Monday, November 4, 2019

Repair work on logic circuits for drive B


Upon inspection of the control logic board I found that an engineering change was made to this board, adding one jumper wire and one 470 ohm resistor. The resistor was broken in half. I didn't have that value on hand but once I picked one up I could repair the board.

Broken resistor on control logic board (05)
Resistor replaced on Control Logic Board

I had to build a replacement for the configuration "chip" that plugs into the socket for U27 on the capstan pre-amp board. It houses five resistors and two capacitors of specific ratings for the 125 inch per second capstan speed:
  • 3.6K
  • 39.2K
  • 2.94K
  • 4.12K
  • 6.8K
  • .01 uf
  • .027 uf
These are placed across the narrow width to opposing pins. In other words, pins 2 and 15 are opposite each other and connect to a 6.8K resistor. Pins 8 and 9 are opposite each other and connect to a 39.2K resistor. Only pins 1 and 16 are unused.

I collected these components at Anchor Electronics and soldered them onto a IC socket that will plug into the socket below it on the PCB. Everything was going well, verifying the values with my capacitor and ohm meters, when I realized that I had bought an .022 capacitor, not the intended .027 uf.

I had to drive out to the store again just to buy the ten cent part I had miswritten on the sheet, having transcribed my sloppy handwriting incorrectly. After the wasted 45 minutes I completed the DIP socket with the proper configuration components and had it plugged into the PCB.

With this installed, the board is complete and configured for the high speed model 6 behavior. Between the broken resistor for the control logic board fix and these parts, including a second try at the .027 capacitor, my bill was just over $4.

Configuration block for 125 ips on Capstan Pre-amp PCB

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