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 Post subject: FM tuner modification
PostPosted: Dec Sat 21, 2019 11:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20869
Location: Warner Robins, GA
I have a Magnavox CR-206 FM tuner that I restored.

Someone had replaced the power switch with one that has a 1 meg pot as I assume the original switch went bad.

I used that pot as a volume control as I decided to try the tuner with the amp/power supply I use for my BC-455A receiver.

It works well with that amp, but I'm directly feeding the phase inverter and there isn't enough signal to drive the amp to full output.

Also Miller capacitance is an issue.

What I've thought of doing is installing a 7 pin socket where the filter cap can is next to the rectifier and using the 7 pin equivalent of the 6SQ7.

I'll set up the circuit the same as the 6SQ7 first audio tube is in the BC-455A.

Now given the 47K plate resistor used for the tube, will the 1 meg volume control cause any loss in treble response?

Also the output of the tuner has a 470pF cap which is now across the 1 meg volume control. For proper de-emphasis should that cap be larger or smaller?

I used a German oval speaker which has surprisingly deep bass response with the amplifier as it sounded good with the BC-455A receiver.

I tried adding an Isophon DP-19 tweeter for the treble using a 6 dB/octave passive crossover and it seems to sound good.

Now if the 1 meg pot isn't suitable I'll do a 10K to 50K pot on the output of the voltage amp tube.

Concerning the heater current I'll replace the dial light with a LED type perhaps similar to what I did with the CR-192 FM tuner. That should allow me to add another tube. I will most likely take the B+ from right after the choke and divide it down with a resistor to 200-250Vdc.

That way I am not affecting the B+ of the tuner.

The output is wired as follows.

Tuner output.png
Tuner output.png [ 16.93 KiB | Viewed 671 times ]

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