Hey guys,

My Magnavox combo when watching a dvd or playing an audio cd kinda sucks compared to the quality of sound I get from the AUX input on the reciever. How would I be able to play a DVD and run the audio straight from the reciever instead of the TV audio? The knob for the unit if switched from the tv turns the set off.
I think I know how I can bypass that but how can I get better sound from the tv? I really dont want to modify a vintage set but if I did I know I could reverse it and bring it ack to stock form..

I think the real problem is that you've got the DVD player connected to the TV through an RF modulator, so your audio (which is already separate) is getting modulated into an RF signal, sent through the tuner, tuned in, then separated back out again. Does the TV have it's own audio output stage, or does it just feed the audio into the amplifier chassis? IIRC, the Magnavox sets always just used the main amplifier for everything - look at the back where the AUX inputs are. Next to it should be the phono and television. Disconnect the television's RCA plug and connect your DVD player's audio. You'll only be able to use mono, because television wasn't stereo back then. But you can connect both right and left channels through a Y coupler. Or, it shouldn't be that hard to modify it to take stereo, I would think.

-Ian

In addition, the FM sound detection systems in older TVs was never very good. Most used the quadrature or ratio detector systems. Almost none used the Armstrong Limiter/Discrminator system which was the best. This was all due to cost-more components and royalties to Armstrong or his widow. (The ratio detector people had to pay at least some money to Armstrong's family after the US Supreme court was finished-it ruled that RCA violated his patents in developing this design).

The old tube type quadrature detector is never found in FM radios, the AM rejection is pretty poor-hence the buzz issues.

I have a Sarkes Tarzian FM table radio which uses a 12BN6 sheet beam quadrature detector tube....I haven't turned it on in about 15 years as I took the 12BN6 out to sub into a TV! Will need to get another 12BN6. As I remember the audio quality was fine...it has a "buzz" control like on Zenith TV's. This is the only FM radio set I've seen which uses the sheet beam tube.

I think the TV's used the quadrature detector so much because it combined a stage of amplification with the detection, allowing the detector to feed the audio output tube directly, thus saving another tube stage.

That is the first FM radio that I have heard of with this type of detector. I have quite a few tube type FM radios and they all have either the Armstrong type detector or the ratio detector. I have also seen the schematics for many more and never seen this. I guess that their are always exceptions to the rule. There were some TRF table radios made until around 1950, long after the superhets were mainstream-again to save money.

Zenith used the "Armstrong System" for their FM radios and paid royalties. Other manuafacturers used both.

Cost was the main reason, the quadruature detector saved 1 tube amplification stage. I think that Zenith first came up with it, but am not sure.

It would be interesting to compare the FM radio with a Quadruature detector against one with the Armstrong System while a thunderstorm was nearby. This would be a good test of the AM rejection.

Another reason that the sound output of a TV detector would not be as good as a CD is that TV sound bandwidth was limited to 10KHZ, not 20KHZ as a good stereo is.

The bandwidth of NTSC TV audio is lousy anyway, or in any event not good enough to do a Maggie combo justice.. I would get out the sams and figure out a way to switch the TV chassis on with the selector in AUX, like you initially suggested.

Best sound from the stock setup would be obtained by aligning the sound IF. It's never gonna touch what youll get pplugged directly in at linelevel. It's also mono, not that thats a big deal. I'm sure Maganvox would have included an input if there were any devices around to use it in the 60's, which there werent. TV audio was the worst source. Sometimes the chassis will be configured for better bass response. They did the best they could.

John

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