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 Post subject: Capacitor choice for a transformer secondary
PostPosted: Mar Tue 04, 2008 9:14 am 
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I want to replace two .003 uf @ 1500 volt capacitors on a power transformer secondary in a Philco Tropic model 42-760. Here is the link to the schematic from nostalgiaair.org (the parts list says 1500 volt rating, and so do the actual capacitors in the set, BTW):

http://www.nostalgiaair.org/PagesByMode ... 013549.pdf

My question is, what type of capacitors do I replace them with? Do I need to replace them with safety caps, or will normal caps be ok? I have some 1600 volt orange drops that I can install, or if I can locate them...I might have two 630 volt poly film caps. If I have to use safety caps, what would be the best ones to install?

Thanks,
John S.

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PostPosted: Mar Tue 04, 2008 12:15 pm 
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use the 1600V. they will be fine that's what i would do SMITHY


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 Post subject:
PostPosted: Mar Tue 04, 2008 5:15 pm 
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I'd be tempted to leave them out altogether. It's unusual to see caps in that position in a power supply.

It might be interesting to experiment with putting caps in or leaving them out and see if you can detect any difference in the operation of the radio.

High voltage ceramic caps (1 kV rating) would work well here. I've never seen one fail to a short. In fact, I'm not sure I've ever seen one fail, period.

A 630 volt poly cap would probably be OK as well. The value is small enough that the current will be quite low, so I would not expect the cap to be stressed by 60 Hz AC. Others will probably note that spikes can show up on the AC line, but I'm not sure this would harm a poly cap.

Don't know if it's easy to find a safety cap with a high enough voltage rating. Most are designed for AC line applications. The reasons for using a "safety" cap here would be different than typical AC line bypass considerations. Here you'd be looking to save your transformer in case of a capacitor short. Not quite the same issue as worrying about a short circuit on the AC line, which is a more serious fire hazard.

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 Post subject:
PostPosted: Mar Tue 04, 2008 5:34 pm 
Silent Key

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Might differ on one point. If you retain the caps in that spot, the peak voltage across each can easily approach 600 volts with a high line. The transformer secondary looks to be about 350 volts each side times 1.4. Go with 1000 volts though the 630 raing would likely last forever!


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 Post subject:
PostPosted: Mar Tue 04, 2008 7:07 pm 
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Thanks for the replies! I'm actually waiting for the delivery of the 1600 volt orange drop caps, and by the time they get here I should be nearly finished restoring the electronics, and be ready to put them in.

Tom said, quote: "I'd be tempted to leave them out altogether. It's unusual to see caps in that position in a power supply."

There must be a reason why the designers put them in there, so I am going to keep them in the circuit (no offense at all to Tom!). If anyone knows, let me know. The point is well taken on worrying about a capacitor short with these, and frying the transformer. I also thought about using ceramics in this application, but I don't have any. Perhaps in the future! I'm not aware of safety caps with a high enough voltage rating either, Tom.

So, I will stick with the 1600 volt orange drop ones. If anyone else has any suggestions, I'm all ears.

Thanks,
John S.

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 Post subject:
PostPosted: Mar Tue 04, 2008 7:47 pm 
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This set has an interesting power transformer set-up. Each side of the primary is connected to the chassis via resistors and the secondary is bypassed to the chassis with those .003uF capacitors. I would be inclined to eliminate the secondary bypass caps and use safety caps on the primary. They serve the same purpose.

The National HRO-50 and HRO-60 used a bypass capacitor on one side of the power transformer secondary to chassis (C-118, if I remember correctly) and when it shorted the power transformer was toast before the primary fuse could blow. I never liked bypass capacitors on the secondary of a power transformer for that reason. My HRO-60 works fine without that C-118 time-bomb on the power transformer secondary. --Ed

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 Post subject:
PostPosted: Mar Wed 05, 2008 7:52 am 
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Ed Engelken wrote:
This set has an interesting power transformer set-up. Each side of the primary is connected to the chassis via resistors and the secondary is bypassed to the chassis with those .003uF capacitors. I would be inclined to eliminate the secondary bypass caps and use safety caps on the primary. They serve the same purpose.

The National HRO-50 and HRO-60 used a bypass capacitor on one side of the power transformer secondary to chassis (C-118, if I remember correctly) and when it shorted the power transformer was toast before the primary fuse could blow. I never liked bypass capacitors on the secondary of a power transformer for that reason. My HRO-60 works fine without that C-118 time-bomb on the power transformer secondary. --Ed


Thanks, Ed (and Tom, too!), I understand what you are saying. Sorry to hear about your transformer, Ed...that must have been a real bummer. The radio I'm working on is a bit tough to find, and I'd rather not have to worry about having to replace a bad transformer for it. In other words, I'd like to keep the original in there, as it has a stamped date code of "8-41" on it. There's another transformer with that same date code in this radio, too. Pretty cool.

As far as the safety caps are concerned, should I connect across-the-line, or one side (or both) to the chassis? Would a .047 uf safety cap be sufficient in either case...and lastly, should I leave those resistors connected to the primary? Those probably won't short out like a normal capacitor would, but would there still be a need for them? I should also think about putting a fuse on the primary, at some time. How do I calculate the fuse rating (amps)?

John S.

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 Post subject:
PostPosted: Mar Wed 05, 2008 4:01 pm 
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I have been using 0.01uF "safety" capacitors for the AC line bypass duty. Recently I have started using two-prong polarized plugs on my refurbished radios and I put one capacitor directly across the power transformer primary and then bypass the neutral side of the line to the chassis with the power switch in the hot side of the line.

The usual arrangement with an unpolarized plug is to bypass each side of the line to the chassis. That will put about 60 volts AC on the chassis due to the "voltage divider" action of the capacitors between the neutral and hot sides of the AC line. I don't like this and have started to use the polarized plugs and the bypass capacitor setup described above. That will keep the chassis potential near ground. The other option is to use a three-prong power cord. --Ed

EDIT: I would remove those resistors from the AC line to chassis on the primary side of the power transformer. I haven't seen resistors used that way before and I have no idea how they would help anything. Literally millions of radios have been built without them. --Ed

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 Post subject:
PostPosted: Mar Wed 05, 2008 9:36 pm 
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Ed Engelken wrote:
I have been using 0.01uF "safety" capacitors for the AC line bypass duty. Recently I have started using two-prong polarized plugs on my refurbished radios and I put one capacitor directly across the power transformer primary and then bypass the neutral side of the line to the chassis with the power switch in the hot side of the line.

The usual arrangement with an unpolarized plug is to bypass each side of the line to the chassis. That will put about 60 volts AC on the chassis due to the "voltage divider" action of the capacitors between the neutral and hot sides of the AC line. I don't like this and have started to use the polarized plugs and the bypass capacitor setup described above. That will keep the chassis potential near ground. The other option is to use a three-prong power cord. --Ed

EDIT: I would remove those resistors from the AC line to chassis on the primary side of the power transformer. I haven't seen resistors used that way before and I have no idea how they would help anything. Literally millions of radios have been built without them. --Ed


Ed,
That's perfect, because someone installed a new power cord on this radio, and the plug is polarized. When you say "and then bypass the neutral side of the line to the chassis" , do you mean with another .01 uf safety cap, or connect the neutral wire directly to the chassis?

Just one more thing, before I remove the resistors. Would the resistors on the primary have anything to do with the dual voltage switch? As a guess, I don't think so, but I just wanted to bring this fact to the table. Having the resistors in there must be putting some kind of voltage on the chassis, and this is not good, IMO.

Oh, and I'll put the capacitor across the line after the power switch, just in case I accidently leave this plugged in. Although I usually unplug all my radios while not in use...I could fail to do so unintentionally. There's no use keeping the bypass cap across the line while the radio is turned off and plugged in.

John S.

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 Post subject:
PostPosted: Mar Thu 06, 2008 12:52 am 
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Yes, bypass the neutral to the chassis with a safety capacitor. Place one safety cap across the power transformer primary and a second one from the neutral side of the AC line to the chassis. Installing the safety cap across the primary insures that it is "off" with the radio, i.e. when the radio is off, there is no AC across the capacitor. When the radio is "on" both sides of the AC line are effectively bypassed to the chassis with the safety caps. The neutral is bypassed to the chassis through one cap, the hot side through 2 capacitors . . . the hot side is bypassed to the neutral which is then bypassed to the chassis. You should have no AC on the chassis with this arrangement.

Those resistors will indeed put an AC voltage on the chassis. You don't want or need that. It is not clear to me how those resistors are supposed to function, but the radio will function fine without them. --Ed

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PostPosted: Mar Thu 06, 2008 1:29 am 
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Kind of reminds me of the people who would use a 200 volt capacitor on the plate of an output tube that is running 150 volts B+. It works fine until you get an inductive kick from the transformer which shorts the output tube and starts melting down the power and output transformers.

The designers are not going to put in parts that are not needed, they built radios as inexpensive as they could. Those parts are there for a reason. Transformers and the inductive kickback have to be dealt with in some way, perhaps this radio had problems in that area and these resistors and capacitors are there for that reason.

I would not try to second guess the designer and would leave it like it was designed, you might save a transformer.

Dave


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 Post subject:
PostPosted: Mar Thu 06, 2008 1:41 am 
Silent Key

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To add to what Dave mentioned, I have a Philco battery portable set here and it runs off a 90 volt battery for the B+. All the paper bypass and coupling capacitors in it are rated at 200 volts. The one from the plate of the output tube to ground is rated at 800 volts! And that is what the 1940 schematic calls out. I am sure it was not selected just because it was handy, as the high voltage caps were quite a bit more expensive back then.
Curt

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PostPosted: Mar Thu 06, 2008 3:43 pm 
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easyrider8 wrote:
Transformers and the inductive kickback have to be dealt with in some way, perhaps this radio had problems in that area and these resistors and capacitors are there for that reason.
Dave


Can anyone explain the utility of connecting a 50K resistor from each side of the AC line to the chassis of a radio? There are millions of radios operating very well without these resistors. What do they do? Inquiring minds want to know. --Ed

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 Post subject:
PostPosted: Mar Thu 06, 2008 4:41 pm 
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I'll second Eds suggestion, scrap the secondary caps and rewire the primary to conform to established standards. Current NEC Code requires no more than a .0047 MF cap across the input line. Those .003's that you purchased will do fine there for a 120V input.

That radio was obviously designed to operate in countries where reliable, clean, and stable AC mains voltage was not the norm in many cases. Thus the method of trying to offer some means of spike suppression.

The National HRO-60 that Ed mentioned was also aimed at the Central/South American and other offshore markets in support of the numerous plantations and shipping lines that moved the goods. Removing the secondary cap in that radio is considered very acceptable by National restorers.

Carl
National Radio 1963-69


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 Post subject: Re: HRO-60 & Capacitor Replacement
PostPosted: Jun Sun 30, 2013 10:40 pm 
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Ed Engelken wrote:
This set has an interesting power transformer set-up. Each side of the primary is connected to the chassis via resistors and the secondary is bypassed to the chassis with those .003uF capacitors. I would be inclined to eliminate the secondary bypass caps and use safety caps on the primary. They serve the same purpose.

The National HRO-50 and HRO-60 used a bypass capacitor on one side of the power transformer secondary to chassis (C-118, if I remember correctly) and when it shorted the power transformer was toast before the primary fuse could blow. I never liked bypass capacitors on the secondary of a power transformer for that reason. My HRO-60 works fine without that C-118 time-bomb on the power transformer secondary. --Ed


I had some trouble with my HRO-60 (it kept blowing fueses and so became a dead radio). Your information was helpful when I attempted to bring it back. I wrote a short blog about my minor repairs ( http://kb1awv.com/?p=624 ). It includes a link to this discussion. Feel free to check it out and a related link which shows the unit in operation.
Thanks for the advice!

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 Post subject: Re: Capacitor choice for a transformer secondary
PostPosted: Jul Mon 01, 2013 12:43 am 
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They didn't put those capacitors in there for the heck of it.

The ones on the primary side are to keep RF from coming in through the power line. Back then there were streetcars and lots of AC/DC motors with brushes. Lots of noise on the AC line. The caps on the primary side are an attempt to attenuate some of that line noise. Also, to a lesser extent, to keep the radio's local oscillator from leaking back out through the power line.

The capacitors on the secondary are for a completely different purpose. The rectifier tube if you think of it this way, is a switch that is opening and closing. When its closed its coupling the secondary to the B+ (and through the first filter cap, to chassis or B-. When the AC voltage is near zero, this switch opens, and unhooks the secondary from chassis or B-. This can cause modulation of the received signal, a 120Hz hum pr buzz, as the receiver, if it's not really well grounded with a ground wire, is being coupled and uncoupled from the secondary ( and primary through capacitance).

So your better radios have one or more caps to couple the secondary to chassis or B-, all the time, not just when the rectifier tube is conducting.

Some radios get away without this by having an electrostatic shield built into the transformer, or by clever winding.


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 Post subject: Re: Capacitor choice for a transformer secondary
PostPosted: Jul Mon 01, 2013 5:21 am 
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There were more little doodads and parts put into vintage standard radios to deal with everything from unstable voltages, interference from navigation beacons, motors, car ignitions, trains, planes, motorcycles, and finally, the bane of near modern times, the television set! Electrical distribution systems so far as I know were better in the US and parts of Europe well before being sorted out elsewhere, so not all of us have experienced the adventures of progress. Guess we all started out wanting to light lamps and run motors, and the rest was "refinement." Nobody under 40 or 45 could possibly make any sense out of this post, so you are excused.

Now at the twilight of the age, new gremlins such as switching power supplies, dimmers, newfangled and some old fangled (flourescent) lamps, battery chargers, appliances, utility meters, cell phones, and noise levels of utility supplies absolutely unprecidented. So those "extra" parts we find in many old sets had a legitimate or at least sincere purpose at the time. So long as they are replaced with modern safety rated components and other appropriate considerations or culled if no apparent benefit is realized, so be it.


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