Marconi TF2163S Switched Attenuator Repair

I sell on Tindie

 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 61Ω  resistors on the two legs. 

I happened to have sime 51Ω resistors, so replaced the burnt one with a 51Ω and 10Ω in series. Checking performance on the VNA showed a loss of 12.8dB and a VSWR of up to 1.3:1at 40MHz; not too good. So I swapped over the two leg resistors, so that one of the original 61 ohm parts was first in line, next to the input. That did the trick. VSWR now 1:1 and a loss of 20dB. Nice.

I had made my own switched attenuator (the red pcb in the top photo), using smd resistors and was happy with its performance - bearing in mind I generally am only working at HF frequencies, but the Marconi goives much finer control and has a nice 'feel'. 

Note the added yellow 'dots' behind the setting points.

My thanks to George and Michael from groups.io Marconi section for their help and advice in getting this fixed.


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