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

[Ken Harpur]

Hi Craig,

I'll get into it in the weekend...I'll connect a speaker to the line output as you say and see what happens. I know that in your case it is neither the IF or Audio Decks causing the loud audio in your radio...but I think I will try swapping them out anyway, just in case the fault with mine is different to yours. Who knows, I might get lucky and find it is in the audio deck (!). I will also check the resistances of the pots just to make sure they are as advertised.

It was interesting to read your account of all the bugs your radio had...you've obviously done a lot of work to get them sorted out. I'm grateful mine are a bit more basic. They all have issues but nothing preventing them from operating. I know that the 2uF AGC caps in three are bad...I rebuilt one using 1.5uF (I made it up by paralleling three 0.47uF...a more permanent solution is pending) and was amazed at the difference it made. One has a couple of bad crystals in the 2nd Oscillator Deck, one has the bad Ant. Trimmer, one has a bad mica somewhere, evidenced by a jump or decrease in gain when I plug something in to the same circuit the radio is plugged into. The EAC has bad AGC...etc
So I have plenty to keep myself occupied! I'm looking forward to the day when I get them all performing like new again...

Also very interesting to read Roger's account of making bugs for the students to find...I would have loved to have been there (I wasn't even born when these radios were made). Interesting to read about painting up 1Meg resistors to look like lower values. 


Kind regards,
Ken 
ZL3AA


On 21 Jul 2014, at 2:00 pm, Craig Heaton <hamfish at efn.org> wrote:

> 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
> 
>