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• Using a hacksaw as a hacking tool, literally

  Ken Yap • 09/21/2024 at 06:27 • 0 comments

  I found another batch of half-century old ICs. These were from the quadraphonic amp that I hacked in A fugly amplifier. The NEC uPC20C power amplifier chips are from around 1973. According to its datasheet, it runs at 20V and produces 1.5W into 8 ohm speakers. The heatsink tabs are at the end of the 14 pin DIP package. (The datasheet labels it NTE1075A, a second-source equivalent.)

  I could find only one schematic on the Internet and it comes from the book 5000 Power IC Amplifiers. Other instances are copies of this schematic. So I decided to reverse engineer the circuit on the board, and in the process discovered some errors in the schematic, mostly in the polarity of the electrolytic capacitors, which I have corrected. Some component values were also changed. It's interesting that NEC tried to minimise the external resistors required but required many bypass and compensation capacitors.

  At first I thought I could desolder the chips from the board and put on a newly designed board, using the same attractive yellow soldermask as for the Motorola MFC9020 ICs. I got as far as creating a new symbol and a new footprint for the IC, including the heatsink tabs, drawing the schematic, and laying out the board. But since I'm the paragon of laziness, I realised that I could reuse the old board by hacksawing off the quadraphonic processing part. It didn't take too long as the board is phenolic rather than the tougher fibreglass. The result is shown above. So this is why this write-up is just a log on an existing project, as it's not enough effort to justify a new project. Notice that the top silkscreen echoes the bottom copper traces which helps reverse engineering.

  I've removed the bridge rectifier and smoothing capacitors from the board as I will probably use a 5V to 20V boost SMPS if I get a round tuit and deploy this board.

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