Corsair II - Troubleshooting Weak Receive on higher bands

From the Ten-Tec Reflector June 8, 2016

The problem typically presents with these symptoms, as Marco IS0KYB described: "on some bands (mostly from 15m upwards) I noticed a weak reception when I use the main antenna input. The problem goes away if I switch to the RX antenna. No, it doesn't seem to be the RX ant switch :) I cleaned it and operated it 100 times… it doesn't even scratch. I believe that the culprit is a faulty switching diode among D1-D6 in the rx/tx switch."

Rick VE3ORY responded to Marco's post:

Like Marco I suspected the problem to be diodes related to T/R switching, however the problem turned out to be in the High Pass Filter circuit on the LPF-TR board (80969). Symptoms were identical to what Marco is indicating, and I found that receive strength was normal if I connected the antenna to the 'RX ANT' connector on the rear panel, which effectively bypasses the SWR and LPF-TR boards (and the High Pass Filter).

My original TEN TEC manual has notation showing "OPTIONAL HIGH PASS FILTER NOT INSTALLED" . However once I got the LPF-TR board removed I discovered that the "Optional" High Pass filter was in fact installed contrary to the notation on the schematic. Trouble shooting eventually led me to disconnecting and bypassing the High Pass Filter and receive performance issue was resolved. I did this by opening the circuit at the input to the High Pass Filter and bypassing it with a capacitor between Q2 collector and the RX connector to avoid interfering with operation of Q2. It has worked fine ever since and no noticeable issue without the "Optional" High Pass Filter.

The biggest challenge in all of this was getting the LPF-TR board out of the chassis which required careful removal of the Band Switch shaft while ensuring that all of the Band Switch wafer sections were in the correct position when reinserting the switch shaft.

While trouble shooting my problem I also discovered some changes on the LPF-TR board regarding biasing resistors for the PIN diodes. I bought my Corsair II new from a dealer at the Dayton Hamfest so presumably these changes were done at the TEN TEC factory. I recall reading somewhere about improvements to performance in PIN diode biasing, however I did not make these changes to my Corsair and so can only presume they were made at the factory but not documented by TEN TEC on the schematic.

Regardless of who did the modification, here is what I found on the LPF-TR board in my radio:

1. 470 ohm resistors were installed in series with D1 and D2

2. R3 had been changed from 330 ohms to a 100 ohm resistor

3. R6 had been changed from 150 ohms to a 47ohm resistor.