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

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.

+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

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.

Look for any leakage from the AC line to the chassis.

Longshot. If it has a two conductor 120 volt plug, try plugging into outlet 180 degrees switching neutral to hot.

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

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

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

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

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