| The HT-37
isn't the first rig that comes to mind when one thinks of getting on
AM. But they can make very nice sounding AM with a few simple
modifications. And, as a bonus, you'll get hi-fi audio on SSB too!
1. Remove all knobs. (use Bristol wrenches for set screws).
2. Remove the top half cabinet, and the 5R4GY rectifier tube.
3. Turn the transmitter over and remove the bottom half cabinet.
4. The sideband generator unit sub chassis has two cinch plugs and a
ground lug to microphone jack. Remove both plugs from the sub chassis
and unscrew the ground lug.
5. Turn the unit back upright.
6. Loosen the audio gain and plate control verniers on their respective
shafts (with Bristol wrenches).
7. Remove the six front panel screws, and gently pull the panel off
just enough to clear the knob shafts. (use something to lean it
8. Pull the microphone input (on sub chassis), and 9 MHz (to main
chassis) RCA jacks out.
9. Remove the four screws surrounding the sideband generator to the
10. Pull the sideband chassis out with the back end angled up slightly.
11. Unscrew the cover from the sideband unit, and study the schematics
12. Snake a thin shielded audio cable through the front of the sideband
generator chassis (you're now holding in your hands) so that it ends up
near the audio gain shaft. Connect center conductor to the top of the 1
Meg audio gain pot which leads to the grid of V2A (12AT7), and shield
to ground side of the pot. Solder on a female RCA jack on the cable end
coming through the chassis and tie wrap it onto the audio gain shaft
sleeve so that it is physically near the female RCA jack on the chassis.
13. Time to remove the low pass filter nightmare in the audio path. It
is located directly below the phasing network, attached to unused lugs
on the socket is L101 (50 mH). Snip that out and throw it into your
junk box. Remove the two (0.1 uF) caps (C108, 109), but keep the wire
leads attached to them intact. Use those leads to connect T101 directly
to pins 1 and 3 of the phasing network. Also, while you've got the unit
in hand, replace the old 10 uF electrolytic (C103), check resistor
14. Button the sideband generator back up and reverse the removal
procedure. Remember to reattach the cinch plugs and ground lug!
15. It goes without saying that even nice audio sounds crappy with 120
cycle hum, so change all old electrolytics in all supplies including
bias. Speaking of bias, (thanks to a suggestion from Chris W2JBL),
string some Zeners across the bias pot to clamp the voltage at -49
volts. This mod is not absolutely necessary but makes things a lot more
linear (especially on SSB).
The RCA female connector (that you ran out through the chassis), can be
fed with any line level mixer, audio chain, etc. You can plug the male
end RCA into your female run and use the front panel mic jack as an
input, or run an XLR to RCA pigtail out the back (as I did) to a
balanced audio source.
In all cases of using line-in audio, remove the 12AX7, (V1A,B).
For stock audio (minus the low pass filter), plug the 12AX7 back in,
reattach the RCA plug to it's normal configuration and talk..
After a few tests, the HT-37 is capable of 130% positive modulation
with VERY low distortion, and sweeps out from 45 Hz to 12 kHz
One last note--- Follow the alignment procedures for sideband
suppression with the phasing method after this mod. And, if you decide
to fall to the dark side and use this on sideband, WIDE is an
understatement!! ALL bandwidth is up to you on the AF side, and sharp
cut off would be highly recommended...