Hi - I am refurbishing my original HW-16 (1970), and am very familiar with its operation.

What kind of noise? Static, thump, growling, hissing? Do you hv a recording of this noise?

I agree with Scott Dorsey that some kind of rx/tx timing issue is at fault. When new, however, my rig never thumped. Transition btwn rx and tx was "invisible." The TR switch is a PNP transistor called a "bias switch". Its operating characteristics could have drifted significantly or, the transistor could be failing. The date code on my transistor is 039, indicating 1970, workweek 39. 44 yrs is a long time. You can find replacements on eBay.

Looking at the "bias switch" ckt, there are no electrolytics, just the PNP transistor, resistors, and bypass caps (0.1uF).

I found a discussion thread (http://forums.qrz.com/showthread.php...it-right/page5) about the transistor replacement as an ECG 102.

Check the RX voltages (key up) on the PCB, around Q1 especially, with a high-impeadance voltmeter. The reference values are on the original Heath schematic. This is important before diddling with replacement parts and such.

Larry, N3CCW, cn87