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Old 08-09-2014, 05:36 AM
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Winky Dink Winky Dink is offline
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Blunderof the Week

I started working on this AC/battery portable, the "Troubador." It's a cheaply-built radio, but remarkably well preserved. The set has no identification as to manufacturer or retail brand and no apparent model or chassis numbers.



The only hint as to its heritage is a standard warning label with "W. M. Co." at the bottom. Fortunately, the original operating and service instructions were inside the cabinet, and the schematic is identical to the Warwick Mfg. Corp. Model 0-50 (Riders Vol. 11 page 7).



Here's what I did yesterday. I decided to remove the shields from the above-chassis IF transformers. When I was spinning the top retaining nut from first transformer, T4--well, I wasn't spinning the nut, I was spinning the core and coils and broke all four coil connections. I found the broken ends of each wire (they're really thin), painted the wires to make them identifiable and soldered a bleb on the end of each one to facilitate handling and soldering. The black stuff is "liquid tape" which I hope will prevent breaking the wire (again) during handling.



So, where should the wires be attached? The two coils seem to be identical--both 34 ohms. All I can figure out is that the ends of the lower coil go across one of the adjustable caps and the end of the upper coil go across the other cap. I don't know if it makes any difference which coil goes to which cap or if +/- order makes any difference.
Here's most of the schematic with the T4 chassis connections shown in their actual colors.



I'd greatly appreciate if someone can help me out with this.
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Old 08-09-2014, 12:01 PM
old_coot88 old_coot88 is offline
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Well, first I would explore the possibility of looking at the 1st IF xfmr as the model. But to take the shield can off, grip the stud firmly with needlenose to prevent its turning, then carefully unscrew the nut, making sure the stud never turns.

But without the model to look at (somebody please double check me on this), i would say the upper coil is the primary, and the lower one's the secondary.

Polarity might (or might not) matter. If it's backwards, the IF stage might go into oscillation. If it does, just reverse one coil or the other to correct it.

A very useful tip for stripping hair-fine wire:
Get an aspirin tablet.
Lay the end of the wire on the tablet,
apply tip of soldering iron so the aspirin melts and boils, producing hot acid which strips the enamel off.
Wipe clean with a damp Q-tip or similar.

But use ventilation as the acetylsalicylic fumes are very nasty.

Last edited by old_coot88; 08-09-2014 at 01:19 PM.
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Old 08-09-2014, 01:41 PM
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Thanks, 'coot. I already took off the second can in a manner similar to what you described. On the first can I used a socket, so I couldn't see what was happening. Also thanks for the aspirin tip. Unfortunately my blunder gave me such a headache that I had already taken all the aspirin in the house. It wasn't easy scraping the wire that I could hardly see. This wire is also covered with what appears to be cotton thread, and I don't know for sure if it is enameled.

I'll follow your advice and see if I can divine a solution.
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Old 08-09-2014, 04:26 PM
old_coot88 old_coot88 is offline
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Originally Posted by Winky Dink View Post
...thanks for the aspirin tip. Unfortunately my blunder gave me such a headache that I had already taken all the aspirin in the house.
Holy mackrel dude. I OD'ed on aspirin once, and went stone deaf with rip-roaring tinnitus for nearly a week. Hearing in left ear hasn't benn normal since.
Quote:
It wasn't easy scraping the wire that I could hardly see.
This wire is also covered with what appears to be cotton thread, and I don't know for sure if it is enameled.
Actually the hot acid will burn the cotton right off too, leaving the wire clean as a whistle (after swabbing the residue off). And there's no mechanical damage from scraping the wire.

BTW, is the top coil the primary or not?
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Old 08-10-2014, 12:14 AM
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Using the other IF transformer as a model and looking at the length of the loose ends, I think I got the thing put back together right. I don't which is primary/secondary but in the schematic posted above, the red & blue are the lower coil and black & white are the upper coil.

I actually didn't find any aspirin in the house except some 10-year-old 81mg tablets. Aspirin doesn't improve with age. I had a patient one morning long ago who was sweating profusely and said that he was in so much pain that his ears were ringing. He'd also taken two aspirin every hour for the previous eight hours.

Cleaning wire is a better use for aspirin. I'll get some and try it out.
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Old 08-10-2014, 12:33 AM
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I did the same thing with my Zenith Trans-Oceanic 3000-1. I was trying to peak the IF transformers for better reception, but I must have broken a wire or two or, Heaven forbid, one or more of the adjustable cores in the transformers crumbled. AM and shortwave are now dead, but FM still works as well as ever. I'm thinking the cores must be crumbled or broken beyond recognition by now; if so, I won't do any more with the radio since I have several sets with working AM, which I don't listen to that much anyway. I'd like to get the AM on my TO working again, though, as this radio has an RF stage ahead of the Wavemagnet antenna and can be a real DX machine when it is working as it should.
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Old 08-12-2014, 04:27 PM
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Quote:
Originally Posted by old_coot88 View Post
Polarity might (or might not) matter. If it's backwards, the IF stage might go into oscillation. If it does, just reverse one coil or the other to correct it.
I'm pretty certain that polarity will not be a concern.
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Old 08-12-2014, 08:31 PM
old_coot88 old_coot88 is offline
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Originally Posted by Tubejunke View Post
I'm pretty certain that polarity will not be a concern.
Probably not, much less so than for the 1st IF xfmr.
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Old 08-19-2014, 04:54 PM
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Down to the Wire

Well, I've finished replacing components, I have the tubes, and I'm almost ready to power up this set. The project is coming down to the wire (pun intended). My last issue is the line cord/resistor combo represented in this part of the schematic.



I couldn't figure out where the 566 ohm resistance was until I started to autopsy the line cord.



The resistance wire, roughly 25 feet, taps into the line cord near the plug end and emerges as a third conductor inside the chassis. I suppose this is the equivalent of a wire-wound resistor to get the rectifier's heater voltage down to about 35V.

I don't want to use this line cord because the insulation is so brittle that it cracks and falls off when it bends. My question is, can I replace the 566 ohm resistance with 566 ohms between the plate pin and the heater? And if that's a good plan, then what power rating would I need? By chance, I have a couple of 560 ohm 7W resistors.
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Old 08-19-2014, 05:11 PM
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To determine the wattage take the heater current and multiply it by (120V - the sum of the heater voltages of the tubes in series with your resistor)...Or, if I'm interpreting the info you posted correctly, 0.15A*(120-35V)=12.75W....Go with a 13W or higher replacement.

There are also ways to replace the cord with a diode or a capacitor to drop the voltage, which have the advantage of not releasing nearly as much heat. You will have to google those as I'm not experienced enough with those methods to specifically advise you off the top of my head.
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Old 08-19-2014, 05:17 PM
Paul Knaack Paul Knaack is offline
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Heres some good information on replacing the resistance line cord using a capacitor rather than resistor to eliminate unwanted heat under the chassis.
http://www.nostalgiaair.org/Referenc...sh/Flash01.htm
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Old 08-19-2014, 06:37 PM
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Thanks for the responses. The nostalgiaair.org article is right at my level. I'll start to work on that tomorrow night.
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Old 08-19-2014, 07:22 PM
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Yes, the way to go is with a non polarized capacitor. It drops the voltage without heat. Be sure of course that you're dropping the voltage for the filament string only, as the B+ supply wants full line voltage. For the record, if using a resistor to drop voltage, it's a good idea to go strong on the wattage rating, even double what the calculation would say, in order that the resistor run cooler. Those Dale resistors that are made for chassis mounting as a heat sink are a good choice when going that route. But I do like the cap method better. Good idea to stick a high value resistor, like 270K, across the dropping cap, to quickly discharge it when the set is turned off. It can be a half watt or so.
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Old 08-20-2014, 08:02 PM
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Or, dispense with the dropper, and use a 117Z4 instead? Same pinout. (DO mark the chassis, for future reference) The Z4 is good for 90mA, filaments draw 50, I suspect the B+ is less than 40mA.
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Old 08-21-2014, 07:33 PM
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Ddn't know this was a common issue.

I read a number of articles and forum posts on resistance line cords.
I had already replaced the two 1200 ohm/3W resistors 1200 ohm/7W ceramics. So I only have to deal with the 566 ohm line cord resistance.



Options that have been recommended include (1) Reproduce a resistance line cord (2) Replace the 35V rectifier with a 117V rectifier (3) Replace line cord resistance with in-chassis resistance (3) Replace line cord resistance with capacitance.

Option 1 is ill-advised (for me) for a number of reasons, and I'll have to pass on option 2 because I want to preserve the original tube line-up.

Option 3 is viable because the heat can be dissipated by cutting a hole in the wooden chassis base to vent the heat into the battery compartment space. I can cobble together 3 or 4 resistors to get 560 ohms at 24 or 25 Watts.



Option 4 is the clear favorite. Informed sources recommend experimenting with estimated capacitance to achieve the desired voltage drop, but I can't do the math to even make an estimate.

Can someone give me an estimate of the capacitance needed to drop 120VAC to 35VAC?

Finally, other than a few motor-start caps, I've been unable to find nonpolar AC capacitors in Mouser, DigiKey, Allied, or the usual vintage suppliers. I'm sure they're there, but I'm probably not using the right search terms.
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