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Amplifier LPF replacement

Question

I have an Endevco® 136 amplifier and would like to replace the low-pass filters. Do you have detailed replacement instructions?

Answer

The low-pass filter (LPF) of a model 136, a 3-channel differential voltage amplifier, can be changed in the field by following these instructions.

Tools required

  • screwdriver, flat-blade
  • screwdriver, Phillips-tip

Your steps

1. Remove the two Phillips screws and washers on the front panel and safely store for reinstallation.

2. Carefully remove the internal ribbon cable which connects the front panel to the motherboard.

3. Remove the two screws and washers from the rear panel (shown removed) and safely store for reinstallation.

4. Carefully pull the motherboard assembly approximately half way out from the cover. The motherboard can be completely removed from the cover, if desired.

5. Using a flat blade screwdriver, begin prying one side of the filter and then the other side, to work the filter loose from the socket without bending the filter pins.

6. Install the new LPF filter modules, ensuring they are oriented correctly, and press each firmly in-place.

7. Reassemble the unit in reverse order. Be extra careful to not over-tighten the 4 screws, as the base material is aluminum. (If the hole is stripped out, simply use a longer replacement screw.)

8. After reassembly, perform a verification test, using a function generator. Turn off the voltage excitation to each channel and inject a signal into pins 4 and 5 on each channel. While monitoring the output, sweep the frequency to the corner frequency and watch the output signal decrease when passing the corner frequency.