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Author Topic: Vintage radio power-up  (Read 5408 times)
N4NYY
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Posts: 4758




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« Reply #15 on: March 02, 2013, 05:02:41 AM »

So back to my original question. I am reading that leaving the tubes in there sockets during a slow power up using a Variac will not hurt anything?

Thanks,

K2OWK


I do not think it will hurt anything that is good. If anything is on the fringe of blowing, it won't matter. I have gotten lucky the 2 or 3 times I have done this. I have had an instant where I fired something up to full AC and POP!
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AC2EU
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Posts: 399


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« Reply #16 on: March 02, 2013, 06:46:44 AM »

There are no guarantees that nothing bad will happen no matter how many precautions are taken. Something could have a sudden random breakdown . The rectifier is the only tube that could be damaged and that would be because of a bad cap.
 However, if you sub the rectifier with diodes, know that this will increase the the voltage seen by the caps at full AC input voltage by 20 to 40 volts, so don't go full scale.
Also, if the diodes are driving a shorted cap, they are  more likely to smoke the power transformer than a tube  ( no current limiting), so I would rather sacrifice a common tube.

I leave the tubes in, but do a quick check for shorts on all of the electrolytics before powering up. Then I monitor the the output of the rectifier as I power up.
I have had no problems with this method

 
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G3RZP
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Posts: 4590




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« Reply #17 on: March 02, 2013, 10:08:15 AM »

Once you've gradually got the electrolytics to accept full working volts using the Variac, you remove the silicon diodes and put the rectifier tube back.

Where there are selenium rectifiers involved, I would change them to silicon with a series resistor, because selenium, when they go, they often produce evil smelling and possibly semi-toxic smoke.
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W2WDX
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Posts: 188




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« Reply #18 on: April 18, 2013, 07:00:59 PM »

Cathode stripping and excess current draw on filaments. Tube filaments are very sensitive to low voltage. Also cathode stripping may actually severely reduce emission of the tube rendering them weak. Remove all the tubes when bringing a radio up on a variac. The only thing you are doing with the variac is not stressing the caps and transformers with inrush currents after sitting idle. The tubes can be damaged by this process however. You do not need the tubes in place to do this procedure, and it is not good for them anyway.
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K2OWK
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Posts: 1061




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« Reply #19 on: April 19, 2013, 05:36:37 PM »

I powered up the National NC=98 with the tubes in the socket's. I used a Variac and stepped up the voltage 30 volts at a time until 120 volts was achieved. I left the receiver for about 3 hours at each voltage level. After the slow power up, I checked the receiver and it operated perfectly. It works on all bands with decent sensitivity for a unit this old. It has some good features I like like a built in antenna tuner, and 2 phasing filters. I am very happy with this restore, and would like to thank all the people on this thread for there advice.

73s

K2OWK

PS: Next restore is an Emerson model # 558. It was one of the first sudo pocket portable radios made in 1948. I had one then and used to take it to the beach in the summer. It has four tubes and operates from a 1 1/2 volt A- battery ("D" flashlite cell), and a 67.5 volt "B" battery which is easy to fabricate from 7 or so 9 volt batteries in series (yes they fit in the case), and should last much longer then the originals.
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KW3U
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Posts: 8




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« Reply #20 on: April 23, 2013, 08:04:15 AM »

I have just obtained a dx20 in good condx, and have new filter caps to install,
but no .1uf @ 1200vdc, but did cobble together two small new types for a reading
of .15uf(both caps are 630v types) would they be an acceptable replacement?
tnx jim kw3u
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K2OWK
Member

Posts: 1061




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« Reply #21 on: April 23, 2013, 11:39:31 AM »

Hello KW3U, To answer your question the capacitors you have will not work. The voltage is to low. A .1uf cap at 1200 VDC is a common unit and available from many sources, "Radio Shack" included. While higher capacitance would be acceptable the 630 volt in place of the 1200 volt would most likely fail.

73s

K2OWK

PS: you should start a new thread for this question as this thread is not for the problem you are asking about.
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KD0REQ
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Posts: 937




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« Reply #22 on: April 23, 2013, 01:12:28 PM »

@W2WDX: if the purpose of this exercise is to reform the B+ filter caps, and there is no rectifier tube in socket, how do you get DC to the filter caps to reform them?

a known alternate is to take one of the 40s/50s capacitor checkers from the cap makers, which were kitted in the 60s by Heathkit as well, that have a neon bulb in series with the cap load as well as a pot to sneak up the voltage.  creep up the volts knob, and if the neon bulb lights, the cap is at breakdown, back off and leave it there for a while.

the modern trick of taking a couple silicon diodes in place of the rectifier tube and creeping up the Variac lacks detection of the current knuckle of a capacitor that is starting to conduct. putting a neon bulb in series with the diodes on an octal (or 7-pin or whatever) plug might be the safest way to go.  if you rigged a jig to slowly ramp up the voltage automatically, use a photodetector at the neon for a safety cutout.
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W2WDX
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Posts: 188




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« Reply #23 on: April 24, 2013, 12:39:50 AM »

Doh!

KD0REQ is absolutely right. I left an important part out, literally and figuratively. Some people usually keep a few spare rectifier tubes around for doing this and I usually replace the rectifiers with NOS anyway on any radio.

In my shop I have a bunch of plugin sockets I have strung some hefty diodes and dropping resistors for this purpose. Basically solid state plugin replacements.

Anyway I still insist leaving in the OTHER tubes in situ on a slow gradual power up may cause the cathodes to strip and significantly reduce emission or damage filaments due to low voltage. You have to remember the filament transformer is a low voltage or part of the main power transformer and a couple of volts under-voltage can significantly increase current draw through the filaments. Simply because some people say they have done this with the tubes in place without a problem doesn't mean it works. In my experience it's generally poor practice.

I can't believe I left out that detail. Thanks for the heads up KD0REQ.

BTW, in radios that have a separate plate transformer, where no other voltages are derived from the same transformer I sometimes wire a varistor (like a CL-60 type) on the primary. This way the "klunk" and stress on the transformer and caps is reduced by this slow start up. If the transformer derives any other voltages other then for plates I may not do this, especially if it has the filament & screen voltages also on the secondary.

John, W2WDX
« Last Edit: April 24, 2013, 12:54:30 AM by W2WDX » Logged

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