need help

[radio nut]

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.

[bob91343]

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.

[radio nut]

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.

[bob91343]

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.

[radio nut]

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.

[bob91343]

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.

[radio nut]

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?

[bob91343]

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.

[radio nut]

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

[radio nut]

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

[bob91343]

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

[radio nut]

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

[DavGoodlin]

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.

[radio nut]

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.

[bob91343]

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.