Marconi TF2163S Switched Attenuator Repair
This unit was an eBay purchase with no known history. These handy boxes have two banks of switched attenuators, one going from 0 to 22dB in 1dB steps (on the right), and the other from 0 to 120dB in 20dB steps. Nice. On this one, the 1dB stepped attenuator checked out fine, but on the other, the first 20dB attenuator had a problem; input impedance was 150 Ohm. Measured on a NanoVNA this gave a poor input match (no surprise) and an insertion loss of 16dB, with the next few attenuators correctly showing 20dB steps. Removing this bank of attenuators from the box was fairly straightforward, that is, once I had tracked down a 1/16th hex key to loosen the grub screws on the front knob (everything on this unit has an imperial thread). It is then a simple matter of removing a bunch of screws to release the bottom panel. Immediately obvious was the burnt out first resistor. The 20dB attenuators are made up as a pi network with 61 Ω resistors on the two legs. I happened t...