[R-390] R-392: My solution to replacing the final audio output tube

[Mack McCormick]

I developed a solution to replace the final audio output tube (26A7) on the
R-392. Very simple using off the shelf components. This mod (completely
reversible) removed a LOT of heat from the radio and reduced the 26VDC
current draw significantly.

I used a voltage regulator to reduce the voltage from 26VDC to 12VDC which
the LM386 requires.
https://www.amazon.com/gp/product/B076H3XHXP/ref=ox_sc_rp_title_rp_9?smid=&pf_rd_p=823698fb-5325-4f6f-9cdd-b754ea810772&pd_rd_wg=5Crpb&pd_rd_i=B076H3XHXP&pd_rd_w=JlgkA&pd_rd_r=e5ed6b98-d679-47bb-921b-f2591173452a&th=1

I piggybacked a LM386 audio board using 3M Industrial double stick tape.
Also provides insulation.

Here is the LM386 board that I used.
https://www.amazon.com/gp/product/B00LNACGTY/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1

Using an octal male connector I pulled GND from pin 2 and +26VDC from pin
4. I connected the input of the LM386 board to pin 3. I bypassed the audio
transformer by opening J613 (the large multi pin connector) and removing
the wire from Pin 1 of the connector. I spliced a separate wire to the wire
that was on pin 1 (properly insulated) and routed that externally out the
top of the jack to the output of the LM386 board. (this mod is easily
reversible).

It provides loud volume with the audio gain pot at about 1/3 rotation.

Here are pics and a short video:
https://drive.google.com/drive/folders/1-1xs5d_ZGLULcxHcTsYHfyY8816RZwv_?usp=sharing

73,

Mack
W4AX
https://www.qrz.com/db/W4AX