UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

PJL · 30th May 2019, 10:33 pm

Done the test with the 2022. It seems the low level from the 2nd LO is a red herring.

Increasing the 236MHz amplitude sees diminishing returns with the increase from +4 to +6dBm having little impact on the amplitude. It starts falling off more rapidly below 0dBm and -6dBm to -8dBm is approximately halving the output.

Even at +6dBm the output is still 10dB down, however, it is difficult to prove that the fault is the mixer as the 200MHz IF will be detuned by the low loss probe (5pF). This is where I could do with the chinese log detector as the intermediate levels are too low for my scope particularly if I use the probe attenuators to lower the input capacitance. The new probe is supposed to be 1pF.

PS: The PP measurements above are readings on the scope but the actuals will be much larger due to the scope bandwidth limitations.

30th May 2019, 10:33 pm	#32
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Marconi TF2370 fault fixing Done the test with the 2022. It seems the low level from the 2nd LO is a red herring. Increasing the 236MHz amplitude sees diminishing returns with the increase from +4 to +6dBm having little impact on the amplitude. It starts falling off more rapidly below 0dBm and -6dBm to -8dBm is approximately halving the output. Even at +6dBm the output is still 10dB down, however, it is difficult to prove that the fault is the mixer as the 200MHz IF will be detuned by the low loss probe (5pF). This is where I could do with the chinese log detector as the intermediate levels are too low for my scope particularly if I use the probe attenuators to lower the input capacitance. The new probe is supposed to be 1pF. PS: The PP measurements above are readings on the scope but the actuals will be much larger due to the scope bandwidth limitations. Last edited by PJL; 30th May 2019 at 10:41 pm.