Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Mar Wed 29, 2017 6:00 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 4 posts ] 
Author Message
 Post subject: Magnavox CR-192 FM tuner question
PostPosted: Mar Tue 21, 2017 12:57 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 16511
Location: Warner Robins, GA
I recently got a Magnavox CR-192 FM tuner that has been recapped, out of tolerance resistors replaced and the conversion to a 1629 eye tube done.

Something was rattling around in the discriminator transformer so I removed the shell and reattached one of the nuts holding the trimmer cap.

The primary coil had to be pushed through the side of its bracket for the shell to come off.

I noticed one wire on the primary was free. Looking at the schematic I see the winding is not tapped and has only two connections which were connected still. Where does that third wire go?

Here's a couple pictures of the transformer. The secondary which is the coil on top does have a center tap, but all three of its wires are connected.

Image

Image


Also I have another question.

This uses a 6X5 tube and the tube in there is not the X plate version.

Is that ok for this tuner or should I replace it with a different 6X5 or possibly go extreme and replace the power transformer with one that has a 5 volt winding and use a 5AR4? I'm not interested in using diodes and a dropping resistor to replace the tube.


Also does this tuner use any sort of de-emphasis circuit?

If not I will need to add one.

The tuner will be feeding the input of a Magnavox CR-198C console which judging by the volume control has an input impedance of 1 meg.

Service manual

http://www.nostalgiaair.org/pagesbymode ... 010327.pdf


Top
 Profile  
 
 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Mar Tue 21, 2017 1:13 am 
Member

Joined: Aug Thu 27, 2009 7:47 am
Posts: 2635
Location: Seattle 98125
Does the free wire wind back into the coil, or go off to another component? Might be related to the 20pF cap between the top of the primary and the center-tap of the secondary?


Top
 Profile  
 
 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Mar Tue 21, 2017 1:38 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 16511
Location: Warner Robins, GA
From looking at it the wire does wind back into the coil I think.

The 20pF cap connects to the top of the primary winding. The line is just missing on the schematic.

Tomorrow I will measure t he ohms of the primary and see if that wire goes to anything.

Radiomuseum has a better schematic and the 20pF cap does indeed connect to the top of the primary.


Top
 Profile  
 
 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Mar Wed 22, 2017 10:23 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 16511
Location: Warner Robins, GA
I checked the wire today and it shows no resistance to anything so I left it alone and put the cover back on.

Decided to do an alignment to see if the tuner worked.

The RF and IF alignment went ok.

The discriminator alignment I had to improvise since I do not have a functional RF generator that can do 10.7 MHz.

What I did is put a scope on the output and feeding the FM signal generator it's built in 400 Hz signal I adjusted the coil until I got a proper sinewave on the scope at maximum amplitude. I then set my VTVM to its zero center scale and adjusted the variable cap for as close to 0Vdc as possible then retouched the coil for a proper waveform and checked that the dc voltage didn't change.

At the moment I don't have access to a high impedance amplifier to check the audio signal quality.

I did notice the output is low at 10KHz but that is probably due to the capacitor used in the output circuit.

What I need is a proper de-emphasis circuit that will work with a 1 Meg load.

The eye tube had already been converted to a 1629 which is good.


Top
 Profile  
 
Post New Topic Post Reply  [ 4 posts ]  Moderators: Marcc, Norm Leal

All times are UTC [ DST ]


Who is online

Users browsing this forum: addyboy, Google [Bot], Norm Leal, Tarpawns, townlinemuseum, valley and 15 guests



Search for:
Jump to:  




















Privacy Policy :: Powered by phpBB