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 Post subject: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Mon 20, 2017 2:38 am 
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Joined: Aug Sat 31, 2013 11:22 pm
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Location: Ft.Worth, Texas
Does anyone have any idea why my Zenith R7000 Transoceanic blows its illumination bulbs when I am using the AC cord? No issues on battery power. Everything else works perfectly. I have not replaced any capacitors. Thanks

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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Tue 21, 2017 4:08 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1121
Location: Las Vegas, NV, USA 89052
Didn't bother to look up the schematic, but I think I remember the dial lights are always ON when unit ON & AC powered? Would guess bulbs are powered straight from 12(?) volt step down transformer... Are lights both burning out, blowing suddenly on power up, or burning out eventually after too short life time? What's voltage at wall socket? What's voltage at bulb socket? Correct bulbs? Is some other heavy use device plugged into the same sub-circuit surging the line? Questions, questions, questions :D

In any case, if over voltage is burning out the bulbs, probably best to correct problem before running the radio much. Over voltage would probably be equally bad for ICs, etc.


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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Tue 21, 2017 4:23 am 
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Joined: Aug Sat 31, 2013 11:22 pm
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Location: Ft.Worth, Texas
Greg Dan wrote:
Didn't bother to look up the schematic, but I think I remember the dial lights are always ON when unit ON & AC powered? Would guess bulbs are powered straight from 12(?) volt step down transformer... Are lights both burning out, blowing suddenly on power up, or burning out eventually after too short life time? What's voltage at wall socket? What's voltage at bulb socket? Correct bulbs? Is some other heavy use device plugged into the same sub-circuit surging the line? Questions, questions, questions :D

In any case, if over voltage is burning out the bulbs, probably best to correct problem before running the radio much. Over voltage would probably be equally bad for ICs, etc.


Greg, after furthur investigation, I am getting 17 volts at all 3 bulb sockets, both with AC and DC power. I would think that I shoule be getting around 14 volts. I buy the bulbs from a supplier on Ebay so, I have to assume they are correct. Onnce, all 3 bulbs blew on initial power up on AC. The second time, they blew prematurely using DC power. I have the schematics but they are too complicated for me to understand.

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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Wed 22, 2017 3:03 am 
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Joined: Oct Mon 26, 2009 10:02 pm
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Location: Indianapolis, IN
Cliff, this may be a "dumb" question since I'm not familiar with the circuit, but how many batteries are used for DC operation ???

I can see 17 volts happening with an unregulated AC power supply, but am surprised that they'd need or use that high of battery voltage...

John


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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Wed 22, 2017 3:17 am 
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Joined: Aug Sat 31, 2013 11:22 pm
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Location: Ft.Worth, Texas
John, it uses 8 D cells for operation. Something, maybe a voltage regulator, on one of the 5 circuit boards is stepping up the voltage to the bulbs. The bulbs are 14 volt. The schematics for this radio are a nightmare to read for me. Its several pages long. On a side note, I ran the radio all day today, turning it off and on several times, and it never blew a single bulb. Maybe it straightened itself out. I dont know. Great radios when everything is working right but a beast to repair.

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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Wed 22, 2017 4:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Utica, NY 13502 (USA)
The bulbs appear to operate directly from either the battery, external DC, or internal AC power supply depending on which one is used. I would measure the voltage being applied to the bulbs in each case.

Attachment:
Zenith R7000 Power Supply Sch.JPG
Zenith R7000 Power Supply Sch.JPG [ 164.71 KiB | Viewed 253 times ]


It is interesting that CR501 and CR503 in combination with R503 and R504 are used only to light the dial lamps during AC operation. If either or both of those resistors have dropped in value and/or if one of the diodes is shorted, the voltage applied to the bulbs could be higher than 14 volts depending on your mains line voltage. Normally, the dial lamps will be off during battery or external DC operation unless the momentary dial light switch is thrown. They will normally be on during AC operation and will go off if the dial light switch is thrown. The dial light switch as shown in the schematic is in the momentary "on" position. Normally, it would be in the middle position.

Dave


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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Wed 22, 2017 11:37 pm 
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Joined: Aug Sat 31, 2013 11:22 pm
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Location: Ft.Worth, Texas
Thanks Dave. I can see that now. When the bulbs were blowing, I had 17v at the sockets with AC or DC. Ive got around 13v now. It seems to have straightened itself out somehow.

I do have another concern. When I leave this radio plugged into the wall and turned off, the transformer on the power supply feels warm. Not hot, but warm. The top of the transformer is seen and can be felt with the battery cover removed. Does this seem normal? Thanks

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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Thu 23, 2017 12:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19401
Location: Utica, NY 13502 (USA)
I was concerned about having the transformer always "on" regardless of whether the radio was on or not. I installed a small, in-line switch on the power cord. But I don't remember if the transformer became warm.

Dave


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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Thu 23, 2017 3:41 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4971
Location: Indianapolis, IN
Cliff,

Have other solid-state radios with built-in AC supplies and it seems normal that they get slightly warm after being plugged in awhile---even with no load and set off (remember, the transformer primary does load the AC line and consumes very small amount of power)... I don't leave any of them plugged in when not in use---even have a power-strip for TV and stereo system which I turn off when not using. Know that they are designed to be left powered, but like to minimize any risk of possible damage from power surges or lightning strikes...

John


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 Post subject: Re: Zenith R7000 T/O Blows Bulbs on AC Power
PostPosted: Nov Thu 23, 2017 3:46 pm 
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Joined: Aug Sat 31, 2013 11:22 pm
Posts: 2306
Location: Ft.Worth, Texas
Thanks John and Dave! I like the idea of just keeping it unplugged from the wall until I want to use it. Have a great Thanksgiving!

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