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 Post subject: 6-S-532 transformer
PostPosted: Mar Thu 14, 2019 10:52 pm 
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Location: saint clair shores, michigan
I picked up a 6-S-532 Zenith today. The radio had some weird "repairs" done, such as the tone control switches replaced with rocker switches and pieces of cardboard covered with fake wood contact paper. The power transformer was also replaced. I need help determining if the replacement is correct for the radio. The label on it reads:
Primary 117v 60hz - black
240-0-240v @ 70ma dc - red
CT - red-yellow
6.3 @ 3 amp - green

The schematic has readings of 15 ohm for the primary
0.5 ohm for the two dial lamps
600 ohm for the center tapped output

The 3 amp capacity for two dial lamps seems to be overkill, while the 70 ma CT seems kind of low. I understand when replacing a transformer extra amperage is not bad, if that is all you can find.

Any education would be appreciated.

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 Post subject: Re: 6-S-532 transformer
PostPosted: Mar Thu 14, 2019 11:32 pm 
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Location: Nipomo, CA 93444
That 3 amps isn't just for the pilot lights. You've got 6 tube filaments to light up as well. If you replace the 6X5 with discrete diodes(as everyone will advise), you can save some filament current.

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 Post subject: Re: 6-S-532 transformer
PostPosted: Mar Fri 15, 2019 12:04 am 
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Location: Annapolis, MD
You are definitely in the ballpark....

Total dial lamp and filament current looks to be close to 3 amps, but look up each tube to be precise---https://frank.pocnet.net/. You can reduce the filament load by bypassing the 6X5 with Si diodes**, and disconnecting the filament. That reduces the load on the power transformer and also helps with problem #2

#2: high voltage....note that the schematic calls for 252VAC at the rectifier plates (wondering about the reference point, but maybe not an issue)
If you do the Si diode mod, rectification efficiency will be higher and so your 240-volt spec may actually put the HV DC on the high side. (not a problem, but a series resistor will reduce it)

HV current: The 6F6 wants about 40mA, so I'm guessing that 70 wlll be adequate. If the transformer does not get too hot to touch, you are probably OK



** Note that the 6X5 is a known weak link, so the Si diode bypass is recommended regardless.

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 Post subject: Re: 6-S-532 transformer
PostPosted: Mar Fri 15, 2019 4:24 am 
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Location: saint clair shores, michigan
Quote:
That 3 amps isn't just for the pilot lights. You've got 6 tube filaments to light up as well.

OOPS :oops:
Too many AA5's lately.
Well I guess the best news is that I won't have to find another transformer.

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 Post subject: Re: 6-S-532 transformer
PostPosted: Mar Fri 15, 2019 4:35 am 
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Location: saint clair shores, michigan
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If you replace the 6X5 with discrete diodes(as everyone will advise), you can save some filament current.


Since the 6x5 is a full wave rectifier, could a full wave bridge be used? I have quite a few in the parts bin, and it seems it would be the easiest replacement.

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 Post subject: Re: 6-S-532 transformer
PostPosted: Mar Fri 15, 2019 4:46 am 
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A full-wave bridge is used with a transformer with no center-tap......in this case, it would have to be a single 240-volt winding with larger wire than what you have now. This is because the winding works "full-time", whereas each half of a center-tapped winding only works half-time.

Just use 2 1N4007 or equivalent (Some of us buy the 1N4007 50 or more at a time---they are cheap)

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 Post subject: Re: 6-S-532 transformer
PostPosted: Mar Fri 15, 2019 5:27 am 
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Location: saint clair shores, michigan
Thanks for the schooling, Mark. The filaments and dial lamps draw 3.06 A. I am pretty sure I can find some 1N4007 diodes in the parts bin.

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 Post subject: Re: 6-S-532 transformer
PostPosted: Mar Sat 16, 2019 12:12 am 
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Location: Near Brandon, Iowa
1968rt wrote:
Quote:
If you replace the 6X5 with discrete diodes(as everyone will advise), you can save some filament current.


Since the 6x5 is a full wave rectifier, could a full wave bridge be used? I have quite a few in the parts bin, and it seems it would be the easiest replacement.

Yes, you can; but since the transformer is center-tapped and the CT is ground-referenced, you will need to clip off the negative output terminal of the bridge and ensure that it can't contact anything in the radio. The outer ends of the HT winding connect to the "AC" terminal of the bridge and the bridge positive output terminal connects to the B+ source node (formerly the cathode of the 6X5 tube). This essentially "throws away" half of the bridge but allows it to function as a full-wave rectifier. Doing this might seem pointless but it might be a little cleaner wiring-wise than mounting discrete diodes on terminal lugs.

Under no circumstances connect the negative DC output terminal to anything. A dab of epoxy over the cut-off stub might be a good idea.

Use a bridge rated for at least 2 amps at 600v or more.

Since forward voltage drop through the bridge (or any silicon diode) is going to be about 20V less than that of a vacuum tube rectifier, you may find it necessary to insert a dropping resistor to bring the B+ voltage down to approximately the correct value.


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