[R-390] R-390A audio enhancements, tone control and hi-fi

[Larry H]

I've been an audio nut for about 55 years now.  My first high end equipment was a pair of Acoustic Research AR-3 speakers I purchased in 1964, which I'm still using today.  They're awesome, but then I like clear clean bass.  Of course one must also have a good amp to go with them.  That was in late 1968, a good friend sold me his McIntosh MA-230 amp.  I use these today with my R-390A and for other stereo repairs I do.  I had no idea the 390A was capable of such good sound.  About 4 months ago a station in my area started broadcasting hi-fi music on the bcb.  I stumbled across it in my car and it really sounded good.  So I hooked up my 390A 'diode load' to my stereo (as I read here to do) and WOW, it sounded great.  But, the highs were a little too high and the lows a little too low.

I got my first R-390A in late 1986 and knew what a great receiver it was (I serviced them in 1961 to 1963).  However, after using it for a few months, I realized that the audio had a large drawback - some stations I'd listen to had way too much base and there was no easy way to compensate for it without using external audio.  Some had so much that it was hard to understand.  After considerable thought, in July 1987 I decided to detach the 800 cps filter and insert an R/C network that would just substantially reduce bass.  That was a big help, but in July 1998 I decided that was not enough and decided to replace the 2 position 'sharp / wide' switch with a 5 position 2 pole.  I found a thin one that would fit and implemented 5 different audio compensations for various levels of bass and treble.  The emphasized treble was used mainly with the 2 kc and 4 kc bandwidths, and the base reduction on 8 kc and 16 kc.  This made an immense improvement in voice intelligibility.  I thought about using a rheostat tone control in its place, but that would not give me the control over the bass and treble at the same time that I was looking for.

Until 4 months ago I had no interest in listening to music with my R-390A.  Then that station came on the air and I thought it would be nice to have another music station I liked to listen to in my shop.  When I hooked it up to the diode load, I noticed 2 things, the audio level was too high and the highs were too high and lows too low.  I decided to change my existing compensation network to make position 5 (wide) hi-fi and feed my stereo from the wiper on the line gain pot.  I ran the audio coax with an RCA jack on it out an existing hole in the back and let it dangle.  This provides the correct signal level to feed most stereos and I like using the volume control on the R-390A.  Of course you know one more change was required to make it good - fix the lack of bass in the R-390A audio amp due to the coupling cap values.  That was an easy fix.  I increased the value of the 5 caps (C602, 4, 5, 7 and 8) to provide good bass (up to the output transformer).  Thank you Chuck Rippel for that info.  The quality of the signal coming out of the line gain pot is now quite good.  It's better than the AM tuner I have that is designed for hi-fi.  Here's a link to the schematic changes I made:

https://s11.postimg.org/p3nw644fn/R390_A-_Aud_Mod6s.jpg

Since both audio 6AK6 output amps are seriously lacking in 2 areas (low fidelity output transformer and single ended design), I decided it was not worth it to me to pursue any farther improvements there.  No matter what I did it would not come close to using my stereo.  Even so, the audio is improved when I use my local speaker or phones.  I'm very pleased with the results I have now.

Regards, Larry