revamping a vintage yamaha t 80 tuner

Posted on Feb 6, 2014

Most FM tuners from this era suffer from two major problems. They have a high and reactive output impedance, and the grounding scheme inside the unit is at best dubious. This is a unit from the era when single sided boards where standard issue. The near total lack of ground plane and high impedance returns to the power supply are very much part of this unit.

revamping a vintage yamaha t 80 tuner
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One wonders what the performance might have been had Yamaha decided to use a double sided board with a fair amount of ground plane on the component side of the board. The following will show an approach to tackle both of these problems. The following schematic is of the T-80 output circuit and the buffer installed to lower the output impedance so I can drive a Jensen Audio Isolator, which is just a pair of Jensen line level transformers in a package. Please note the feedback network from pin 2 to pin 6 of the op amp. The 2. 55k ohm resistor is the approximate resistance match of T-80 output circuit and the two capacitors in parallel represent the capacitance match of the T-80 output circuit. Although it is not shown here, when the T-80 has the high cut filter added the capacitance will double. The high cut circuit is only activated when you find a very noisy station that is not coming in well in local mode. For my present antenna configuration I really don`t need this and the output circuit is optimized  for local stereo mode. This circuit will reduce upper order distortion, especially when connected to a relatively low impedance input and lower the output impedance to 100 ohms to drive the Jensen Audio Isolator. For technical information on the Jensen Audio Isolator Go Here: Although transformers certainly have some drawbacks, they are very effective in isolating the ground of one unit from another. Given the grounding...

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