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 Post subject: Magnavox CR-192 FM tuner alignment issue
PostPosted: Mar Tue 21, 2017 12:57 am 
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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


Last edited by Tube Radio on May Fri 05, 2017 6:46 pm, edited 1 time in total.

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


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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Mar Tue 21, 2017 1:38 am 
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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.


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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Mar Wed 22, 2017 10:23 pm 
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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.


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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: May Mon 01, 2017 3:39 am 
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Location: Warner Robins, GA
An update.

I just found the cable I had made for an external input to my CR-198C console and connected the tuner to my McIntosh C-24 preamp and set the preamp to mono.

The tuner receives great and sounds quite nice with excellent sensitivity and selectivity.

Concerning the de-emphasis circuit I do plan on adding tweeters back to my CR-198C so it will be full HI-FI again.

Would cable capacitance be an issue with the de-emphasis circuit?

That said when the tuner was connected to the preamp without the cap disconnected it sounded ok, although it has 47uS de-emphasis.

I've got to take the tuner to work tomorrow so I can install the LED dial lamps so I may play around with it using the RF generator I have at work and see how well the higher frequencies pass through the tuner without the capacitor. I may also compare it to my stereo tuner and see how it compares then insert the capacitor and see if the output is reduced by the same amount as the stereo tuner with and without the capacitor installed. If it is not right with the cap installed or removed I may try a 750pF cap and see how well it then performs.

That said 470pF may have been correct when the cable capacitance from the terminal strip to the output jack, the cable from it to the input of the CR-198C and the internal cabling from the input to the switch and volume control is taken into account. Then again the circuit might use a lower time constant to optimize it for the console which didn't reproduce the treble all that good stock.


That said I may leave the 470pF cap installed and once I get the tweeters back in the CR-198C I will see how it sounds. If it is good enough as is I'll leave it alone.

EDIT:

Here's a picture of the dial lamps installed.

Image

Didn't take any pictures of the dial lamps, but next time I have the tuner apart I'll take pictures and post them here.

I just so happened to have some black double sided tape so I used that on the top and bottom of the dial glass to block the light from going anywhere but through the edge of the dial.

The dial isn't quite as bright and the color shown is almost correct. The dial lights up a whitish color with a hint of light yellowish green.

I like the tuning eye, but it is a bit bright for my liking. I may disconnect it and use the tube with my RCA K-80 console.


Last edited by Tube Radio on May Fri 05, 2017 6:49 pm, edited 1 time in total.

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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: May Wed 03, 2017 3:46 am 
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Location: Warner Robins, GA
Since I had to work on the chassis I decided to take pictures of how the dial lamps are mounted.

Image

Image

Image

I also removed the eye tube which to me made the dial look better.


Last edited by Tube Radio on May Fri 05, 2017 6:47 pm, edited 1 time in total.

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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: May Fri 05, 2017 12:37 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17484
Location: Warner Robins, GA
Found the problem.

There's some distortion on some bass so the discriminator must not be aligned right.

Will take it to work tomorrow where I got an RF generator that will do 10.7MHz so I can adjust the discriminator by the instructions instead of the improvised way I did it.

Will have to use a scope for DC measurements as I don't have a VTVM or high impedance analog meter at work.

I'll just go through the whole alignment procedure again.


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 Post subject: Re: Magnavox CR-192 FM tuner alignment issue
PostPosted: May Fri 05, 2017 7:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17484
Location: Warner Robins, GA
Well everything was going good then I noticed I could no longer get the discriminator set right.

Turned off the RF and found a sinewave around 10.7MHz still present. Disconnected the connection to pin 8 of the 6SBY7 and the sinewave went away.

Connected just a regular lead and the sinewave was back.

Checked the other pins of the 6SB7Y and I get the sinewave also when connecting to pins 3 & 4

Don't know what the problem is as this morning it was fine.


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