NAD 4300 tuner-Digital tuner with a Knob. - UK Vintage Radio Repair and Restoration Discussion Forum

djsbriscoe · 19th Apr 2019, 5:08 pm

Hi,
Bought myself an NAD 4300 FM/AM tuner. I am curious how the tlp801 photo interrupter (which works with the weighted flywheel tuning knob) works with the 4011 logic chips, TC9147AP synth chip and upc4558 chips to derive the tuning voltage for the front end in this tuner? There appears to be 2 phase comparator outputs on the synth chip but I'm not clear how the tuning voltage is arrived at. I'm guessing there is some kind of phase locked loop arrangement. Just not sure what the op amps do and how these components work together.
I'm just doing some research in case I need to fix it. Helps if I can understand how it all works first. I hope someone can help me learn something. Thanks.

David.

Terry_VK5TM · 20th Apr 2019, 2:19 am

The tuning voltage is derived from the 35v rail the 4558's are connected to.

Those 4558's are amplifying the output of the PLL/tuning IC which runs from the 5v rail.

That's the simple explanation

Radio Wrangler · 20th Apr 2019, 6:46 am

As you turn the tuning knob, segments alternately light and darken a light sensor. There are two light sensors, mounted in staggered positions, so by looking at the sequence from the pair of them, a small logic circuit can determine whether the knob is being turned clockwise or anticlockwise and it feeds pulses to either the 'up' or 'down' inputs of the PLL synthesiser chip. This counts up or down the frequency number stored in this chip. It goes to two places; the display to show you where you've tuned to, and it is translated to the appropriate division number for the PLL.

The PLL gets a sample from the tuner front-end's local oscillator, which it divides by that number. A phase detector compares the frequency out of the divider with a reference from a crystal oscillator. The phase detector steers the tuning voltage to the local oscillator to get its two inputs in step. So the LO is tuned to and held on the exact wanted frequency. The LO tuning voltage is shared with the tuner's RF stage tuned circuit, which are arranged to track the LO with the correct offset.

So turning the knob changes the frequency number displayed, and the phase detector makes whatever voltage is needed until it sees the LO is on the right frequency.

There are two of these systems, one for the FM front end, one for the AM.

David

20th Apr 2019, 2:19 am	#2
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	Re: NAD 4300 tuner-Digital tuner with a Knob. The tuning voltage is derived from the 35v rail the 4558's are connected to. Those 4558's are amplifying the output of the PLL/tuning IC which runs from the 5v rail. That's the simple explanation __________________ Terry VK5TM https://www.vk5tm.com/

20th Apr 2019, 6:46 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: NAD 4300 tuner-Digital tuner with a Knob. As you turn the tuning knob, segments alternately light and darken a light sensor. There are two light sensors, mounted in staggered positions, so by looking at the sequence from the pair of them, a small logic circuit can determine whether the knob is being turned clockwise or anticlockwise and it feeds pulses to either the 'up' or 'down' inputs of the PLL synthesiser chip. This counts up or down the frequency number stored in this chip. It goes to two places; the display to show you where you've tuned to, and it is translated to the appropriate division number for the PLL. The PLL gets a sample from the tuner front-end's local oscillator, which it divides by that number. A phase detector compares the frequency out of the divider with a reference from a crystal oscillator. The phase detector steers the tuning voltage to the local oscillator to get its two inputs in step. So the LO is tuned to and held on the exact wanted frequency. The LO tuning voltage is shared with the tuner's RF stage tuned circuit, which are arranged to track the LO with the correct offset. So turning the knob changes the frequency number displayed, and the phase detector makes whatever voltage is needed until it sees the LO is on the right frequency. There are two of these systems, one for the FM front end, one for the AM. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done