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 Post subject: My first TRF (output transformer)
PostPosted: Apr Sat 03, 2010 2:54 am 
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Location: Chillicothe, Ohio 45601
I’m getting ready to work on my first TRF radio a Stromberg Carlson 641B and I wonder if there is anything to look out for? It looks like this radio doesn’t use electrolytic capacitors so I assume I can use two 1mfd capacitors in parallel to replace the 2mfd capacitors. I also have a Bremer Tully speaker on the way and it will be nice to get this 80 year old radio working.

Here is the schematic
http://www.nostalgiaair.org/PagesByMode ... 040636.pdf

Thanks
Rod


Last edited by hotrod on Apr Tue 06, 2010 11:15 pm, edited 3 times in total.

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 Post subject:
PostPosted: Apr Sat 03, 2010 3:30 am 
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Rod

You can parallel 1 mf if you have them. Radio uses a lot of 1 mf caps. Those .3 mf caps can be .25 or .47 mf.

Other than checking continuity of the audio interstage transformer and power supply divider resistor, shouldn't have problems.

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 Post subject:
PostPosted: Apr Sat 03, 2010 7:56 am 
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Location: Chillicothe, Ohio 45601
I think I have enough 1 mfd caps and I stock .33. I need to read up on how to restuff the square cans. The first problem I have to fix is the volume control is actually to different controls on for the antenna that is supposed to be 20K and measures 18.5. and one for the audio. They are joined my a shaft running from the front to the back of the radio. The problem I have is that the audio control is missing it's shaft and that shaft goes through the audio control to a coupling that controls the antenna volume control. However. I think I can make on out of some brass rod. The wiper for the front control has a set screw and I think it will work when I get it back together. It measures just under 800 Ohms. Maybe it was a 1K, does this sound like a number that would have been used in a radio like that? It seems low to me and I hope it is not bad.

Rod

P.S
Anyone know what color code Stromberg used because the resistors are all a single color.


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 Post subject:
PostPosted: Apr Sat 03, 2010 9:57 am 
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Looks like I am going to have to do more mechanical work on this one. The pot-metal piece that held the dial onto the tuning shaft crumbled on me. I'm not sure how I am going to fix that and I doubt I can find another on. I think a machine shop would charge me more than the radio is worth.

Rod

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 Post subject:
PostPosted: Apr Sat 03, 2010 2:01 pm 
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The principal concern with the caps is getting the voltage & current rating. In order to stay with originality of type I have used mains MKT and Motor run (not start) capacitors.

Remember if you sneak in electrolytics, they do not pass RF well. Do not be tempted to change the values as they could be resonant with the coils.

Coupling:--

Only for originality would you attempt to recut to that shape. A profile cutter would do it (as said..... at what cost). All you need to to get you out of trouble is use bar stock.

Face it off & size it first (the lathe will give you the centre). You can mark the centre with a centering drill but do not bore the centre hole until you mark the position for the 3 radial holes.

Mark out the 3 holes, 120 degrees apart, drill out to tapping size, on a press to keep them straight.

The lock screws can be replaced by grub screws, preferrably to take an Allen Key. Use the ones from machine pulleys as they are high tensile & do not break up like the ones normally in knobs.

Note that the lathe will mark a circle for the screw centres on the face. NB how the lock screws are in relation to the webs. Drill the same. Bore the centre hole before these.

I would be cautious taking the hole positions from anything other than what ever bolts to that coupling.

Marc


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 Post subject:
PostPosted: Apr Sat 03, 2010 2:42 pm 
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hotrod wrote:
Looks like I am going to have to do more mechanical work on this one. The pot-metal piece that held the dial onto the tuning shaft crumbled on me. I'm not sure how I am going to fix that and I doubt I can find another on. I think a machine shop would charge me more than the radio is worth.

Rod


Rod, does it NEED to be metal? Could a high density polyester work? Maybe you can make a molding/casting using this kind of technique?

http://www.radiomuseum.org/forum/replic ... stors.html

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 Post subject:
PostPosted: Apr Sat 03, 2010 2:58 pm 
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Yes it can be plastic. I've also thought about threading the shaft and putting a "bracket" between two nuts or finding a dial pulley with set screws for a 5/16 shaft and modifying it to mount the dial on but all I have are press on pulleys.

Rod


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 Post subject:
PostPosted: Apr Sat 03, 2010 3:47 pm 
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That's what I love about having a Farm with a workshop. Things like that just get made. It's pretty straight forward.

Aluminium bar stock will be good; Don't use plastic that is soft, or capable of distortion.

Marc


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 Post subject:
PostPosted: Apr Sat 03, 2010 6:17 pm 
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Yeah, I don't think I will have any problem making the volume control shaft. The pot metal piece is going to be a little more difficult though.

Rod


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 Post subject:
PostPosted: Apr Sun 04, 2010 4:45 am 
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I fixed the volume control shaft tonight. I wanted solid stock but my hardware store only had tubing in 3/16" unless I wanted steel. It seems like it is going to work find. I still haven't decided how I am going to fixed the cracked pot metal piece.

Rod


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 Post subject:
PostPosted: Apr Sun 04, 2010 7:09 am 
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I have a question about the resistor value on this radio (SC 641B). The resistor goes across a 25pf mica capacitor but no value is given for the resistor in the schematic. However, The 642 which is on the same schematic has a parts layout sheet and it states that this resistor is 2 Ohm but the use the new Ohm symbol only this resistor. All of the other resistors use the Greek letter that looks like a w(Omega). Anyway this resistor that is listed as 2 Ohm measures at about 9 megs. Is this a 2 Ohm resistor or did they use the new ohm symbol because it was a 2 meg resistors? What would normally be used in this part of a circuit?

ImageImage


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 Post subject:
PostPosted: Apr Sun 04, 2010 8:07 am 
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According to the data that looks like a leaky grid detector and the data says 2Meg for the control grid of the detector.

Two choices with the "pot metal" If its the bit in the picture? Glue it, Which I consider an exercise in futility.

Remake ... fraction of the time ... permanent and despite reservations, not that complicated. Try an Engineering or Bearing supplier for the bar stock they may even have a bushing close?

Think outside the square, we're not doomed yet.

Marc


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 Post subject:
PostPosted: Apr Tue 06, 2010 6:16 am 
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Thanks Marc, I didn't think 2 Ohms sounded correct.

I also have a couple of new questions. I took the Output transformer can off tonight to replace the caps in it and I realized that I only have 1 mfd capacitors in 400 Volts and I planned on using these to replace both the 1 and 2 mfd capacitors. Will these be good enough in the output circuit from a single ended 45? What about the power supply it has an 80 rectifier should I order some 600/630 volts capacitors for the power supply?

In addition, the output transformer measure 2.0 ohms resistance across the primaries and 1.7 ohms across the secondary. The data sheet states 1.7 across the speaker terminals so that sounds good to me. Does 2 Ohms sound about right for the primary?

Finally, I think I am going to look for a shaft collar to try and replace the broken pot metal piece.

Thanks,
Rod


Last edited by hotrod on Apr Tue 06, 2010 8:17 am, edited 1 time in total.

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 Post subject:
PostPosted: Apr Tue 06, 2010 6:40 am 
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Note the difference in the lower case omega "w" vs upper case omega.

One means ohms, the other means Megohms. Atwater-Kent did it in their diagrams too.

Primary of your output xfmr should be a couple/few k-ohms of DC resistance.


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 Post subject:
PostPosted: Apr Tue 06, 2010 2:21 pm 
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For the most part I tend to keep the parts inventory down by stocking mainly 630V capacitors, as I principally repair tube radio's. Like I ordered 30 0.047's today as a Midwest 34-16 took a toll on what I had.

The 1 & 2 mfd are not that common here. If you are going to stay true to the circuit, then 500V minimum is what is required.

For the filtering a Cap capable of high current is in order, you may have to take other advice but I would be thinking something like Solens, Pulse grade, or Motor type run caps.

The thing with transformers is, unless a DC resistance is specified, it is usually meaningless as transformers are inductors & what is normally quoted is impedance.

I would not stress with the "Pot Metal" thing. It's the sort of thing one could "knock up" casually on a slow afternoon.

This from experience, not otherwise. As I have the Taps, Dies, Drills & Lathe on site that would make short work of it. Catch 22 Australia is not close to you.

Marc


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 Post subject:
PostPosted: Apr Tue 06, 2010 2:25 pm 
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If you are showing only two ohms across the output transformer primary, it is either internally shorted, or one of the .01uF capacitors in the filter network that are connected in series with the center point grounded and across that choke are shorted. Most likely the one on the right hand side.

Unsolder one lead of the primary of the output transformer and measure it directly. Like Jack said, it should measure at least several hundred ohms. Be sure you are reading your meter correctly.
Curt

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 Post subject:
PostPosted: Apr Tue 06, 2010 6:26 pm 
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Thanks everyone, When I got the output transformer out of the can and everything disconnected from it I find that the primary is actually open. Will one of the Universal output transformers work on this set with a high impedance 1920's speaker?


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 Post subject:
PostPosted: Apr Tue 06, 2010 6:34 pm 
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Hi Rod

Yes, you can use an universal output transformer but there is a less expensive way. If your radio only uses primary of the output transformer and has cap coupling to the speaker you can use a choke. Most likely already have a choke in your collection?

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 Post subject:
PostPosted: Apr Tue 06, 2010 9:22 pm 
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Thanks Norm, It looks like the secondary is connected to the speaker to me. Here is a layout of the audio transformer can.

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 Post subject:
PostPosted: Apr Tue 06, 2010 10:01 pm 
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Hi Rod

Schematic shows a 2mf cap off of plate of output tube. You can connect a high impedance speaker from this cap to ground but also need B+ on plate of the output tube.

Can't tell by the schematic what's used in your specific radio. Secondary of the output transformer is shown with dotted lines. If secondary is needed requires an output transformer. If secondary isn't used a filter choke will work.

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