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


It is currently Jul Sat 11, 2020 1:01 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 64 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Sun 29, 2019 2:35 pm 
Member
User avatar

Joined: May Sun 13, 2012 8:12 pm
Posts: 13923
Location: Central PA 16801
for the 120s that i have serviced, i was never impressed with them, especially in sensitivity.

they were a basic entry level set that probably disappointed people in SW, LOL.

steve

_________________
You have enemies ? Good. That means you've stood up for something, sometime in your life.
-Churchill


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Sun 29, 2019 3:35 pm 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
Quote:
I had a chance to work on the poor performing (AM Band) S-120 again last night. First I did check and compare the ferrite rod antenna to the better performing S-120. Resistance readings with the antenna disconnected are the same. About 1.2 ohms from green to orange, 1.0 ohms from green to black, and .2 ohms from black to orange.
The schematic posted earlier in this thread didn’t show wire colors. But you could compare the wiring between the two S-120 radios that you have.

Comparing your resistance measurements to the schematic suggests the following is the correct way to connect the ferrite rod:
    Black wire to chassis ground
    Green wire to trimmer capacitor C1 and terminal 8 of S1B (rear) wafer of band switch
    Orange wire to terminal 3 of S1B (front) wafer of band switch.

Trimmer C1 should have a strong effect for peaking the AM broadcast band sensitivity at the high end of the tuning dial (1400-1600kHz). But if trimmer C1 has little or no effect, then there is still a problem somewhere.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Sun 29, 2019 7:21 pm 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
Dutch Rabbit wrote:
for the 120s that i have serviced, i was never impressed with them, especially in sensitivity. they were a basic entry level set that probably disappointed people in SW, LOL.steve
What would be your ranking of all of the Hallicrafters general coverage receivers? Those that include AM broadcast band? Both older and newer models.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Sun 29, 2019 9:22 pm 
Member
User avatar

Joined: May Sun 13, 2012 8:12 pm
Posts: 13923
Location: Central PA 16801
i can't answer that.

the s120 seemed to be the only model that ever came into the shop and i was never impressed. i don't have any experience with the bigger hallicrafters sets.

to me, the 120 just had a basic setup with a basic tube lineup and their performance indicated that.

i serviced the cousin of the 120 once. this model had a wood grain case, a black dial, with white numbers that was beautiul when lit up. that radio was more silent than the 120. it was horrible, but there was nothing wrong with it. it was even a more basic and cheap setup than the 120.

i have more of a liking to National, especially the big boat anchors.

steve


Attachments:
National%20NC-173%20(2).JPG
National%20NC-173%20(2).JPG [ 82.12 KiB | Viewed 1137 times ]

_________________
You have enemies ? Good. That means you've stood up for something, sometime in your life.
-Churchill
Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Mon 30, 2019 2:58 pm 
Member

Joined: Sep Thu 26, 2019 1:56 am
Posts: 194
electricboyo wrote:
Quote:
I had a chance to work on the poor performing (AM Band) S-120 again last night. First I did check and compare the ferrite rod antenna to the better performing S-120. Resistance readings with the antenna disconnected are the same. About 1.2 ohms from green to orange, 1.0 ohms from green to black, and .2 ohms from black to orange.
The schematic posted earlier in this thread didn’t show wire colors. But you could compare the wiring between the two S-120 radios that you have.

Comparing your resistance measurements to the schematic suggests the following is the correct way to connect the ferrite rod:
    Black wire to chassis ground
    Green wire to trimmer capacitor C1 and terminal 8 of S1B (rear) wafer of band switch
    Orange wire to terminal 3 of S1B (front) wafer of band switch.

Trimmer C1 should have a strong effect for peaking the AM broadcast band sensitivity at the high end of the tuning dial (1400-1600kHz). But if trimmer C1 has little or no effect, then there is still a problem somewhere.

-EB


I agree and that is how my ferrite rod antenna is connected. Yes, C1 has a lot of effect when adjusting. I went through alignment again (actually several times) for the AM broadcast band and now have it working about as well as my other S-120.

Not being a fan of talk radio, it's been years since I really tried to pull in AM broadcast stations. I notice our newer clock radio doesn't work any better on AM, and neither does my weather radio. Maybe it's just where we live here in New Hampshire. We moved here from the midwest 3 years ago. Years ago, at least in the midwest, we could get tons of AM stations really well.

_________________
KC1LML
Peterborough, New Hampshire USA


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Mon 30, 2019 3:03 pm 
Member

Joined: Sep Thu 26, 2019 1:56 am
Posts: 194
Dutch Rabbit wrote:
i can't answer that.

the s120 seemed to be the only model that ever came into the shop and i was never impressed. i don't have any experience with the bigger hallicrafters sets.

to me, the 120 just had a basic setup with a basic tube lineup and their performance indicated that.

i serviced the cousin of the 120 once. this model had a wood grain case, a black dial, with white numbers that was beautiul when lit up. that radio was more silent than the 120. it was horrible, but there was nothing wrong with it. it was even a more basic and cheap setup than the 120.

i have more of a liking to National, especially the big boat anchors.

steve


Understandably, S-120 radios get no respect... LOL. Call me nuts, but for me, it's more fun trying to get a poor performer working better than say a high-end radio.

_________________
KC1LML
Peterborough, New Hampshire USA


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Mon 30, 2019 4:28 pm 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
Quote:
I agree and that is how my ferrite rod antenna is connected. Yes, C1 has a lot of effect when adjusting. I went through alignment again (actually several times) for the AM broadcast band and now have it working about as well as my other S-120. Not being a fan of talk radio, it's been years since I really tried to pull in AM broadcast stations. I notice our newer clock radio doesn't work any better on AM, and neither does my weather radio. Maybe it's just where we live here in New Hampshire. We moved here from the midwest 3 years ago. Years ago, at least in the midwest, we could get tons of AM stations really well.
Could your AM reception be weak because of localized RF interference?

One thing I've been tracking down lately is learning how to suppress all the sources of AM broadcast band radio interference that exist inside my house.

Listening to AM DX at night requires turning off certain appliances and lamps in my house. Also my radios perform differently in different rooms of the house.

If the radio is working well and properly aligned, and local RF interference has been controlled well enough, one thing stands out:

    At night the "background noise" on AM broadcast band should consist of ocean-wave sounding white noise in between stations. There should not be a lot of 60 or 120Hz buzzing noise. Of course there will be intermittent pops and cracks if there is thunderstorm activity in the region. For non-clear-channel frequencies, there will be fading in and out and sometimes 2 or more stations will be heard at the same time. But there should be something there. Not dead silence.

With my more sensitive radios I can often hear at least occasional audio at nearly every 10kHz licensed frequency all across the AM broadcast band. It helps to have the list of 50kW clear-channel AM broadcast stations on hand while doing this.

As far as local RF interference is concerned, the best investigative tool is a traditional handheld transistor radio with "analog" (knob) tuning. As you take it closer to the room or source of the interference, then you can switch off items in that room until you locate the source. Most local interference sources have a 60 or 120Hz component, along with some swishing or whooshing sounds. These whooshing sounds often come from DC-DC switching power supplies that aren't well shielded or filtered internally. The handheld transistor radio will also help identify whether any other nearby wireless or electronic devices, such as BT, WiFi, mobile phone, tablet, laptop, video monitor, are too close to the AM radio. Most local RF interference sources are somewhat "broad band" in nature. By this I mean to say that their noise pollution is heard over a range of frequencies; not necessarily all the way across the entire AM broadcast band, but typically the noise will be heard over a range of 100kHz or more on the dial (say between 1000 and 1100).

So far all of my AM DXing is only with the internal ferrite rod or wire loop antenna inside my radios. I haven't yet decided which type of internal antenna works better (ferrite rod vs. standard back-panel wire loop antenna). I haven't yet experimented with accessory tunable loop AM broadcast band antennas either, but I plan to do so very soon. I'm in the process of putting about 10 more vintage radios into operation this month. One of those is an S-120; it appears to be in undisturbed original factory condition. I know that I will recap it and replace the selenium rectifier before I try listening to it. It also needs a new internal speaker (old cone is totally ripped to shreds). I have a replacement speaker already.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Last edited by electricboyo on Oct Thu 10, 2019 3:13 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Mon 30, 2019 4:35 pm 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
Quote:
Quote:
Trimmer C1 should have a strong effect for peaking the AM broadcast band sensitivity at the high end of the tuning dial (1400-1600kHz). But if trimmer C1 has little or no effect, then there is still a problem somewhere. -EB
I agree and that is how my ferrite rod antenna is connected. Yes, C1 has a lot of effect when adjusting. I went through alignment again (actually several times) for the AM broadcast band and now have it working about as well as my other S-120.

It's great to hear that C1 has a big effect. This proves the front end L-C tuned circuit (loop antenna and main tuning capacitor) can be resonated.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Mon 30, 2019 8:50 pm 
Member

Joined: Sep Thu 26, 2019 1:56 am
Posts: 194
Ah, yes, I did more than my share of AM Dxing back in the early 1960s. Now that I’m retired, I plan to give it another go, if I can get a decent AM broadcast band radio working. I do have a couple of 1930s era radios that work pretty well, at least on stronger AM stations.

Funny you should mention RF noise. I do have a lot of it at my house, although it's not coming from inside my house. A couple of weeks ago I did some investigating and turned off the main breaker at my AC power panel. It didn't reduce the RF noise at all. The noise is pretty much all up and down the AM broadcast band but not in the shortwave bands.

I got in the car and drove around the neighborhood. The epicenter seemed to be on the next block over from me. I suspect there’s a bad transformer somewhere on that street. I did call the power company who surprisingly has a process for radio/TV interference. They said they would send me a package I would need to fill out and return.

Despite all that, I was thinking I still had an issue in the S-120, because my car radio can override the neighborhood RF noise when I tune in a strong station. The S-120 doesn’t and seems to mix in the noise with the station. My weather radio on AM doesn’t work any better than the S-120. My 1930 AM radios (an RCA Victor and a B.R. Co.) sort of override the noise on a strong station but not entirely.

I suppose I should hold off doing anything else to the S-120 until the power company resolves this interference issue.

_________________
KC1LML
Peterborough, New Hampshire USA


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Mon 30, 2019 9:40 pm 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
I often use a battery-powered transistor radio when searching for AC power line related noise.
It is certainly possible for bad utility company equipment to generate RF noise. Especially on the AM broadcast band. The utility companies are usually cooperative with follow-up when you report this to them. Fixing the issue might save them from a future power outage in the neighborhood.

I continue to be amazed at the performance of car radios, both old and new.
    My wife drives a 2013 Nissan Juke with the premium-grade factory audio system. Performance on both AM and FM is terrific.
    I myself have a 2005 Toyota Prius. Unfortunately the hybrid drive system inverter does generate noise and hash that is easily heard alongside weak AM stations. The Prius converts a 240V DC input into 0-240V variable frequency AC to drive the electric propulsion motor/generators (there are 2 of these). It's not surprising that RF noise is generated considering the power level: The maximum current to/from the 240V DC battery pack can reach 100A. That's >20kW.

Getting one of those tunable AM loop antenna accessories might be helpful.

Another line of thought:
    How well does your ham-band reception work?
    What type of ham antennas do you have and what bands do you work most often?
    Is your house in a valley or on top of a hill?
    I think there are different types of RF interference noise once you get to frequencies at 80 meters and beyond on the ham bands. The higher the frequency, the lower the noise.
    I don't know whether you do anything with 160 meters or not, but I would expect its performance to be similar to AM broadcast band.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Last edited by electricboyo on Oct Mon 28, 2019 3:53 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Sep Mon 30, 2019 9:50 pm 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
Dutch Rabbit wrote:
i can't answer that. the s120 seemed to be the only model that ever came into the shop and i was never impressed. i don't have any experience with the bigger hallicrafters sets.to me, the 120 just had a basic setup with a basic tube lineup and their performance indicated that.i serviced the cousin of the 120 once. this model had a wood grain case, a black dial, with white numbers that was beautiul when lit up. that radio was more silent than the 120. it was horrible, but there was nothing wrong with it. it was even a more basic and cheap setup than the 120.

i have more of a liking to National, especially the big boat anchors.
steve
That NC-173 looks gorgeous.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Tue 01, 2019 1:28 am 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
Tbone wrote:
EB that’s a good idea for testing. On other AC DC sets with a rectifier tube, one could substitute in a 1N4007 diode to accomplish the same thing. The rectifier filament is out of the circuit so tubes are dark.
I have a variable 0-200V DC power supply on my workbench. This works very well as an alternate source of B+ voltage for detecting DC leakage paths through IF transformers and other components too.

I connect the + output of the DC power supply through a mA meter to the + terminal of the first electrolytic filter capacitor. This is usually the same as the cathode pin of the rectifier tube. The - output of the DC power supply connects to the common - terminal of the electrolytic filter capacitors in the radio under test.

The radio under test is NOT receiving AC power at this time. Therefore the tubes aren’t receiving filament power. In other words the tubes are dark and cold. This means the tubes can be kept in their sockets. No need to pull any out. UPDATE: After testing several radios I discovered that dark/cold tubes can have intermittent shorts/leakage that behave just like SMD in IF cans. So I am changing this instruction to: "Remove ALL tubes from radio before making this test."

Most AA5 radios will have <500uA of B+ current during this test. This should drop as the B+ filter capacitors “re-form” themselves. For a radio that has already been recapped the B+ current may gradually drop to <100uA.

I plan to test this on several radios and will report the results. This might be the start of a whole new method to painlessly evaluate vintage radios for SMD and for other DC leakage issues too.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Last edited by electricboyo on Oct Mon 28, 2019 4:02 pm, edited 2 times in total.

Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Tue 01, 2019 2:02 am 
Member
User avatar

Joined: May Sun 13, 2012 8:12 pm
Posts: 13923
Location: Central PA 16801
electricboyo wrote:
Dutch Rabbit wrote:
i can't answer that. the s120 seemed to be the only model that ever came into the shop and i was never impressed. i don't have any experience with the bigger hallicrafters sets.to me, the 120 just had a basic setup with a basic tube lineup and their performance indicated that.i serviced the cousin of the 120 once. this model had a wood grain case, a black dial, with white numbers that was beautiul when lit up. that radio was more silent than the 120. it was horrible, but there was nothing wrong with it. it was even a more basic and cheap setup than the 120.

i have more of a liking to National, especially the big boat anchors.
steve
That NC-173 looks gorgeous.

-EB

thank you.

i fully restored it about 15 years ago it is very minty.

steve

_________________
You have enemies ? Good. That means you've stood up for something, sometime in your life.
-Churchill


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Tue 01, 2019 2:09 am 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
Dutch Rabbit wrote:
thank you.
i fully restored it about 15 years ago it is very minty.
steve
That National NC-173 is a “keeper” for sure.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Tue 01, 2019 2:26 am 
Member
User avatar

Joined: May Sun 13, 2012 8:12 pm
Posts: 13923
Location: Central PA 16801
yes, it isn't going anywhere, LOL.

it aligned and performs perfectly.

i believe i paid around $250 for it in richmond, va when i lived down there around y2k.

it was untouched.

steve

_________________
You have enemies ? Good. That means you've stood up for something, sometime in your life.
-Churchill


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Tue 01, 2019 3:28 am 
Member

Joined: Sep Thu 26, 2019 1:56 am
Posts: 194
electricboyo wrote:
Another line of thought:
    How well does your ham-band reception work?
    What type of ham antennas do you have and what bands do you work most often?
    Is your house in a valley or on top of a hill?
    I think there are different types of RF interference noise once you get to frequencies at 80 meters and beyond on the ham bands. The higher the frequency, the lower the noise.
    I don't know whether you do anything with 160 meters or not, but I would expect its performance to be similar to AM broadcast band.

-EB


I haven't done anything with the HF ham bands, other than listen in a little every now and then. I have a technician's license which allows me to transmit on VHF (2 meters) and UHF (70cm). As far as reception in the HF area, I'd say 41 meters through 16 meters seems normal. That's using the two Hallicrafters S-120s with just a 25' piece of wire. I've thought about putting up a long wire in my backyard, but suspect it will just pick up more interference.

My 2 meter/70 cm antenna is up on my roof. Coverage is about what it should be. I have access to 3 repeaters on nearby mountain tops that lets me get out farther. My house is in a valley about half-way up one side. If it wasn't for those repeaters, I'd be limited to local around town ham radio communications. I have a dual-band base station and two hand-helds. Most people around here use these bands for emergency type communications. It's pretty quiet most of the time. Of course, that's not true in the larger cities.

You're correct, the RF interference does extend almost up to 160 meters, but past that, it's quiet. None of this RF noise is a problem on FM, shortwave, or my ham radios. In fact, I didn't even realize it was that bad until I started working on this S-120.

I was reading an article some guy wrote after a power outage. He impressed his kids by digging out his old AM radio, putting in some fresh batteries, and pulling in stations crystal clear from hundreds of miles away. He said his kids never did quite understand how he could do that since the Internet was down. But he was shocked to discover he could hardly get anything on AM as soon as the power was restored.

On another note, I pulled out my 1939 RCA Victor tonight and gave it a try. It picks up some RF noise, not quite as much as the S-120. But within a half-hour, I had logged five 50,000 watt stations out of NY. So, that tells me the S-120 still has a problem. Yet, it doesn't seem too bad on shortwave.

_________________
KC1LML
Peterborough, New Hampshire USA


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Tue 01, 2019 3:21 pm 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
moallen wrote:
On another note, I pulled out my 1939 RCA Victor tonight and gave it a try. It picks up some RF noise, not quite as much as the S-120. But within a half-hour, I had logged five 50,000 watt stations out of NY. So, that tells me the S-120 still has a problem. Yet, it doesn't seem too bad on shortwave.
What model is your RCA radio?

By the year 1939, radio technology, especially for the AM broadcast band, had developed to a level that could provide excellent AM DX performance.

I also think that the S-120 might have inferior AM broadcast band performance because it is “trying” to be a full range communications receiver. But the S-120 is still based on the standard AA5 circuit design: There is no tuned RF stage, and only one IF stage.

Some of my favorite AM DX radios are the 1950’s era Zenith AM/FM radios that have a 3 section tuning capacitor with tuned AM RF stage and two IF stages. These Zenith radios also appear to have AM band IF transformers that perform better than most. That said, those Zenith IF transformers do have the unfortunate tendency to develop SMD. I collect 1950’s 1960’s radios, most of which have slug-tuned IF transformers. So I always watch for SMD, but luckily haven’t encountered it recently.

As you suggested earlier, it would be an entertaining project to take the S-120 and find a way to wring out “maximum” AM broadcast band performance from it.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Tue 01, 2019 5:10 pm 
Member

Joined: Sep Thu 26, 2019 1:56 am
Posts: 194
electricboyo wrote:
moallen wrote:
On another note, I pulled out my 1939 RCA Victor tonight and gave it a try. It picks up some RF noise, not quite as much as the S-120. But within a half-hour, I had logged five 50,000 watt stations out of NY. So, that tells me the S-120 still has a problem. Yet, it doesn't seem too bad on shortwave.
What model is your RCA radio?

By the year 1939, radio technology, especially for the AM broadcast band, had developed to a level that could provide excellent AM DX performance.

I also think that the S-120 might have inferior AM broadcast band performance because it is “trying” to be a full range communications receiver. But the S-120 is still based on the standard AA5 circuit design: There is no tuned RF stage, and only one IF stage.

Some of my favorite AM DX radios are the 1950’s era Zenith AM/FM radios that have a 3 section tuning capacitor with tuned AM RF stage and two IF stages. These Zenith radios also appear to have AM band IF transformers that perform better than most. That said, those Zenith IF transformers do have the unfortunate tendency to develop SMD. I collect 1950’s 1960’s radios, most of which have slug-tuned IF transformers. So I always watch for SMD, but luckily haven’t encountered it recently.

As you suggested earlier, it would be an entertaining project to take the S-120 and find a way to wring out “maximum” AM broadcast band performance from it.

-EB


I’ve been trying to figure out why my RCA 45X13 (AM only) works so much better on the standard AM broadcast band than either of my Hallicrafters S-120s, or my weather radio for that matter.

For one thing, the RCA has a simpler RF front-end and a loop antenna. Both are AC/DC, although the RCA has a 35Z5GT tube instead of a selenium rectifer.

Grounding is different between the two radios. The RCA is from the era when which way the AC plug was inserted in the wall socket made a big difference in reception. The S-120, although not polarized, makes no difference in reception.

The way the RCA works best is with neutral going through the on/off switch to chassis ground. The only ground in the RCA is the chassis ground. B- is also connected to the chassis. So, with the AC plug inserted this way, the hot AC wire goes through the tube filaments and is then wired to the chassis.

Regardless of how the AC plug is inserted in the wall outlet, this arrangement is obviously a hazard, but hey, so are a lot of things in life. Personally, I’m ok with it.

Getting back to the S-120, it has both a chassis ground, as well as a B- ground. They are connected to each other through a 470 K ohm and .0047 uf capacitor in parallel. Depending on how the AC plug is inserted, B- ground will be either hot or neutral. Its metal cabinet is isolated from the chassis with plastic insulators. This is a little safer than the RCA but not idiot-proof. But I don’t see a reason to install a polarized plug unless the line cord is frayed.

In any case, I wonder if this has something to do with why the S-120 (AM band) is bothered so much by RF interference, or why reception makes no difference which way the AC plug is inserted.

_________________
KC1LML
Peterborough, New Hampshire USA


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Tue 01, 2019 5:16 pm 
Member

Joined: Sep Thu 26, 2019 1:56 am
Posts: 194
electricboyo wrote:
Tbone wrote:
EB that’s a good idea for testing. On other AC DC sets with a rectifier tube, one could substitute in a 1N4007 diode to accomplish the same thing. The rectifier filament is out of the circuit so tubes are dark.
I have a variable DC power supply on my workbench that can cover 0-200V. This works well as a source of B+ voltage for detecting DC leakage paths through IF transformers and other components too.

I connect the + output of the DC power supply to the + terminal of the first electrolytic filter capacitor. This is usually the same as the cathode pin of the rectifier tube. The - output of the DC power supply connects to the common - terminal of the electrolytic filter capacitors in the radio under test.

The radio under test is NOT receiving AC power at this time. Therefore the tubes aren’t receiving filament power. In other words the tubes are dark and cold. This means the tubes can be kept in their sockets. No need to pull any out.

Most AA5 radios should have <500uA of B+ current during this test. This should drop even lower as the B+ filter capacitors “re-form” themselves. For a radio that has already been recapped, the B+ current may get as low as just a few uA.

I plan to test this on several radios and will report the results. This might be the start of a whole new method to painlessly evaluate vintage radios for SMD and for other DC leakage issues too.

-EB


I'm sold on the idea. It saved me from having to drill out the grommets holding them together, since I didn't measure any leakage. I just pulled the 50C5 tube and used the radio's B+, as you suggested. But I have a variac that I could use with a bridge and set of filters.

_________________
KC1LML
Peterborough, New Hampshire USA


Top
 Profile  
 
 Post subject: Re: Hallicrafters S-120 has no stations, just static
PostPosted: Oct Thu 03, 2019 3:23 am 
Member
User avatar

Joined: Sep Wed 10, 2014 7:13 pm
Posts: 1169
Location: Urbana, Illinois
electricboyo wrote:
I have a variable DC power supply on my workbench that can cover 0-200V. This works well as a source of B+ voltage for detecting DC leakage paths through IF transformers and other components too. I connect the + output of the DC power supply to the + terminal of the first electrolytic filter capacitor. This is usually the same as the cathode pin of the rectifier tube. The - output of the DC power supply connects to the common - terminal of the electrolytic filter capacitors in the radio under test.The radio under test is NOT receiving AC power at this time. Therefore the tubes aren’t receiving filament power. In other words the tubes are dark and cold. This means the tubes can be kept in their sockets. No need to pull any out. Most AA5 radios should have <500uA of B+ current during this test. This should drop even lower as the B+ filter capacitors “re-form” themselves. For a radio that has already been recapped, the B+ current may get as low as just a few uA.

I plan to test this on several radios and will report the results. This might be the start of a whole new method to painlessly evaluate vintage radios for SMD and for other DC leakage issues too.
-EB
Update: Today I'm using this test method on a Zenith H845 ch 8H20.

I've had this radio for a while but haven't restored it yet. It plays weakly on AM. FM is fine. I do recall hearing a burst or two of static that might have been SMD (silver mica disease).This radio contains several wax paper capacitors that I will replace. It still has the original selenium rectifier. I will be changing that too.

This Zenith H845 ch 8H20 is a great candidate to demonstrate the diagnostic method described in the above quote. This radio contains a total of 7 IF transformers (3 for AM, 4 for FM). A lot of time will be saved if it turns out that only 1 or 2 of them need to be rebuilt. I decided to perform the tests described in this thread before changing any components inside this radio.

Initially the "DC leakage current" was about 800uA ( 0.8mA). That's a bit higher than the 500uA "go/no-go" level. I also observed that the value goes up and down over time. This is a sign of something intermittent, such as SMD.

Bingo! The final AM IF transformer that feeds the AM detector is CONFIRMED to have SMD. The voltage on its secondary varies between 250mV and occasionally rises to >5V. But this measurement should be 0.0V.

I added one more item to the test rig:
    I connected my Heathkit IT-12 signal tracer to the secondary of the final AM IF transformer. Guess what? Every few minutes there is a tell-tale burst of SMD noise. At the same time, the DC voltage at this point goes up. While writing this post I am hearing bursts of SMD noise about every 5 or 10 minutes. The noise continues for a few seconds up to a minute, and then goes away. It is quite definitely intermittent, as expected. That's the way SMD is.

So now I know that I will need to rebuild this IF transformer.
    Fortunately this radio contains the "large format" Zenith IF transformers which are attached to the radio chassis with bolts and nuts. These IF cans are relatively easy to take apart. I've done this several times in the past with complete success. They contain a large circular mica sheet which has all of the internal capacitors "printed" on it. Previously I cleaned off the silver tarnish deposits from these large mica sheets and put them back together. That was sufficient to restore perfect operation. I plan to try that same "simple" repair this time too.

I also identified DC leakage in the wax paper coupling capacitor that goes from the plate of the 19T8 to the control grid of the 35C5. The voltage at the 35C5 control grid measures about 650mV and is steady. This represents DC leakage in the coupling capacitor ( .022uF ). I already planned to replace this capacitor. The measurement confirms that it really does need to be replaced.

I intend to work through this radio step-by-step, measuring the total DC current on the B+ rail (with radio otherwise unpowered). I'm expecting to see the value drop below 500uA after I identify and repair items like leaky wax paper capacitors and IF transformers with SMD.

-EB

_________________
I’m learning to be a leading-edge designer of trailing-edge technology.


Last edited by electricboyo on Oct Tue 08, 2019 12:10 am, edited 2 times in total.

Top
 Profile  
 
Post New Topic Post Reply  [ 64 posts ]  Moderator: Sandy Go to page Previous  1, 2, 3, 4  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 8 guests



Search for:
Jump to:  


































-->


Privacy Policy :: Powered by phpBB