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 Post subject: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Fri 02, 2018 9:01 pm 
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Joined: Mar Thu 03, 2011 8:53 pm
Posts: 916
Location: Davis, CA
Circuit diagram for this radio is at Riders Montgomery Ward page 9-57. See http://www.nostalgiaair.org/pagesbymode ... 011526.pdf.

The two electrolytics in this radio are 5mfd-200v. I have 4.7mfd-450v and 10mfd-450v in my stash of parts. Are the 4.7mfd caps adequate?

Thanks, Bob


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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Fri 02, 2018 9:08 pm 
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Yes; 4.7 is essentially the same as 5.0mfd for this application.

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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Sat 03, 2018 4:30 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4199
Location: Near Brandon, Iowa
While using two 4.7uF caps in place of the original 5uF units would probably be fine, you should find that the set will have less residual hum if you use 10uF caps in both positions. 10uf is well within what the 5Y3 tube can tolerate.

If it were me, I would install a 20uF cap in the C10 position and a 40 uf cap in the C9 position. 450V rating would be acceptable for both.

When this set was manufactured, high-capacity electrolytics were expensive and so the designers tried to strike a balance between optimum performance and minimizing recurring cost. Not to single out Airline but if push came to shove, radio manufacturers usually chose the latter over the former since per-unit-sold profit was a lot easier to quantify than customer satisfaction.


Last edited by lorenz200w on Mar Sat 03, 2018 7:04 pm, edited 1 time in total.

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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Sat 03, 2018 6:53 pm 
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Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
+1 to what lorenz200w said, with a small note of caution. My Airline is a bit newer and when I bumped up the relevant position of C10, mine was marked differently of course, I inadvertently bumped up my B+. Your print says they are looking for 135VDC off C10. I would monitor that after the fact to make sure you are still where you need to be.

Tomie.


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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Sat 03, 2018 8:58 pm 
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Joined: Mar Thu 03, 2011 8:53 pm
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Location: Davis, CA
Attachment:
Airline 62-425, resize.jpg
Airline 62-425, resize.jpg [ 184.93 KiB | Viewed 281 times ]


Thanks for the comments and suggestions. I have a few more questions regarding the polarity of C10 and the values of R9, 10, & 11. Here is a photo of the circuit diagram for easy reference.

Is the negative side of C10 attached to the field coil at the point labeled F' on the circuit diagram?

The candohm resistor, R9, 10 and 11, has values of 204, 42, and 119 ohms. Design values are 200, 33, and 100 ohms respectively. What is the reliability of these old candohm resistors and should I replace it?

Bob


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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Sun 04, 2018 6:30 am 
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Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
Hi Bob,

The short answer is yes, the negative side of the electrolytic is tied to the point marked F' on your copy of the print. Someone has penciled in a, "+" at that point and that is backwards if that is referring to the electrolytic. B+ comes off the rectifier, the high voltage center tap is the most negative point in the radio. Hook the negative side of the electrolytic to the center tap string ahead of the field coil and the resistors like it is shown on the print, the positive side of the electrolytic to the rectifier.

You are within tolerance on the candohm except the center section is a bit high. Some have said +- 20% and that section is a bit high even at that. On old stuff I try to stay closer than 20% if possible. In any case it would probably work like it is.

That said I don't like candohm resistors. That's just my opinion. I try to swap them out with individual resisters tied in a string. If you do that don't use the existing candohm terminals as a tie point, you'll be paralleling all of your resistors. Install a terminal strip of some kind, or point to point depending on the physically layout of the chassis.


Tomie


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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Sun 04, 2018 10:12 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4199
Location: Near Brandon, Iowa
Candohms are nasty devices because they not only can fail open (the usual failure mode for resistors, at least those other than Candohms), they can fail shorted to ground as well.

Failing open is no big deal- since the Candohm is in the current return path for the high voltage winding center tap, if an open happens the radio will just stop working. No fireworks, just a dead radio due to no B+.

If one of the Candohm elements shorts to ground (the resistance wire segments are insulated from the steel case by fish paper, which can burn through) more insidious consequences can result... the radio may still play, but its biasing scheme will be upset, possibly leading to tube overcurrent (especially on the high-power audio output tube) or, in the most benign scenario, distorted sound. Many restorers customarily replace these parts with discrete wirewound resistors in order to eliminate the possible "time bomb" effect of an original Candohm part.


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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Sun 04, 2018 9:02 pm 
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Joined: Mar Thu 03, 2011 8:53 pm
Posts: 916
Location: Davis, CA
I am going to replace the candohm but want to confirm the wattage of the resistors. The current through the candohm is 12.5/333= .0375 amps. Using the formula for wattage, W=IsquaredxR, I determined that 1/2 watt resistors would be more that adequate for R10 and R11 and a 2 watt for R9. Does this look ok? Do the resistors need to be wire wound?
Thanks, Bob


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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Mon 05, 2018 1:19 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4199
Location: Near Brandon, Iowa
rck46 wrote:
I am going to replace the candohm but want to confirm the wattage of the resistors. The current through the candohm is 12.5/333= .0375 amps. Using the formula for wattage, W=IsquaredxR, I determined that 1/2 watt resistors would be more that adequate for R10 and R11 and a 2 watt for R9. Does this look ok? Do the resistors need to be wire wound?
Thanks, Bob

One industry practice is to derate the resistors calculated-power-wise by 70%, and then use the next-higher standard power rating. I generally derate by 100%. You don't have to use wirewound resistors in this application but wirewounds are less susceptible to value drift over time than other types... probably not a big concern here.


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 Post subject: Re: Substitue Electrolytics for Airline 62-425
PostPosted: Mar Tue 06, 2018 9:01 pm 
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Joined: Mar Thu 03, 2011 8:53 pm
Posts: 916
Location: Davis, CA
I finished the electronic restoration of this radio replacing all the paper caps and out of spec resistors. I used 22 and 10 mfd electrolytics for C9 and C10, respectively, and was able to stuff them inside the old electrolytic case. I also replaced the candohm resistor using a 200 ohm 2 watt resistor for R9, a 33 ohm 1/2 watt resistor for R10, and a 100 ohm 1 watt resistor for R11. B+ voltage is 143v. Voltage along the candohm string starting with point F on the circuit diagram is -10.9, -4.3, and -3.2. Radio is playing nicely. Thanks to all for the help.

Bob


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