Anyone want to try this one? I could use the help.

I placed a friend's local auction find on the bench as a favor: a near mint SX-110...just to string the dial, align, and pop in filter caps. Radio plays just great as long as you don't wear headphones or deal with low level signals. Replacing the filter caps only got rid of about 20% of the hum. It is present and at a constant level no matter what the settings of volume or sensitivity.

Here's what I've done:
Pulled tubes one at a time starting at the front end. Hum always there until I pulled the audio output tube.

Disconnected the coupling cap from first audio to audio out tube...no change.

Checked all tubes and then plugged in NEW rectifier and audio tubes. No change.

Checked grounding of electrolytics; checked electrolytics; moved electrolytic and tie points away from power transformer bell. No change.

One at a time, I parallelled up another 10mf to each of the three electrolytics. Very little change until I got to the last one: the one that goes on the "leg" headed up to a grid of the audio output tube. Big, big change! At least gets it down to a "normal" level like I am used to hearing on these old Hallicrafters superhets.

Any ideas? What am I missing here? Thanks in advance.

Jim

Oh, yes....and I also lifted the hot side of the filament string to listen as the filaments cooled. Hum stayed thru the fade out.

Jim

Hi,
You may have a tube with a "K" short, or there may be a loose element that is picking up a stray AC signal, or possibly even a tubular cap, or resistor have opened, and is acting as a floating ground. There is a slight chance too that the secondary of the last IF is open.

:
: Oh, yes....and I also lifted the hot side of the filament string to listen as the filaments cooled. Hum stayed thru the fade out.

: Jim

It sounds from your comments that you're familiar with these sets so this may not apply but you're listening to the hum on headphones. Are they modern HiFi type phones? perhaps they have very good bass response and you're hearing the residual hum that's normal for these sets and not audible on a small speaker or telephone type headphones.
Don Black.

: Hi,
: You may have a tube with a "K" short, or there may be a loose element that is picking up a stray AC signal, or possibly even a tubular cap, or resistor have opened, and is acting as a floating ground. There is a slight chance too that the secondary of the last IF is open.

:
: :
: : Oh, yes....and I also lifted the hot side of the filament string to listen as the filaments cooled. Hum stayed thru the fade out.

: : Jim

Hey, guys! Thanks for the input. Very perceptive Don! Yes, I listen on a great big monster pair of 30+ year old super-cushioned hi-fi Koss phones. I use this pair all the time...on everything, (and it's all old stuff)so I expect some residual hum. The shack speaker is a big Hallicrafters R46B. What I am trying to do is get the hum down to what I(rightly or wrongly)call the "usual Hallicrafters hum level".

Norm, I re-checked the grounds on the electrolyics again. They are good. I originally had them soldered to one of the twist-lock pins on the original three section can that was already well soldered to the chassis, and just happened to have a mounting hole available on the tip. So, for now I've gone with the "patch" of an extra 10uf on the screen of the audio out tube. This works. I'm going to go ahead with alignment and let it "cook" for a few days. Maybe another clue will turn up. At least I'll get a change of scene.

John, a tube with a "K short". Would that be a cathode-heater short? I would think any heater problems would show themselves when the voltage is cut. I've done that to no avail. I don't think the IF is open: the radio works too darn good. An open last IF would really stick out, wouldn't it? I assume any resistor/capacitor problems would be confined to the area of the audio out...as the problem stays as all tubes are pulled, until I pull the Audio out tube. Is this sound thinking? Checked the audio section resistors and they are all very close. Replaced several tubular caps going to ground. What do you think, John?

Hi,

While such an occurance would be rare, if the secondary winding were burned out near the middle, you would still have a capacitive coupling effect between stages, and it would be one possible source for noise. Based on the level of the noise, it should be confined to the audio, back to second detector.

I have had to deal with strange hum sources on an SX-42, and an SX-99, and they can be very tough to isolate. In both cases, the problems were not with a single component either.

Some other things to try would be to make sure that the rivits that hold the tube solckets in place are soldered to the chassis itself (one is sufficient), as well as any terminal strips that use one terminal as a ground to the chassis via the support mounting hole. I have actually had tubular capacitors pick up hum, and those have shown up by striking the side of the capacitor with a screwdriver.

In any event, it sounds like something is acting like an antenna to pick up the hum. Much like using "noise injection"; which might be worth a try too. Turn the set on, and place a screwdriver on various points that should create noise within the audio stages, and look for the spot where there is no change, or minimal change, and check for bad components in that immediate area, or to see if anyone had done any work prior to your acquiring the radio.

If you need a copy of the operating manual, or schematic, check here:

ftp://bama.sbc.edu/downloads/hallicra/sx110/

http://bama.sbc.edu/hallicra.htm

: Hey, guys! Thanks for the input. Very perceptive Don! Yes, I listen on a great big monster pair of 30+ year old super-cushioned hi-fi Koss phones. I use this pair all the time...on everything, (and it's all old stuff)so I expect some residual hum. The shack speaker is a big Hallicrafters R46B. What I am trying to do is get the hum down to what I(rightly or wrongly)call the "usual Hallicrafters hum level".

: Norm, I re-checked the grounds on the electrolyics again. They are good. I originally had them soldered to one of the twist-lock pins on the original three section can that was already well soldered to the chassis, and just happened to have a mounting hole available on the tip. So, for now I've gone with the "patch" of an extra 10uf on the screen of the audio out tube. This works. I'm going to go ahead with alignment and let it "cook" for a few days. Maybe another clue will turn up. At least I'll get a change of scene.

: John, a tube with a "K short". Would that be a cathode-heater short? I would think any heater problems would show themselves when the voltage is cut. I've done that to no avail. I don't think the IF is open: the radio works too darn good. An open last IF would really stick out, wouldn't it? I assume any resistor/capacitor problems would be confined to the area of the audio out...as the problem stays as all tubes are pulled, until I pull the Audio out tube. Is this sound thinking? Checked the audio section resistors and they are all very close. Replaced several tubular caps going to ground. What do you think, John?

Hi Jim

The hum is still in your power supply section. Was negative ends of filter caps connected to the same place as originals? Even a ground point a few inches away can cause hum.

It's ok to add filter capacity to screen grid of the audio output tube.

Norm

: Anyone want to try this one? I could use the help.

: I placed a friend's local auction find on the bench as a favor: a near mint SX-110...just to string the dial, align, and pop in filter caps. Radio plays just great as long as you don't wear headphones or deal with low level signals. Replacing the filter caps only got rid of about 20% of the hum. It is present and at a constant level no matter what the settings of volume or sensitivity.

: Here's what I've done:
: Pulled tubes one at a time starting at the front end. Hum always there until I pulled the audio output tube.

: Disconnected the coupling cap from first audio to audio out tube...no change.

: Checked all tubes and then plugged in NEW rectifier and audio tubes. No change.

: Checked grounding of electrolytics; checked electrolytics; moved electrolytic and tie points away from power transformer bell. No change.

: One at a time, I parallelled up another 10mf to each of the three electrolytics. Very little change until I got to the last one: the one that goes on the "leg" headed up to a grid of the audio output tube. Big, big change! At least gets it down to a "normal" level like I am used to hearing on these old Hallicrafters superhets.

: Any ideas? What am I missing here? Thanks in advance.

: Jim

: Anyone want to try this one? I could use the help.
If all else fails, why not test? With the volume turned down completely, put your scope on the screen grid of the output tube and see what your B+ looks like there. You should have below 1 volt p-p ripple. If your ripple is higher, then your power supply is certainly the culprit and not the signal chain. Also, scope the other main B+ points to see how much ripple is hanging around post filtering. The fact that your addition of capacitance affected the hum leads me to believe there is ripple here. Your new filters may exhibit a high power factor even though they are new units. If you do not have a scope, a good AC meter can be used but is not as effective. Good Luck
: I placed a friend's local auction find on the bench as a favor: a near mint SX-110...just to string the dial, align, and pop in filter caps. Radio plays just great as long as you don't wear headphones or deal with low level signals. Replacing the filter caps only got rid of about 20% of the hum. It is present and at a constant level no matter what the settings of volume or sensitivity.

: Here's what I've done:
: Pulled tubes one at a time starting at the front end. Hum always there until I pulled the audio output tube.

: Disconnected the coupling cap from first audio to audio out tube...no change.

: Checked all tubes and then plugged in NEW rectifier and audio tubes. No change.

: Checked grounding of electrolytics; checked electrolytics; moved electrolytic and tie points away from power transformer bell. No change.

: One at a time, I parallelled up another 10mf to each of the three electrolytics. Very little change until I got to the last one: the one that goes on the "leg" headed up to a grid of the audio output tube. Big, big change! At least gets it down to a "normal" level like I am used to hearing on these old Hallicrafters superhets.

: Any ideas? What am I missing here? Thanks in advance.

: Jim