Saturday, January 16, 2016

Victor SEA-50 graphic equalizer restoration

Few weeks back I got Victor SEA-50 stereo graphic equalizer from one of my friend. This is 10 band stereo graphic equalizer manufactured by Victor Company of Japan (now JVC Kenwood) around 1970 - 1980. SEA-50 consist with 63 transistors and it provides ±12dB control range for 32 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz, 12 kHz / 16 kHz / 20 kHz center frequencies.

The SEA-50 which we got is not working properly and most of its controls may generate distortion above +8dB and below -8dB. Apart from that left channel of this receiver has low volume and distortion.

Defective transistors in control board (next to 2SA493).

After few hours of checkup we identify few defective transistors (2SC982) in control board and we replace those with 2N3904 NPN transistors. Subsequent to this fix, all controls of SEA-50 start to work again on its full range (±12dB) by without producing any distortion or clipping.

Sunday, January 10, 2016

Mullard 3-3 Amplifier Project (Part 1)

Mullard 3-3 is quiet popular 3W tube amplifier introduced by Mullard Ltd in 1956. A schematic and design detail of this amplifier is available in "Mullard Circuits for Audio Amplifiers" book and in National Valve Museum article. This amplifier is based on EF86, EL84 vacuum tubes and EZ80 full wave rectifier tube. In this project we decided to construct this original Mullard 3-3 Amplifier with some slight changes and commonly available electronic components.

Top view of Mullard 3-3 amplifier with 6P14P and 6J8 tubes.

In our prototype we replace EZ80 tube with 400V 5A bridge rectifier which is commonly available in many electronic spare parts shops. Also we replace EL84 with 6P14P pentode and EF86 with 6J8 pentode. Both of these valves can directly use with this circuit and those values are available for lesser price than EL84, EF86 tubes.