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.

https://lh6.googleusercontent.com/-i...ub%2520010.jpg

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).

https://lh6.googleusercontent.com/-b...ub%2520102.jpg

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.

https://lh3.googleusercontent.com/-r...ub%2520400.JPG

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.

https://lh3.googleusercontent.com/-W...ub%2520404.jpg

I'd greatly appreciate if someone can help me out with this.

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.

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.

Holy mackrel dude.:eek: :DI 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.
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?

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.

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.

I'm pretty certain that polarity will not be a concern.

Probably not, much less so than for the 1st IF xfmr.

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.

https://lh6.googleusercontent.com/-W...ub%2520602.jpg

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

https://lh6.googleusercontent.com/-W...ub%2520609.jpg

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.

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.

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

Thanks for the responses. The nostalgiaair.org article is right at my level. I'll start to work on that tomorrow night.

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.

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.

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.

https://lh5.googleusercontent.com/-4...ub%2520612.jpg

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.

https://lh3.googleusercontent.com/-4...ub%2520015.jpg

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.

I've approached that problem a different way.
Install a standard line cord and use a one amp silicon diode and about a 150 ohm 5 watt resistor. Leave the tube in there, for appearance.
BTW, if you look at the newer Warwick models, they switched to a 117Z4, in the later runs.
I've also used 117Z6's, as well, but the socket has the be rewired.

I messed up on my last entry!
The 150 ohm and the diode was meant to be connected from the plate and the cathode of the rectifier tube, to furnish B+. Leave the heater disconnected. :scratch2:

Hey Tom!
Look up WA2ISE's website.
His formula for cap droppers is real easy to use and I've had great sucess with my applications. :thmbsp:

I read a couple of descriptions of using a diode, but the authors came back with corrections so often that I figured I would be venturing too far into the "I Don't Really Know What I'm Doing" territory. I will look for WA2ISE's formula.
Thanks.

...but my antivirus blocks me from wa2ise.com.

"Website blocked!
The site contains infected code: Script.Packed.IFrame.K@gen
Virus: Script.Packed.IFrame.K@gen"

I expect that this is a false positive, but I'll have to stay off the site for now.

Ehhh, it's not impossible that's not a false-positive, better safe than sorry.


I fiddled around with things to get a preferred number value here, so if your line voltage is lower than 125.5 the current will be less than 150mA.

3.3µF works fine, 3.47µF is "exact" at 120v/60hz for 35v 150mA...

http://www.funkengine.net/vk-img/201...ic-blunder.png


I'll leave the exercise of doing the reverse calculations up to you to figure out exactly what the current would be with a 3.3µF E6 series cap at your particular flavor of local mains. :yes:

Gotcha! Thanks for the lazy man's answer.

I rather like the idea of replacing the rect with a silicon diode, and doing away with the tube rect alltogether. Add series resistance to get the B+ (and fil) voltage you want..

There's so much irony here... http://www.ebay.com/itm/NTE-MLR335K2...item540204913c


You can get them cheaper if you don't mind the slow-boat from Shenzhen, but I'll gladly pay the asking price for something that ships stateside at a reasonable price.

There's a learning curve to parametric searches, here's a brief crib-sheet:


The trick with Mouser's search for caps is as follows:

Head to the capacitor category, choose your flavor (electrolytic/film/ceramic/mica/lambskin/etc...) then tick the stocked box at the top, hit apply filters.

Now you pick your value, or possibly your ideal value plus one on each side. Again, hit apply filters. (Sometimes you'll pick 0.5x your value because 1x isn't a sane number, just parallel them up.)

Now we choose our voltage. I usually go with selecting the DC voltage, as many suitable caps don't actually have up-front AC rating values on them. In your case we'll start at 250VDC, then hit the > to pick anything 250 or higher. Again, apply filters. (Are you sensing a pattern here?)

Now we'll say we only want radial caps, that are good for 105C minimum, applying filters. (This is one of the few times where you can enter more than one requirement between hitting apply filters.)

Finally, since this *is* a current limiting cap, we're going to go for 5% tolerance!

Gives us 5 results, across the normal spectrum of "I'd trust it in my own home." to "Only the best will do here, Cornell Dubilier please!" pricing...

http://www.mouser.com/Passive-Compon...r&Ns=Pricing|0

In the Matsushita affordability range you even get to choose between 10mm and 15mm lead spacing! How bourgeoisie!:king: I'd go for the 15mm unit myself.


Resistor hunting is a similar path, but you pick through-hole first, then your tolerance, then wattage, then value. I usually pre-select only Vishay branded items, resorting to random sources only if I can't find what I need from Vishay.

I still say the free shipping unit linked at the top is your best investment unless you've got a dozen or so things on your perpetual Mouser saved shopping cart ready to purchase. The Mouser order will arrive in the shortest time though, that I can assure you.

I checked my website with several URL scanners, none found anything. Doesn't mean that yours was a false positive, but I couldn't find anything. About a year and a half ago there was an infection, but I believe I was able to remove it. Did that a year and a half ago.

The result above for the cap is consistent with what my excel spreadsheet says, 3.5uF. Use one for 120VAC service.

Couldn't find what I needed from Mouser because I was stuck on "nonpolar electrolytic" as referenced in the Nostalgiaair link:

"The capacitor must be a special type. It's a non-polarized electrolytic. It's non-polarized to handle the AC current, and electrolytic because of the relatively high capacitance value required."

But that article was written over 20 years ago!

Regarding w2ise.com, I think my G-data software is quirky. I get blocked occasionally from innocuous sites.

The last cap-dropper I made up was from a scrap ceiling fan.
When I see one on trash day and I have time, I remove the switch and cap assembly and leave the rest there. They have two AC caps of different values. :thmbsp:

The cap should be a film or such, electrolytics may overheat, and tend to not be an accurate value. And get one designed for service across the powerline, usually they say "250VAC". Fan caps are good for this, think I saw them at Home Depot.

I'll take a look in the home stores. In the meantime, I hooked up the set with ceramics, powered it up with the variac. The 7-watt ceramics I used to replace the original 3-watt resistors inside the chassis got much hotter than I would have expected. I think that so much heat in a small chassis really cooked all the components that I had to replace.

Look at Parts Express, Electronic Goldmine, and the other semi-surplus sites. I have often found them there, cheap cheap.

Thanks for the advise. I've never worked on a resistance line cord set before so I've never needed to do this. Odds are that on a good day I could remember the correct formula and solve it, but I don't want to advise someone off the top of my head on something critical like that, and lacked the time to confirm correctness through research at the time of that post...

Return to Troubador
 

I went to the "surplus" sites. It was very expensive because I bought a lot of other stuff I don't need. But the capacitor was cheap enough.

Finally back to the Troubador transformerless, looks-like-a-suitcase, AC/DC, portable radio. I'm trying a 3.5uF 250VAC motor run capacitor to reduce the AC voltage to the rectifier. From the cathode(?) of the 35Z4 the voltage is further reduced to the four 1.4-volt tubes in series.

After patching in the speaker and loop antenna, with line voltage 124VAC, the radio operates with 38 VAC to the rectifier and 1.8 VAC to the other tubes.

https://lh5.googleusercontent.com/-I...ub%2520730.JPG

Is 38 volts too high for long-term operation of the 35Z4? With 38VAC to the rectifier, I think I have the wherewithal to reduce the A voltage to the rest of the radio--if that's worthwhile.

BUT...this is what the radio sounds like: (link to 10-second video)

https://picasaweb.google.com/coldrb/...72964473148050

I've heard this type of audio before, so I assume it's a common issue that someone can explain to me. What should I look at to fix the audio?

Thanks much, and
Shanah Tovah

Sounds like 60Hz. hum to me... did you replace the filter caps?

jr

Plus 1, bad filter cap or connected wrong.

Greg

I didn't think this was hum. My impression was that 60-cycle hum would sound like it was imposed on top of the audio, but this sounds like the audio is rapidly cutting on-off. The prominence also seems to be frequency-dependent. At one end of the band it's barely noticeable. This sounds like someone talking through the turning blades of a fan.

Filter caps were replaced with new electrolytics very close to original values, but I'll re-check my connections.

Thanks.

...and I tried reversing the polarity of the plug, but that made it sound much worse. Also took away the variac and isolation transformer, but that didn't change the sound.

Oh, now that is interesting! Possibly an indication that the 60 Hz is getting into the radio at the converter stage... open antenna coil ? How is reception... weak perhaps? are you using an external antenna wire or ground to boost the signal?

jr

WD,
You should definitely get the AC heater voltage down a few volts on the 35Z4. Try a resistor in series with the dropper cap. Maybe start with a 22 ohm and see which way it needs to go. (Make it 2 watt or higher rating.) Heating won't be much of an issue.

The 1V DC tube filaments want to see no higher than 50 ma. So the DC filament dropper should be delivering 50 ma. to the string. Since it's series, always go by the current, not voltage per se.

The video won't play on my rig (iPad). But what you describe sounds like 'tunable hum'. That's when the received signal is getting 'modulated' at the powerline rate. It can be caused by a coupla things..

1. An open antenna coil causing the converter tube's grid to 'float', thus making it hypersensetive to ambient powerline fields.

2. The received signal being re-radiated by house wiring, usually due to power diodes in some piece of equipment switching the re-radiated signal on and off.
Try disconnecting (not just turning off) all appliances and electronic gear one-by-one and see if the problem clears.

Immediately after my last post I was playing with antenna configurations. When I returned the original setup with the loop antenna, the problem was gone. The only change I made was a tiny tweek on the loop antenna trimmer, just to see if the screw would turn easily. Now I have strong, clear reception with lots noise between stations.

The hum (or whatever) was familiar to me because I had in VCR that did that back in the 1980s.

I will work on the filament voltage/current issues, then see if I figure out how to align the set. Thanks for the help.

Just in Time for the Playoffs
 

I finished the Troubador yesterday just in time for the National League playoffs.
Most of the previous audio issues resolved when I got rid of the jumpers and temporary connections and did the final hard wiring. I also replaced the 1T5GT substitute for the detector/audio amp with the specified 1H5GT (although it probably didn't make any difference). The filament voltage/current came out perfectly with no changes--34V to the rectifier and 46mA to the other tubes. The last step was to give the tuner plates a thorough spray of CRC Cleaner.

https://lh4.googleusercontent.com/-c...ub%2520850.jpg

I chose this set because it's uniquely different from everything else I have. I was surprised that any prewar radio could have been produced so cheaply. The dial face/escutcheon is cardboard stapled to plywood, and the dial is a cardboard disk with a red stripe.

https://lh6.googleusercontent.com/-1...ub%2520855.jpg

The covering material is textured paper. I reaffixed the loose paper with rubber cement. Not taking chances with wetting the paper, I cleaned and finished it with plain paste wax.
The original wooden base for the chassis was getting loose, so I replaced it and added some perforations to help ventilate the two power-dropping resistors.

The set gets good clear reception of all local stations, and there's no noise at all between stations. I'm a Washington baseball fan by birth, so this afternoon I'm going to put on my lucky t-shirt (purchased at Cooperstown 25 years ago) and listen to the game.

https://lh6.googleusercontent.com/-c...ub%2520860.JPG

Thanks for all the help in get this set up and running.