Some users complained of interferences caused by strong local broadcast station on LW/MW.
Boris, LZ2JR designed 7th order Chebyshev high-pass filter which was connected in line with the RX path. Filter effect though was inadequate and I was forced to look closer at receiver front end especially input point at RX/TX switch. Then realized that LPF diodes are not effectively cut off on RX so large signals can make them act like AM detector causing IMD problems. Adding pulldown resistor 100kohm would be enough for diodes to be reverse biased. (On RX cathodes see +13V). This way diodes and band LPF’s will be fully isolated from receive path and will no longer interact with incoming signals.
Needed parts:
1. SMD resistor 5% case 0805 value 100kohm
2. Firmware update to version 2.20A for front panel and main board processors
Added pulldown resistor is 100kohm SMD case 0805 and is easily placed onto the two vias in LPF board. LPF board is located on back panel PA assembly and can be accessed after removing PA shield and then completely pulling off the PA module.
100kohm pulldown resistor
This modification needs new firmware 2.20A to be uploaded in RGO ONE FP and MB processors. FW 2.20A completely disengage band LPF’s from receive path. Current band LPF is switched on only on TX.
Has this mod been implemented in new production units?
ReplyDelete73 - Rick, DJ0IP