Blunderof the Week

[Electronic M]

Originally Posted by dieseljeep

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.

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...

[Winky Dink]

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.



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

[jr_tech]

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

jr

[Gregb]

Plus 1, bad filter cap or connected wrong.

Greg

[Winky Dink]

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.

[Winky Dink]

...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.

[jr_tech]

Originally Posted by Winky Dink

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.

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

[old_coot88]

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.

[Winky Dink]

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.

[Winky Dink]

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.



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.



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.



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

[Winky Dink]

I’ve rewired my transformerless sets to eliminate hot chassis. This is the last one I have, but I’m not sure what to do with it. This is an AC/battery portable in which we replaced the original voltage-dropping resistance with capacitance.

Link to schematic:

https://goo.gl/photos/EMuXaAP2JWrNPPWo6

As it is now, the chassis is at 45VAC with switch off and 27VAC with switch on (or vice-a-versa with a non-polarized plug). Do I need to address the hot-chassis issue? It’s a small “suitcase” portable, and a small child can easily unlatch and open the back to stick a hand inside.

The other sets were straightforward, but this one puzzles me. What can I do to fix this?

[dishdude]

The chassis looks like it is decent quality, but wow that case is the cheapest of the cheap.

[init4fun]

Originally Posted by Winky Dink

I’ve rewired my transformerless sets to eliminate hot chassis. This is the last one I have, but I’m not sure what to do with it. This is an AC/battery portable in which we replaced the original voltage-dropping resistance with capacitance.

Link to schematic:

https://goo.gl/photos/EMuXaAP2JWrNPPWo6

As it is now, the chassis is at 45VAC with switch off and 27VAC with switch on (or vice-a-versa with a non-polarized plug). Do I need to address the hot-chassis issue? It’s a small “suitcase” portable, and a small child can easily unlatch and open the back to stick a hand inside.

The other sets were straightforward, but this one puzzles me. What can I do to fix this?

Hi Winky ,

Two solutions come to mind ;

# 1 , a small 1:1 transformer to isolate the set from the line .

# 2 , keep it the Hell OUT of small children's hands so no modifications to the radio are needed .


(only partially joking with suggestion # 2 , some things ya just can't "idiot proof" no matter how ya try)

[Winky Dink]

Thanks for the suggestions. I’m trying to follow the simple method of polarizing the plug and putting the hot line through the switch and the neutral to ground--best explained by Phil Nelson here:

http://antiqueradio.org/safety.htm

I’ve done this successfully on several AA5’s, but the layout of this set confuses me. Here’s what it looks like to me (after erasing the battery connections from the schematic):



You can see where I get lost from not knowing what to do with the capacitors.

This is the cheapest-looking “vintage” radio I’ve seen. It must have had little use to have survived in such good condition. But it’s cute, and it adds some variety to my very small collection.

- Henry

[init4fun]

Winky your drawing looks perfect to me , you have the neutral connected straight to the chassis and your hot going through the switch to the tube's heater (via the dropping capacitor) and it's plate . The across the line cap C4 .05 MF is electrically identical in both it's originally drawn position on the schematic and where you drew it in with a question mark . Drawn either way , it's got one lead on the hot side and one lead on the neutral side , just as the circuit needs it .