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 Post subject: marconi model 157
PostPosted: Nov Fri 11, 2016 12:46 am 
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I picked up a Marconi model 157 and when I plugged it in it made a little hum but no reception so I went ahead and did a total recap one cap and one end at a time and now I have a little hum at low volume and when I turn up the volume I get a loud motor boating sound any idea where I went wrong thanks a0e 1k0 nl


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 12:55 am 
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Location: Hudson, MA
Are you sure you got the electrolytics polarity correct.
I also noticed the radio has a 1.2V bias cell , have you replaced this.

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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 1:02 am 
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yes the polarity should be right and I do not have a 1.2v bias cell


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 1:15 am 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2140
Location: Santee Calif. 92071
Boil a cup of water. Drop bias cell into the hot water. There will be little bubbles come from the cell. Let cool off the water will be sucked back into the cell. Use tooth pick with a little super glue around the edges. Don't get any in the middle of the carbon disk. Cell will now have voltage again. Positive end goes to ground.


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 1:26 am 
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two things do I leave the cell in the water to cool off or do I take it out right away and let dry off and second is there any way to test the bias cell


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 2:02 am 
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Location: Santee Calif. 92071
Leave the cell in the hot water until the water cools off. The expanded hot air inside the cell leaks out as little bubbles. When the water cools off with the cell still in the water it will pull water back into it. You can test the cell with VTVM. Only a little over a volt is what to expect. Will last a long time.


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 2:11 am 
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thunderbird281 wrote:
Leave the cell in the hot water until the water cools off. The expanded hot air inside the cell leaks out as little bubbles. When the water cools off with the cell still in the water it will pull water back into it. You can test the cell with VTVM. Only a little over a volt is what to expect. Will last a long time.

I have never worked with these, it would be interesting to know the cell voltage before and after rejuvenation.
Could a 1.5V watch battery be used instead?

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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 2:19 am 
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Location: Santee Calif. 92071
Watch battery is normally 3 volts. To much for bias. The original cell by now would be dead. Read zero. That leaves a floating grid. Tube won't work that way. The cell just needs a little water as like a car battery would. Have two radios I have done this to. Both still play after being on display for 5 years or more.


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 2:22 am 
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that being said I should be able to test the bias cell in the radio before I try to rejuvenate it correct or not


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 2:25 am 
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Location: Santee Calif. 92071
Yes that's right.


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 6:47 am 
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Joined: Nov Mon 05, 2007 11:08 pm
Posts: 2372
Location: Calgary Alberta
I have restored many Marconi and Rogers radios and I just use a 1.5 volt watch battery.
I put it in a battery holder because I don't want to solder the battery itself. It gets too hot using solder directly on the battery, then the battery is no good.
Here are some photos of how I use the battery holder, using a bit of glue gun glue to hold the battery in the holder.
Dan , in Calgary


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 9:27 am 
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perfect job but can I use a 1.5 volt flashlight battery if when I test mine I need to replace it or at least temporarily hook up the flashlight battery


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 10:32 am 
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Location: Victoria, Australia
As long as you get the polarity right any cell of 1.5V can be used. The thing with the button cell and any other, the current drain from the grid is infinitesimally small, so any cell is going to go storage flat. So a "D" cell is an impractical choice long term.

Marc


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 11:09 am 
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so the positive side goes to pin 6 of tube 6k7 correct


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 11:40 am 
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I see no schematic: The cell negative end goes to the TC (grid 1) possibly via the IF transformer secondary. Pin 6 is a dead pin on 6K7, it not connected to anything inside the tube. So in the absence of a circuit it could have anything on it (node of convenience). The positive end of the cell will end up going to chassis.

Marc


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 12:11 pm 
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Joined: Mar Fri 07, 2014 10:20 am
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I just hooked up a 1.5 v battery with two jumper cables and still got the same motor boating sound so this sound came in the radio after I recapped it so could there be a bad cap or something I did but I took each cap off one end at a time and replaced it that wat


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 1:14 pm 
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Joined: Mar Fri 07, 2014 10:20 am
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if I disconnect one side of the bias cell should the motor boating go away


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 1:25 pm 
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Location: Annapolis, MD
spsquires wrote:
if I disconnect one side of the bias cell should the motor boating go away

Not possible to answer that...
When something is not working properly, the first step is to be sure everything is wired correctly and that there are no bad components.
Since you have been replacing things, I would start by checking everything against the schematic. In the process, be sure there are no bad solder joints.

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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 1:32 pm 
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Location: Annapolis, MD
PS....
We normally do not trouble-shoot by disrupting the operation of a circuit---especially doing something like removing bias, which puts a tube in an unknown state.
It can, however, be appropriate to isolate a problem by disabling the transmission of a signal. For example, you might short out the primary or secondary of an IF transformer to see if it stops an oscillation problem.


Check everything against the schematic first.......

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: marconi model 157
PostPosted: Nov Fri 11, 2016 6:43 pm 
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Joined: Mar Fri 07, 2014 10:20 am
Posts: 1903
I started my trouble shooting and found that when you take the wire off the top of tube 6r7 the noise goes away and I noticed that wire is connected to the negative side of the 2 electrolytic caps 10 and 20 uf with a resistor and I have the negative of those two wires joined together and connected to the wire that came from the canister with the caps in would that cause the problem


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