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

[Ken Harpur]

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