced the hum.
:
:Going backwards to the IF stages didn't improve anything... so I stuck with the audio section.
:
:I noticed that as I lightly squeezed my two fingers over the body of C8 the .01 coupling cap to the grid of the final audio amp the hum got louder a tad.
:So it seemed the location of that cap was picking up some AC.
:I decided to try shielding that cap by making a grounded coil of wire over the cap's body from end to end. Well that seemed to make a nice dent in the hum level so I was encouraged.
:
:I did the same thing ... touching the body of C5 the input to the 2nd audio... and that acted similarly so I also used a grounded coil of wire as a shield on that cap and again had some improvement.
:
:When I tried it with C4 the coupling cap from the volume control there seemed to be an improvement again... but I wasn't completely satisfied.
:
:So I started poking around the volume control and checking the layout.
:I noticed that the ground wire on the low end of the pot was a little long... so I removed it and instead ran a very short little jumper right over to the ground side of the AC power switch.
:
:I also found a mystery 47k resistor going between the top or high side of the pot and the junction of C3 and R4 where they meet the IF lead.
:
:It was probably a factory production line change part but I pulled it out anyway and made a direct connection from the high side of the pot to the junction of the IF lead and C3-R4.... as it's shown on the print.
:
:Then I tested everything again and found that about 80% of the hum was now gone.
:
:As my hand or fingers came anywhere near the body of the coupling cap C4 from the volume control, it seemed to increase the hum level again.
:
:So this time I re-wound another nice coil of insulated wire completely along the cap's body and a little down each lead and soldered BOTH ends of the coil wire shield to ground (B-).
:WELL... that finally did it... SUCCESS !!
:Virtually no hum now at all... not on the bench or even back in the cabinet.
:
:I feel so GOOD now because these are the most irritating kinds of problems and we rarely if ever REALLY get them solved.
:
:And this one is [b]solved[/b] ...beautifully.
:
:But it wasn't [b]just one[/b] single simple thing.. it was a combination of ALL those solutions/fixes.
: ... but at least I have the consolation of knowing that it wasn't an unknown strange kind of "magic" cure.
:
:Nope.. this one was fixed... step by step... but fixed for sure.
:
:Now [b]why[/b] was this a problem in the 1st place I ask myself?
:
:Myself answered ...and me thinks that the extra stage of audio amplification probably makes it all more susceptible to the slightest noise... since it's designed to really amplify more than a typical AA5 set.
: (I think.. that's why)
:
:Ahh.. let's see.. hummm 5:17 am?... ok
:Time to go to bed... lol
Terry F
Using a single point as ground reference.
http://svconline.com/news/avinstall_technical_grounding_theory/