Audio Mods for HW100/100 and SB101/102

by Chris, KD2XA

From Smithsonian Museum of American History, Edmund LaPort Archive (ca 1920-1950)
Scanned by John Lyles, K5PRO Edmund LaPort, RCA

I have two Heath rigs I use on AM: a HW101 and an SB102. Both are used as exciters only. I replaced the crystal filter with a 0.001 uF cap, unbalanced the balanced modulator to give about 15-20W carrier, shorted the ALC jack (must be done!) on the back panel, and I'm on the air with excellent AM.

A few simple audio mods are in order. In the speech amp change R2 to 6.8 Meg, C2 to 47 uF, C9 to 0.05 uF, C11 to 0.05 uF, and C13 to 10 uF LOW LEAKAGE nonpolarized electrolytic. If C13 is not stable, your carrier balance setting will drift. While you are in there replace the balanced modulator diodes with new 1N4148 or similar being careful to match the forward and reverse resistance to get a matched set of four. **The component designations (C2, R2 etc) for this mod are good for all HW100, HW101, SB101 and SB102 rigs.

To operate, switch to CW or TUNE, load for maximum power out, then increase the loading until the output drops about 5-10 percent. Switch to LSB, set the carrier balance pot to give about 15-20W carrier, and advance to mic gain until voice peaks just kick the wattmeter up a bit. Best operation is done by monitoring with an oscilloscope (strongly recommended). If your audio is out of phase (scope shows no/low positive peaks) reverse the phase by unbalancing the carrier balance to the same output level, but at the other side of the null.

For line-level audio just couple direct to the high side of the mic gain pot through a 0.47 uF cap. If done properly, fed with processed audio and carefully tuned, these mods produce audio that rivals the best you hear on the air. If you switch the crystal filter back in, you will now sound darn good on sideband too.





5 June 2005