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 Post subject: Help with B+ voltages on a homebrew TRF
PostPosted: Feb Mon 26, 2018 7:57 pm 
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Joined: Aug Fri 21, 2009 7:45 pm
Posts: 785
Location: Port Dover, Ontario
http://www.nostalgiaair.org/pagesbymode ... 002718.pdf

Some time ago I purchased a homebrew TRF but was never able to get it up and running. Amongst other problems the field coil in the speaker was open and I tried to use a PM speaker.

Eventually, I decided to start from scratch and removed most of the parts with the exception of the tube sockets, the hand wound coils, the choke and the tuning assembly (which came from a Spartan set).

I used the schematic in the link to rebuild the set. My set has a 25Z5 rectifier and no pilot lights.

It is working reasonably well but with lower volume than I would like. I have a question about the B+ voltages.

On the 6D6 and the 43 I get about 117V but on the 6C6 I measure about 80V. Is this as it should be or is it too low?

I also notice that the volume is much better when I connect the aerial directly to the grid cap of the 6D6 rather than to the aerial lead.

Any advice would be much appreciated. Thank you.

Joseph


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 Post subject: Re: Help with B+ voltages on a homebrew TRF
PostPosted: Feb Mon 26, 2018 8:14 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37065
Location: Livermore, CA
Joseph

Voltage will be lower on 6C6 as the plate is supplied through a resistor.

Radio may need alignment, peaking? Two stages should be peaked to the same frequency. Use a station at high end of the band when adjusting trimmers. They aren't shown on your schematic but should be there.

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 Post subject: Re: Help with B+ voltages on a homebrew TRF
PostPosted: Feb Wed 28, 2018 5:28 pm 
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Joined: Aug Fri 21, 2009 7:45 pm
Posts: 785
Location: Port Dover, Ontario
Hi Norm

Thank you for the reply. As you noticed there are no adjustment spots on the circuit diagram.

There is one on each gang of the tuning capacitor. One of these has a noticeable effect but the one which is part of the antenna coil has no effect. I am going to go over the connections in that part of the circuit and make sure that I have things right.

Thank you for your help.

Joseph


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 Post subject: Re: Help with B+ voltages on a homebrew TRF
PostPosted: Feb Wed 28, 2018 6:19 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 33574
Location: Maryland 20709, USA
munali wrote:
There is one on each gang of the tuning capacitor. One of these has a noticeable effect but the one which is part of the antenna coil has no effect.
Hi Joseph,

This, plus the fact that sensitivity is worse when connecting the antenna to the antenna input, suggest that the antenna coil is damaged (probably an open winding).

That's a very common problem on older sets that used external antennas. Nearby lightning strikes would cause the antenna transformer primary to blow open.

- Leigh

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