[R-390] Too Loud Amelco is Dead: Long Live the Amelco

[Craig Heaton]

To All,

It’s time to report my findings. First a little flash back to refresh
memories. I believe this R-390/A followed me home sometime in February 2007.
It went thru the usual recap & etc. After a good alignment or two the loud
audio from either the line or local gain became obvious. Loud audio is
better than no audio, thus the issue was pushed down the list of thing to
do. I’d get to it


.mostly on rainy days in Oregon.

In the past several years everything has been tried many if not dozens of
times. Tubes were swapped, old tubes replaced with NOS tubes from the junk
box, tube voltages measured &  compared with a good (to me) Motorola
R-390/A, IF decks swapped, AF decks swapped, C603’s swapped, another set of
caps for C603, different speakers, different matching transformers between
the receiver & speakers. The wiring harness was checked for shorts, opens,
and grounds. Every wire between the IF going to the AF deck was checked. All
the plugs/wiring coming or going to the AF deck was checked. Even back
shells of the plugs were unfastened to look for issues, nada. At this point
in time I can proudly say all was tried that I could think of, or read.

The problem, cause, or whatever was on the panel; front or back of the
Amelco. Both R104 & R105 were good and within spec of 20%. I had a few more
in the junk box. So I picked the two with the greatest resistance and tried
them, nada once more. 

Art Collins please forgive me, I have sinned. R-390/A lovers, you read it
here first. Both 2.5K potentiometers for R104 & R105 have been replaced with
5K audio taper pots. Life is now good. When the phone rings, just turn down
the local gain. Did I treat the symptoms or find the cause. Darned if I
care, it works. At the min setting on local gain, audio from the speaker is
almost gone. One has to listen closely to hear the little sound coming out.
At a setting of 1 to 1-1/2 the audio is comfortable. Crank the local gain up
and that ½ watt of audio power rocks the speaker. 

73’s
WD8KDG
Craig

PS: ? By the way, the 5K RV4 pots were found on a new internet surplus
website. Ten bucks a pop and they have 2.5K audio taper pots for that price!