MC3470 floppy disk read amplifier pinout
DIP-18. Read amplifier, peak detector and pulse shaper
Amplifies the millivolt head signal, filters and differentiates it, and puts out one TTL pulse per flux reversal after a time-domain mask (one-shot 1, pins 6-7) that rejects narrow noise. On the Disk II card R33 (or R21) on that one-shot is the comparator offset adjustment at TP5; R28 trims the output at TP7. Runs on +5 V (pin 11) and +12 V (pin 18). A bad one gives repeated recalibration and I/O errors.
Open the interactive 3470 pinoutPins
| Pin | Name | Type | Signal |
|---|---|---|---|
| 1 | IN A | io | Amplifier input A from the read/write head. |
| 2 | IN B | io | Amplifier input B from the read/write head. |
| 3 | OFFSET DEC | ctrl | Offset decoupling / gain select; an external capacitor sits between pins 3 and 4. |
| 4 | OFFSET DEC | ctrl | Offset decoupling / gain select (pair with pin 3). |
| 5 | GND | ground | 0 V. |
| 6 | MONO1 | ctrl | One-shot 1 (time-domain filter) timing; the external RC sets the mask width (about 3 µs on Apple 5.25 drives). Disk II: C7 330 pF to pin 7. |
| 7 | MONO1 | ctrl | One-shot 1 timing (pair with pin 6). Disk II: TP5; R33 10K pot (or R21 8.2K) to +5 V, C7 330 pF to pin 6. |
| 8 | MONO2 | ctrl | One-shot 2 (output pulse width) timing. Disk II: C8 330 pF to pin 9. |
| 9 | MONO2 | ctrl | One-shot 2 timing (pair with pin 8). Disk II: TP6; R20 4.7K to +5 V, C8 330 pF to pin 8. |
| 10 | DATA OUT | io | Digital read pulse, one per flux reversal, to the 74LS125 and ribbon pin 16. TP7. |
| 11 | VCC1 | power | +5 V (7 V absolute maximum). |
| 12 | DIFF | ctrl | Differentiator components: L5 68 µH, C13 3000 pF, R24 91 Ω across pins 12 and 13. |
| 13 | DIFF | ctrl | Differentiator components (pair with pin 12). |
| 14 | DIFF IN | io | Active differentiator input from the head filter. |
| 15 | DIFF IN | io | Active differentiator input (pair with pin 14). |
| 16 | AMP OUT | io | Amplifier output into the L3/L4/C10/C11 filter; TP8/TP9 side. |
| 17 | AMP OUT | io | Amplifier output (pair with pin 16). |
| 18 | VCC2 | power | +12 V (10-14 V). |