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 Post subject: Magnavox CR-192 FM tuner question
PostPosted: Nov Tue 28, 2017 1:49 pm 
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Location: Warner Robins, GA
I have a Magnavox CR-192 FM tuner I restored to working condition including adding an AFC circuit and getting the alignment pretty much perfect so that the IF has enough bandwidth to feed a MPX decoder and it gives pretty good stereo separation.

The problem with it is the audio seems a bit bass heavy when feeding the MPX decoder and also when directly feeding an amp with a high impedance input both without the de-emphasis circuit connected.

Any idea what could cause that?

One would think based on how it sounds that the treble is being cut, but that doesn't seem to be the case.


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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Dec Sat 02, 2017 3:35 pm 
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Could it be something that is reducing the treble some as I go higher in frequency while still allowing for decent stereo separation?

Maybe it is a problem with the MPX decoder.

I did find the solid state MPX decoder I made several years ago. I might try that one with a buffer between the high impedance output of the tuner and the lower input impedance of the MPX decoder and see if it sounds any better.


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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Dec Sat 02, 2017 4:24 pm 
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How does the tuner sound with its original mono output circuit connect to your amplifier?

Dave


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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Dec Sat 02, 2017 4:43 pm 
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I think it sounds a bit heavy in the bass using the mono output, but I cannot remember as it's been several months since I've used the tuner.

This was originally meant to connect to a Magnavox CR-198C console which had no tweeters so it wouldn't have much mattered if the sound was a bit on the bassy side and might have even made the console sound better.

Here's the discriminator circuit.

Attachment:
Discriminator.jpg
Discriminator.jpg [ 68.7 KiB | Viewed 334 times ]


I don't see anything that would bwe causing the apparent overly bassy sound.

If needed I can email you the full schematic.

I don't think I have a buffer amp I can use to connect the tuner's output to my McIntosh C-24 preamp unless one isn't needed for that preamp. I woudl also have to make up a proper de-emphasis filter.

When getting the alignment perfected for stereo I did replace the shielded audio wire with a good quality low capacitance shielded cable to ensure there was minimal cutting of the highs by the cable given the tuner needed to pass up to a 53KHz signal for proper stereo decoding.


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 Post subject: Re: Magnavox CR-192 FM tuner question
PostPosted: Feb Fri 09, 2018 1:38 pm 
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Back at this.

I don't see anything that would make the tuner sound overly bassy.

I did a test as I recall which was to switch the AFC on and off with no apparent change in how bassy the audio sounds.

I do know when fed to my Ampex 520 MPX decoder the audio seems to be in stereo far as I can tell, although I don't know how good the stereo separation really is.

What I really need to do is bring the tuner back to work and do an audio sweep from 20-20KHz and see if the audio signal remains at the same voltage level which would mean a flat audio response.

If the audio does start to reduce in level in the treble there's nothing I can really do as I got the IF aligned just about as perfectly as I possibly could which was necessary for the MPX decoder to even produce anything even remotely resembling a stereo output. I could try stagger tuning the IF stages for better bandwidth and have a reduction in sensitivity, but at one point during the alignment I accidentally found out how to display the S curve and it seemed like it had enough bandwidth.


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