need help
 

I am helping a friend with a Stromberg Carlson radio. I had a hard time finding a diagram for it. I found one 8 years ago at my library in one of two green books with diagrams in it. fixed the set and it was used regularly. Well it quit recently. He lost the diagram for it and I have no idea what model it is. The library sold the books since then.

It is a trf set and the #27 tubes are not getting b+ voltage. but there are two many questions for me to just dive in. As an ex. there is a pretty big can that has 8 pins and 6 are disconnected and have electro. caps hooked to them going to ground. What are the other two pins for? one has 187 volts and the other is dead. That and there is two small boxes that I don't know what is in them but I didn't touch them to begin with for a reason. Darned if I remember now

Any help is greatly appreciated... This set was purchased according to my friend the day his dad was born and has been in his family since 1928 so he really wants it running. He says he misses playing it.

Tubes are; 80,71a and 27's The 80 is top right and you really can't see it. the tubes in a line and one to the left closer to the front are 27's and the left back is the 71a

The metal tag on the back has a " piece" number of 17897 but no model number.just the usual info.

Pictures too dark to see anything.

The 27s get their B+ through the coils so see if a coil has opened. You may have to open the can.

coils checked as far as having resistance. The dead pin in the box I am thinking is the filter choke. I went through beitmans diagrams and I pulled up one that looks the same electrically from what I looked at today and if I am correct its the filter choke thats out. It had 187 volts on one side and nothing on the other. But at this stage I am just guessing. I will need to recheck it and in the meantime I am hoping that someone will recognize this radio.

If that is true, an open choke isn't a big deal. First make sure there is no short (to ground) on the side with zero potential. Sometimes you can repair a choke if you can get it open; the problem is most often at the end of the winding or the actual terminal.

Failing that, you can replace it easily and tuck the replacement into an inconspicuous place.

well Its a Model 636 I did find it. ok, leads to a question, what value caps should I put in in parallel with the audio filter assembly? There is not a value on the diagram and the ones in the set are shorted.

That is very strange. I would suggest you don't need a filter at all, just connect the load to the capacitor. It may have been a 10 kHz beat filter, something unnecessary due to FCC's channel spacing system.

Otherwise you can calculate the filter based on whatever characteristics you deem it should have.

I am learning as I go along (zero electronics education ) so I would have no idea how to calculate the filter needed if I wanted too. But you are saying that it should not need the audio filter that looks like a choke?

I think it should work fine without the filter. If you want to compute a filter, there are online setups that allow you to enter the impedance, cutoff frequency, and such, and will tell you inductance and capacitance needed. But I don't think you need it.

However, I see the audio output is coupled through a 2 microfarad capacitor. Knowing that the 71A can only put out probably less than a Watt, you need to come up with the optimum load (speaker?) impedance for this stage. The lazy man's way would be to allow the capacitor to make a filter with the load such that the corner frequency would be, say, 100 Hz or even 50 Hz. So you calculate the reactance of the 2 microfarad capacitor at that frequency and that will tell you how many Ohms speaker to use.

Let me see, my memorized rule is that 1 microfarad at 1 kHz is 159 Ohms. So 2 microfarads would be 80 Ohms. But it needs to be at least one tenth that frequency, making for a load impedance on the order of 800 Ohms. I would suggest they expected the user to use a 2000 Ohm speaker or headphone.

The speaker is an original Stromberg 24 inch exposed cone speakers. Not sure what the impedance of the speaker is though...

Here is a picture off of the net that looks like his speaker

Use your Ohmmeter; I would guess you would find 2000 Ohms.

Ok , I will check as soon as I can. My friend will be out of town for a couple of days.

My AK-30 has the same output tube, 71A.
A very simple circuit, it uses the speaker as the plate load, no capacitors or inductors in the circuit.
The Radiola 100A speaker I use measures close to 1400 ohms DC resistance.

I am working on this radio literally in front of the owner. He is a friend but picky in that: there were caps in this set.....so they were there for a reason......so why would you bypass them? He watches everything I do. That is why I am trying to come up with a value for the caps I need to get.

Well you can make some assumptions. First, that it's a 10 kHz whistle filter. Second, the impedance level is 1500 or 2000 Ohms. Then look up a filter design website to find a low pass LC filter with that configuration and you will have the numbers you seek.

Make the cutoff frequency 8 kHz as an example and it should work okay.

Okay I found a web site http://www.calculatoredge.com/electr...low%20pass.htm that will calculate it for you.

I tried it, selected 8kHz and 1500 Ohms and three components, and got two 0.44 microfarad capacitors and a 21.2 Henry inductor. Oh and I selected 3 dB ripple, sort of arbitrarily.

Thank you, you are obviously electronics trained or understand far more than I do. I really am trying to learn more about how a radio works instead of just being a cap replacer and resistor checker. Sometimes it is hard wihout a teacher..... I just can't raise my hand and get the answer I need. So When I do get a book sometimes it helps me sometimes not.......

If you are serious about learning electronics, you need to make a plan. You could go for a BSEE but that's kind of ambitious; perhaps a technician course would be better. But they don't have them the way they used to. One of the old courses from CIRE or Sprayberry or National or DeJur would be a possibility. Browse the web.

Sit quietly and read from page 1 in a basic textbook. Read the first few pages over and over until it makes sense. Do some experimenting. For instance, get an Ohmmeter and measure a few resistors. Calculate what resistance they would be in series or in parallel and then measure them that way to see how you did.

Humility is important. There is no end to the learning process so don't look for results, just understanding. Read the Radio Amateur's Handbook, for instance. Over and over.

A few posts up I think I mentioned DeJur but what I meant to say was DeVry.

Ok, I have probaly repaired over 100 radios and 5 b&w tube tv's now. With an occasional exception I am getting better at diagnosis. With my Capehart 21n2 I had a distortion problem on am and fm. Knowing that it has seperate fm and am up to audio I figured it was an audio problem Knowing that the paper caps in the bass switch were replaced I looked the diagram and figured it was one mica tied to the volume control.....and was right. It works well now. The owner of Fair Radio here in lima can look at a diagram and tell you what exactly each cap is doing and why. The man just blows my mind with his knowledge.

I have been there when a technician couldn't figure out what was wrong with a military transceiver and handed the diagram to the owner... he looked it over asked what was going on and said check this transistor or whatever part it was and sure enough was right. I guess it may take me a LONG time to get there but WOW..... thats what I want.

Maybe I am trying too hard !

Just accumulate knowledge and experience. Nothing else will do.

I too can diagnose without seeing the gear; I have done it many times. And yes, I can tell the function of each part, and even critique the design. And make suggestions for change.

Electronics has been my hobby and vocation for 70 years or so. I am always learning something new.

thank you for your help. I love all things old and wish I had grown up back then with the tube radios and tv's. old cars...........you name it.

Most of us who did this back when good old stuff was tossed thought we were the only ones who cared. Now we know there are others, of all ages, who can learn the art of conservation.

I just hope as an example with my pride and joy Capehart by Farnsworth 21n2 that as much as I play it that tubes will still be available and not cost and arm and a leg when I am old.
Yes some are being made new now but 30 years from now who will have a 6af6 tuning eye or 1l6 for the Zenith sets? are all the tubes we need going to be out there?
What about crt's..... will the color roundies be out of luck?

If I would ever hit the lottery I would love to start making some of these hard to find items like the early color crt's.......but is it financially possible now?

Some of the rarer tubes will get harder or impossible to find but there are ways to adapt to others. Whenever this topic comes up the fact is raised that millions upon millions of tubes were made during their heyday and caches of them are still being discovered, so there's a likelihood that most common tubes will be readily available for a couple of lifetimes.

As to learning about radio, there's nothing like reading to understand, and experimenting. There are many ways to gain experience but as a kid I scrounged old junkers and used the parts to build sets copying schematics out of books and Popular Electronics. My uncle and I built a crystal set, not a very good one, that picked up two locals, when I was in the third grade. After that I got into working with tubes. I never could get a regen to work. Finally one day that I remember vividly it squealed into action and was picking up DX. I ran outside my shack around the yard exulting: I felt like Armstong himself!

One good way is to take a simple working set like an AA5 and go through it with a signal tracer, following the signal stage by stage. If you don't have a signal tracer you can make one quickly using a spare amplified speaker left over from a previous computer, making up a couple of probes: one for AF and one with a germanium diode for RF. One site that I have quoted frequently takes you through an AA5 and I think is helpful as it explains each stage in turn so the whole mysterious thing is not staring at you all at once. Some may want to bookmark or print out this site:

http://www.angelfire.com/electronic/...bes/AA5-1.html

You can use a second AA5 as a signal tracer. Just use the hot side of the volume control as a probe and, if you want rf, add a diode.

Just watch out for those hot chassis jobs! An isolation transformer is a very good adjunct to your shack if you're going to work on this stuff.

I do not understand the reason for the line isolation transformer.... the set is still powered up so what is the difference?

The isolation transformer makes the gear safer for the technician. In the nonisolated set, one side of the power line connects to the active circuitry and, sometimes, also to the chassis. This is not a safe situation. The danger varies widely depending on the orientation of the plug to the power line; one way is asking for trouble, the other may be safer until you connect a second unit.

Well, the multi section bar resistor had a bad section so I replaced that, bypassed the audio filter and The radio plays well. I showed my friend who owns it your posts and he is allowing me to leave the audio filter bypassed. He did this because of your reply and the other videokarma member's opinion. I have 2 more radios to work on for him so I will document the diagram with what I have done so a future generation will understand my work years from now. But I would like to say thanks for your assistance on this project. Thanks to everyone who assists on any posting I put up.

Re: Isolation Transformers -- there is a dangerous shock potential existing between the hot side of the power line (the black wire, brass screw, shorter slot in a receptacle) and ground. If you're touching that line and some other part of your body is grounded, you complete a circuit and receive a shock, which could be fatal. With an isolation transformer, there is no direct connection between the secondary of the transformer and the primary, so the danger of a fatal shock between ground and a device plugged into this transformer is eliminated. Other precautions to be observed: rubber mat or dry wood floor (not bare concrete), rubber sole shoes, long pants (dropped solder burns!), and keep one hand in your back pocket when probing live chassis.

oh okay. That makes sense now. I figured power up equals powered up the same either way. I guess I will need to go to Fair Radio and get one. My next project is a transformerless ac/dc Meck radio so I will need an isolation transformer quick.

one last question. in the same room plugged into the same outlet is a zenith 10s ? that I repaired years ago. If my friend turns on his 6 or 8 foot fluorescent shop light the zenith sounds like the lytics are really bad. only picks up local and still hums very badly and very loudly. turn the light off and it is fine. The 1928 stromberg picks up more stations when the light is off but is not really affected otherwise. Any ideas?

I guess my point is that certain things interfere with a radio's performance but I have never seen it so extreme before

In the old days, manufacturers were not compelled to make their stuff immune to RFI and they were not regulated regarding generating it.

The serious susceptibility is testimony to the lackadaisical engineering, making it work at the lowest cost without regard for such things. It was expected that the user find a solution. They sold line filters to the consumer, similarly to surge protectors today. Waste of money, patching poor engineering.

Many of the sources of interference today didn't exist when antique radios were new. Switching power supplies in modern electronics come to mind as bad interference sources. On the other hand, if you read old manuals, they write about interference from brushes on motors of all sorts, electrical signs, streetcars, vacuum cleaners, and so on. Sometimes a line safety cap or two added to a set, or at the noise source, will help, if the noise is coming in over the power line.

http://www.justradios.com/safetytips.html

Sometimes the lamp radiates the harmonics that hum, buzz and/or hiss (like a weak signal) on AM and FM. A line filter cannot always fix that, only a well-grounded light fixture may help AND physical separation.

I repaired a friend's Philco 37-650 by adding a line filter I salvaged from a monitor. It had an M-filter, that is a cap across the line, two inductors with common core (like in an old Zenith TV) and a cap on the radio side across the line. It really cut down on interference from dimmer switches (buzz).

Ironically, old T12 lamps and magnetic ballasts seem to generate very little interference compared to electronic ballasts driving linear OR compact fluorescent lamps. The higher the wattage or number of lamps driven, the worse it gets.

I've had problems with large linear fluorescent lamps before too, but it was with 40's era sets. Not terrible, but there was enough hum with them on for me to wonder what was still wrong with recapped sets, that is until I turned the fixture off.

So if I understand it correctly the 1928 stromberg isn't affected that much since it has line to ground caps and the zenith probably doesn't have any. Since line to ground caps are dangerous if they short I believe I took the originals out and never replaced them because 8 or nine years ago when I worked on it I did not know about safety caps. And I sure don't want to be liable for a house fire if a normal cap ran from line to ground went bad.

I will have to get safety's and put them in!