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 Post subject: Zenith Radio Nurse
PostPosted: Sep Sat 29, 2018 9:42 pm 
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Joined: Dec Tue 26, 2017 5:16 am
Posts: 15
After years of searching, I finally acquired one of these Noguchi-designed baby monitors:

http://www.slate.com/blogs/the_vault/20 ... _baby.html

As the one I got did not come with its transmitter, I thought I could just tap into its amplifier and attach it to something else, like an Echo Dot. (It normally works on a 300kHz signal transmitted over the power line, like a one-way version of intercoms made back then.)

Over the past few days I recapped it, measured all the voltages, then gave it a try.
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When I introduce a line level signal (from an iPhone headphone jack over a 0.1mF capacitor) at the grid of the 41 tube (the penthode), I get a clear but very low level output. If I input the same signal at the grid of the 6F5G triode, I get a slightly louder output but it distorts and levels off before it gets to an acceptable volume.

I have to believe this device was probably made to be louder, given its purpose. Also the output penthode is rated at over 5W and was originally intended for car radios.

Should I try it with a new set of tubes (probably not too easy to find. Also, I figure, unlikely to be bad, given how little use it seems to have had) or seek a defect somewhere else, like the field coil of the speaker, the potentiometer?

I would very much appreciate any suggestions.


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Sep Sun 30, 2018 7:49 am 
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Joined: Dec Tue 26, 2017 5:16 am
Posts: 15
Actually I did a whole bunch more measuring. The voltage at 6F5 screen is about the same as B+ (175 rather than the 82 shown on the schematic) but both R1 and R2 are new and at correct values. All other resistor and capacitor values are also correct.

Does this mean that the 6F5 is faulty?


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Sep Sun 30, 2018 9:16 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2222
Location: Saskatoon
If the plate voltage is the same as the B+ voltage then the tube isn't drawing any current. This can be caused by any of the following:
- The tube is bad,
- Bad socket contact,
- No continuity from cathode to ground,
- Tube is cut off due to negative grid voltage.


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Sep Sun 30, 2018 4:34 pm 
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Posts: 13260
Probably original coil is loading down the input. Try this...

Disconnect coil & cap connected to 6F5 grid.

Add a 4.7meg from grid to ground and use a .05 cap between input and grid.

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Tom


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Sep Sun 30, 2018 5:04 pm 
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Joined: Apr Thu 14, 2016 8:25 pm
Posts: 416
Location: pensacola fl
You may have to bypass the cathode resistor or change out the capacitor to a larger value. This would be necessary if there is distortion and or the treble is too high.
The mate to your receiver is called the The Guardian Ear Transmitter I think and it would be something if one could be found to go with your receiver. The schematic for it could be interesting to see.


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Sep Sun 30, 2018 6:25 pm 
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Joined: Dec Tue 26, 2017 5:16 am
Posts: 15
Here is the schematic for the Guardian Ear transmitter as well as description of how it works.

The grid voltage reads correctly, 2.7V (as fas as I can read on the schematic — there is a decimal point there).

— If the anode or cathode did not contact the socket, I thought it would not work at all.
— I did try lifting the grid wire and use a signal there.

In the end my suspicion is with the tube although how the tube on such a little used and clean device would be faulty, I am not sure. Unless that is why it was not used. I found a metal tube for $7 on eBay.

Incidentally, with or without the triode in place, there is a small voltage drop across R2, so it is not exactly B+ I cannot figure that out. All capacitors surrounding it are new and not leaky. Plugging the tube in or out does not change the voltage.

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 Post subject: Re: Zenith Radio Nurse
PostPosted: Oct Mon 01, 2018 11:08 am 
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Joined: Dec Tue 26, 2017 5:16 am
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While I wait for the triode to arrive, I tried using a 1:26 ratio interstage transformer to drive the pentode directly from my outside line-level source. Actually works very well.


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Oct Sat 06, 2018 2:00 pm 
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Joined: Dec Tue 26, 2017 5:16 am
Posts: 15
Incidentally, without the 6F5 being plugged in (that is, in the receiver, the “Nurse”), there is still a small voltage drop across R2. Since the only paths from R2 to the ground are two capacitors, which should normally block the DC, does this mean there may be a DC leak across one of them, or is it normal because the incoming DC is not completely filtered and the AC element does waste over the capacitors?

I am not exactly new to tubes, but it is fascinating how much there is to learn and understand!


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Oct Sun 07, 2018 3:04 am 
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How much voltage drop are you talking about?? Just saying it has drop doesn't tell us much.

Low impedance meters can cause a few volts drop in readings across resistors of 60-80K or higher.

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Tom


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Oct Thu 11, 2018 5:26 am 
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Joined: Dec Tue 26, 2017 5:16 am
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Wow, I did not think about that, thank you! That resistor is 330K so that is very possible, isn’t it?

I certainly need a better VOM.


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 Post subject: Re: Zenith Radio Nurse
PostPosted: Oct Fri 12, 2018 9:29 pm 
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Joined: Apr Thu 14, 2016 8:25 pm
Posts: 416
Location: pensacola fl
All the digital meters I own are in the megohm range above even my vtvm's which come in at 11 megohms. My two fluke digital meters are in 50 and 100 meg range depending on which one.


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