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Reprinted here with permission from the author
HW-101 Relay Filter Mod
Steve Ray K4JPN

This mod was inspired by the article in August 1985 73 Magazine titled "Modification Mania". The problem I faced with my HW-101 is when I using the CW filter transmitting in CW the output would be greatly reduced on 40 meters and slightly reduced on the other bands, yet if I switched in the phone filter I could easily get the full output. After trying all the Service Bulletins and tweaking and retweaking the driver coils nothing seemed to solve the problem of reduced output with the CW filter in. I could measure the input and output of the CW and phone filter and saw that the drive was down considerably when the CW filter was in versus the Phone filter. Finally I decided that rather then solve the problem of reduced output with the CW filter and full output with the phone filter, the solution was to simply use the phone filter when transmitting in CW and be able to select which filter I wanted when receiving.

The circuit is quite simple, take 12.6 VAC from a convenient location on the rig rectify it and have it hold in the CW filter in receive and in transmit drop back to the phone filter. Instead of the SPDT and DPDT switches use relays instead to switch the filters and use the now unused SPDT to turn on the relays.

S1 is simply the SPDT switch previously wired to switch the Phone/CW filter in and out; RL1 is the transmit/receive relay in the HV cage of the HW-101. Fortunately it has a spare terminal just the way it is needed it for the mod. I used Radio Shack miniature relays where the present SPDT and DPDT switches are used to switch the Phone and CW filter in and out.

The schematic diagram shows the CW filter in and the HW-101 in the receive mode, for the phone filter it is only necessary to open S1. The circuit operation for the CW filter is simple. When you want the CW filter in you close S1, this grounds relays RL a and RL b through RL1 connecting the CW filter into the circuit and grounding the Phone filter output. When you transmit RL1 is opened and the relays RL a and RL b drop out connecting the Phone filter and grounding the CW filter, the same as with the old switches.

Parts List

C1 100 mfd at 25VDC

C2 & C3 .01 at 100VDC These are bypass caps I installed one right at the relays and the other right where the line comes out of the HV cage from RL1. Do not omit these Caps.

D1 & D2 1N4002 or similar diode

R1 1200 ohm ½ W

RL a Radio Shack SPDT 12 VDC Relay Part Number 275-248

RL b Radio Shack DPDT 12 VDC Relay Part Number 275-249