Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jan Sat 16, 2021 1:11 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 11 posts ] 
Author Message
 Post subject: National RAO Hum
PostPosted: Nov Wed 18, 2020 7:43 am 
Member

Joined: Jan Thu 12, 2017 5:08 am
Posts: 36
Hi all.

I'm working on another RAO receiver, a National. It's been re-capped all resistors checked, tubes tested on my Hickok 800 tester. Weak tubes replaced and no heater to cathode shorts indicated.

The problem I'm having is eliminating a loud hum I hear when I use a pair of 600 ohm headphones in the headphone jack.
I get the same hum if I connect the headphones to the speaker terminals. I have already replaced the 4 Mfd paper oil filled caps with 10 Mfd electrolytics, but get the same thing to the same degree. The negative side of the power supply is not grounded to the chassis in this radio. It's floating. Filter chokes are not shorted or open.

It sounds like AC hum pickup from the filament string. None of the heater wires supplying the tubes are twisted together.

All chassis grounds are good, they were checked with ohm meter to the chassis. I'd like to get rid of the annoying hum.

Could it be the 1st audio amplifier picking up hum?
Any ideas?

Thanks

Mike


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Wed 18, 2020 8:03 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 1965
That is usually the source of hum pickup.

Hum is generally caused by one or more of three things.
1. Pickup - capacitive usually.
2. Poor power supply filtering.
3. Power line getting into the audio system.

My question is, what sort of hum? A whiny buzzing hum comes from pickup, capacitive. A very smooth clean hum, power line getting into the audio. A rougher sounding hum, no high frequencies, comes from poor filtering.

How this works is that capacitive coupling tends to favor the harmonics, the high frequency components. Direct coupling is clean sounding, no harmonics. Poor filtering allows harmonics from the rectifier to enter the audio system.


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Wed 18, 2020 4:08 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3659
Location: Seattle WA US
+1
Also, one problem unique to radios like the RAO that have chassis ground at a potential above B- : There are some filter capacitors with negative terminal at chassis ground, and some with negative terminal at true B- . If any of these get mixed up, which is easy if you replace more than one at a time or replace cans with radial capacitors, you get hum.

If this is your problem, voltages at the AGC amplifier tube will probably not be normal.

-Chuck K7MCG


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Thu 19, 2020 8:43 am 
Member

Joined: Jan Thu 12, 2017 5:08 am
Posts: 36
Hi Bob

The hum is an ac hum from the power line. It's just like when you place a screw driver on the grid of an audio amplifier tube and touch the screw driver with your finger, it gives an AC hum from the AC field. It's a low frequency hum.

I checked the filter chokes and they are not shorted and I also replaced the 10ufd caps I had in there with 22ufd just to see if it would make any difference. It was the same hum level.

I'm sure the radio should not be humming this loudly in the headphones, unless the phones used back then in WW2 did not have much bass response so maybe they would not pick up the hum.


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Fri 20, 2020 5:20 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 9804
Location: Beaver Falls, PA. USA
Look for any leakage from the AC line to the chassis.

_________________
Tim KA3JRT


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Fri 20, 2020 6:15 pm 
Member

Joined: Jan Tue 07, 2020 1:41 am
Posts: 3567
Location: Fenton, MI 48430
Longshot. If it has a two conductor 120 volt plug, try plugging into outlet 180 degrees switching neutral to hot.

_________________
NAVY JACK FLAG- 'Don't Tread on Me'


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Sun 22, 2020 9:19 am 
Member

Joined: Jan Thu 12, 2017 5:08 am
Posts: 36
As always, thank you to all who replied to my post.

I was able to almost eliminate the hum by placing an electrolytic capacitor of 10 uf from the B- to the the chassis. I guess this filters the B-. The only hum I hear now is the slight hum from the transformer to audio transformer coupling when the radio is in B+ off position. The cap has it's negative terminal to the B- and the positive to chassis.

If anyone has thoughts on this please let me know.

Thanks

Mike


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Sun 22, 2020 5:26 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3659
Location: Seattle WA US
Your added cap is in parallel with C196 and C197 - which clearly aren't doing their job. Suggest you find them and replace.
-Chuck K7MCG


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Sun 22, 2020 9:19 pm 
Member

Joined: Jan Thu 12, 2017 5:08 am
Posts: 36
Hi Chuck

Yes the hum would indicate a power supply filtering problem.
On my schematic, the filter caps in question are numbered c 153, 154, and 155.
These are oil filled paper at 4UF each. They are connected in the circuit by solder lugs
that are on threaded posts with nuts, as you probably know well.

I disconnected these, on both the B- and B+ sides, and soldered 10UF electrolytic caps onto the lugs
with the proper polarity observed. So the 4 UF caps were completely out of the circuit.
I got the same level of hum. I even used 22Uf caps and the same thing. Strange. I checked the DC resistance of the
chokes and they are in spec according to the manual, which is an original navy copy. The chokes are not
shorted to the case as I checked that with my wind up megger. hmmm, weird. I thought for sure the 10Uf caps would fix the issue. Maybe there's AC getting into the audio from some where?

I must be missing something somewhere.

Mike


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Sun 22, 2020 9:31 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3659
Location: Seattle WA US
When you understand the purpose of C196, and C197, which are between B- and chassis ground, you will understand why adding a capacitor between B- and chassis ground reduced your hum.
The filter caps between B+ and B- are only part of the filtering in your radio.
-Chuck


Top
 Profile  
 
 Post subject: Re: National RAO Hum
PostPosted: Nov Tue 24, 2020 8:39 am 
Member

Joined: Jan Thu 12, 2017 5:08 am
Posts: 36
Chuck

Thank you. That helps me quite a bit. That makes a lot of sense as to why there's a hum.
I will check those caps out.

Mike


Top
 Profile  
 
Post New Topic Post Reply  [ 11 posts ]  Moderator: Sandy

All times are UTC [ DST ]


Who is online

Users browsing this forum: joelandjoyce and 15 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB