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 Post subject: Correct B+ voltage for Detrola 10ZM3
PostPosted: Jun Tue 18, 2019 4:39 pm 
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Joined: Jan Sat 12, 2019 9:50 pm
Posts: 8
I have a Detrola 10ZM3 that I have partially restored. I have the online Rider schematic, but it doesn't give the B+ voltages before and after the speaker voice coil. It's obvious that someone previously tried to rehab it, as it has a replacement power Xfmr. Of course the 5V and 6.3v windings are ok, but I only have 218V on the B+ to the CT of the output transformer. I've recapped the electrolytics, so there is zero hum. The speaker and output transformer are also not original, but they halfway work. I receive 3 local AM stations but have about 50% audio distortion. An external audio source injected into the audio preamp at the volume control has the same distortion. So my questions are:
1. What should the B+ voltage be at the CT of the Speaker?
2. If my speaker turns out to be unusable, and I replace it with a choke and permanent magnet speaker, what Henry size should the choke be?
3. The Speaker I have has the expected choke coil, and the expected voice coil, but has another coil inside the choke in series with the voice coil. Anybody know what that is?
ANY help is appreciated.
Jodie


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 Post subject: Re: Correct B+ voltage for Detrola 10ZM3
PostPosted: Jun Tue 18, 2019 4:49 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 34372
Location: Maryland 20709, USA
Jodie wrote:
3. The Speaker I have has the expected choke coil, and the expected voice coil, but has another coil inside the choke in series with the voice coil. Anybody know what that is?
Hi Jodie,

An electrodynamic speaker has two or three coils.

The large one (usually at the back) is the field coil.
The voice coil is obvious.

A small one concentric with those at the front of the field coil is a hum-bucking coil.
It samples the AC hum from the field coil (when it's used as a choke) and feeds that
sample to the voice coil to cancel the audible hum. The connections are important.
If it's connected in reverse, the audible hum will be increased rather than reduced.

- Leigh

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http://www.AtwaterKent.info
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 Post subject: Re: Correct B+ voltage for Detrola 10ZM3
PostPosted: Jun Tue 18, 2019 5:03 pm 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 1617
Location: Austin, Texas
B+ at 218V will be OK for the radio to operate.

Check the grid voltages on the 6F6s. It should be in the -10 to -20 volt range when measured relative to the chassis. If out of that range, look for the cause.

Jay


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