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

[Ken Harpur]

Hi Larry and Bill,

Thanks for the suggestions...

First the trimmer...I did an ohms check from the gear to ground and the meter started at around 500k then climbed to around 1100k. That was not moving or touching anything else, just holding the two probes still until everything stabilised. I haven't yet checked AGC voltage on the top of the Cap via the gears as this particular receiver is semi-disassembled undergoing a thorough clean. By next weekend it should be all back together...

Regarding the Calibrator...I found in the circuit diagram C-313 which goes from the plate of V-205A to the grids of V-206 via 22k resistors. That was my first suspect, I don't have any 220pf so I made one by temporarily connecting two 110pf in parallel. This has made a slight improvement so I will order some suitable replacements and do a permanent repair. 
Now...I unpacked my Scope and fired it up...please bear with me regarding using a scope because it's a recent purchase and I'm still learning how to use it.  Anyway...these checks resulted in a clean sine wave output...the PTO, E-402, E-211 and E-210. However checking E-209 I see a really ugly waveform, I don't know how to describe it but there are two signals there and both waveforms look like they are out of phase with one another? Not at all looking like a sine wave, kind of really dirty looking. At E-208 I cannot get a trace whatsoever...nothing I adjust on the scope or receiver will either produce, or show a waveform at this test point. Yet the calibrator does work. I see that E-209 is in the AGC line and could be a clue why I am getting distortion on strong signals, but that is an issue for another time. 

So this takes me back to why I am seeing nothing at E-208 at all. This particular radio does have a number of small issues that I'm slowly working through. Replacing C-313 has helped somewhat, the raspiness has gone and what I'm hearing now is slight distortion on the calibrator signal. The radio does distort on strong signals as said earlier, so I'm wondering if work on the AGC would help. 


Thanks once again and kind regards,
Ken Harpur


On 6 Jul 2014, at 1:46 pm, Larry H <dinlarh at att.net> wrote:

> Hi Ken,  
> 
> On the trimmer, the gear mounted (pinned) on the top of the trimmer cap is at agc potential.  Check to see that it is and the mating gear on the horizontal 6" shaft is also (it should be isolated from the 6" shaft).  You should measure about 420k ohms from both gears to ground.
> 
> On the cal osc, Bill is probably right on the micas.  You might have a bad pin contact on one of the two tubes - try wiggling them quite a bit and see if that helps.  If not, check the voltages.  The cathode follower should smooth off the corners of the square waves from the multivibrator a little via L210.  Can you scope the cal output at E208?
> 
> Good luck.  Regards, Larry
> 
> 
> On Saturday, July 5, 2014 6:11 PM, Bill Riches <bill.riches at verizon.net> wrote:
> 
> 
> In the cal crystal circuit there are a few low value mica caps that become
> leaky and make a racket.  Look for the ones going from plate to grid of the
> osc/mult tube.  I do not have a print in front of me.  Suggest you change
> the 3 or 4 that are there.  An ohm meter will not measure the leakage but
> when you put voltage on the cap you can see the leakage.  You have to remove
> the rf deck to do this.  Yippee.
> 
> 73,
> 
> Bill, WA2DVU
> Cape May, NJ
> 
> My second issue is on an EAC model. The Calibrator is very raspy (dirty)
> sounding, not the nicer, more sine-wave like sound from the Teledyne. Work
> in this area has been limited to visual inspection and re-doing potential
> cold solder joints. The crystal oven has been swapped over from the Teledyne
> with no difference...still raspy and the EAC crystal oven put in the
> Teledyne and the note is clean, so I'm sure it is not the Crystal oven.
> V-205 and V-206 have been replaced...nope, still raspy. Of note is when I
> wiggle V-205 the pitch of the note changes slightly (this does not happen on
> the Teledyne) so I re-soldered all tube socket pins...no change. I suspect I
> have a bad mica in this area. All resistors are within spec and so far no
> components have been replaced in this deck.
> 
> Any ideas or advice would be much appreciated!
> 
> Kind regards,
> Ken Harpur
> ZL3AA
> 
> 
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