old RCI 2970 - solved: distorted receive in SSB mode with moderate to strong signals.

[BayouRadioAmplifier]

I have this old 2970 (with the -013 board) that had many problems. it was worked on by others,
sometimes making things worse. (solder blob shorts across circuit traces., etc)

But the last thing I was trying to repair was while in SSB receive, with even a moderate
strength signal, the audio was distorted. The RF gain had to be turned way down
to hear a clean SSB signal.

But clean signals received in AM, and the AGC is working. and in SSB the AGC is working.
I could see the bar graph meter move and also measure the AGC voltage with a
scope at IC1-14, the voltage going up to about 3 with a strong signal.

I have read posts about this problem. Nasty SSB receive audio. in various radios.

I see in the design of these RCI radios the receive SSB detector is separate
from the transmit path.

I was thinking- maybe there is not enough carrier oscillator injection power
to properly detect a stronger SSB signal. And with the scope, I could clearly
see this was the problem.

I removed capacitor C90 which is only 10pF and put in a 100pF capacitor,
and I could see a larger level of carrier signal inserted at the detector.

And with the antenna connected, now I can have the RF gain up all the way,
and receive clean SSB signals. Also the receiver hiss noise is different,
and sounds like a "normal SSB radio".

So anyone else out there have this problem? Distorted SSB receive audio
in one of these RCI/Galaxy radios? Lets try this repair on another radio.

.....

 

[nomadradio]

Seems to me there was a swaperoo of both C89 and C90 The wrong cap at C90 with too little capacitance reduces the 10.695 MHz carrier drive level to the sideband detector. For clear sideband audio, this carrier has to have a level higher than the sideband IF signal. I have a vague recollection that C89 would turn out to have to much capacitance. Making C90 larger restores the balance by increasing the carrier drive to the sideband detctor. C89 feeds the IF signal, so making it smaller tends to help if you find a cap bigger than 5pf in that spot.

This should have been posted as a web page 25 years ago, but I haven't found it.

73

 

[Onelasttime]

Freq. it is the voltage regulator or the crystal filter if the problem comes and goes.

I am one of those people that think after 10-15 years all of the low voltage caps should be replaced in anything made in China anyways!