[R-390] [MRCA] Working on the R-392

[Charles Steinmetz]

David wrote:

> Maybe I will replace the audio amp at some point.
> You are sure right that it does get warm.

Getting rid of the heat is certainly a good idea, but an even better 
reason to do something about the 26A7 output amplifier is that it 
improves the audio quality by about a million percent.

There is a very simple modification that works extremely well, my "super 
simple audio mod." Documentation is posted at 
<http://www.ko4bb.com/getsimple/index.php?id=manuals> [file name "R392 
26A7 super simple audio mod.pdf"]. Whether you want to do this mod or 
not, it may be worth a read -- this file explains what is wrong with the 
existing 26A7 amplifier and why it sounds so very, very bad.

The best fix, IMO, is to replace the 26A7 with two power MOSFETs as 
drawn by someone named "S. Johnson" in 1991.  That schematic, with my 
annotations, can be downloaded from 
<http://www.ko4bb.com/getsimple/index.php?id=manuals> by searching for 
the file named "R392 26A7 MOSFET replacement Johnson annotated 
updated.pdf."  [NOTE: there is an older file posted there that does NOT 
have "updated" in the file name.  You want the updated file.]

An annotated schematic of the stock audio amplifier is posted there as 
well, for reference [file name "392 audio amp schematic V606 607 608 
annotated.pdf"].

Best regards,

Charles