[R-390] R-390A Noisy Ant. Trim and Raspy Calibrator revisited

[Craig Heaton]

Ken,

If you have time try connecting a speaker to the line output. See if the
audio is loud there also. I swapped IF's, AF's, etc between a Motorola and
the Amelco, always had loud audio on both Line & Local audio.

By the way, if I didn't mention it before..................The original
audio pot for the local gain on the Amelco had 2.5K on the cover of the pot!
But when checking the pot with a VOM, it was a 5K audio taper pot. The guts
were switched, solder connections looked original. The acceptance seals were
still on the top & bottom covers, stuck to the sides of the RX. Kinda makes
you wonder. 

Anyway, the Too Loud Amelco has been playing nice for the last couple of
weeks. I have to ask the rest of the gang here on the R-390A e-mail
reflector if the mil tech schools had booby trapped receivers for students
to trouble shoot?

Craig,  

-----Original Message-----
From: Ken Harpur [mailto:igloo99nz at yahoo.co.nz] 
Sent: Sunday, July 20, 2014 2:44 AM
To: Larry H; Roger Ruszkowski; hamfish at efn.org; bill.riches at verizon.net;
R-390 at mailman.qth.net
Subject: R-390A Noisy Ant. Trim and Raspy Calibrator revisited

Hi Larry, Bill, Craig and Roger,

Firstly, I am sorry it's taken me a while to respond to all your
suggestions...it's been busy here and I haven't been working on the radios
until this weekend.

As far as the noisy Antenna Trim on the Teledyne I haven't made any progress
on this at all. No amount of exercising the trimmer seems to be cleaning up
the scratchiness...so I think the next step is to have a look inside.
Something completely unrelated came up on this radio though...While
reassembling the front panel I noticed the Local Audio pot had been changed
to 5k. I installed a correct value NOS 2.5k and now I have the "Too loud"
audio problem that Craig had. So someone in the past had tried to fix the
issue with a larger value pot. I have yet to try substituting AF
decks...that is something I will try next time I am in the shack.

As far as the Calibrator on the EAC goes. The raspiness issue was resolved
by replacing C-313. I believe the issue with the remaining distortion is the
AGC and swapping out the IF deck with two others seemed to confirmed this,
both with the calibrator signal and listening to on-air broadcasts. With the
EAC deck back in...on a strong signal it distorts heavily with the RF gain
full up (just like it does if you switch to MGC).  If I turn the RF gain
down the signal cleans up nicely.

I feel a bit stupid re trying to Scope E-208...I thought I'd see the output
from the calibrator there but of course we are talking about micro-volt
level signals so it makes perfect sense why I wouldn't see anything. While
having another look at E-209 I discovered my probe is faulty which could
explain what I was originally seeing, if I wiggle it just right, it works.
For fun I had a look at J-221 and I could see the 17 Mhz signal from the
first crystal oscillator...nice and clean sine wave. I scoped the IF out for
the calibrator signal...a sine wave but a little blurry. I also had a look
at the IF out with a normal broadcast station and same result. Unsure of how
to interpret what I was seeing, as this is the first scope I've ever owned I
went to youtube to learn. I found the blurred waveform was just the sine
wave being modulated by the signal. So still tuned to the broadcast station
I slowed the timebase way down and could see the modulated audio, just like
I was seeing in the videos.

So...I had fun, got a little side-tracked and when I get more time to get
stuck into it again I'll go hunting for the AGC problem. Thanks for the
responses, comments and advice...as always I've learned a little more about
these radios.

Very best 73
Ken Harpur 
ZL3AA