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Old 11th Sep 2018, 6:13 am   #70
Radio1950
Hexode
 
Join Date: Apr 2017
Location: Buderim, Queensland, Australia.
Posts: 428
Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

That is quite an interesting suggestion Jeremy.
I did look at this possibility very early in the process, and dismissed it after checking for vibration, but to be absolutely certain, I did some more tests last weekend.

The receivers now have so small a degree of FM, that it is impossible to see any FM, or indeed any more improvement, on a CRO, so I reverted to using Audacity Frequency Plots of the demodulated signal.

I made a reference recording and plot.

I then removed the transformer 25 mm away from the chassis and repeated the test.

I then used 12 mm of soft material to make a platform base for the transformer in its original position, taking care to avoid any touching, and repeated the test.

No real improvement was noticed in either case.

The oscillator section around the 24 MHz VCO is very slightly microphonic, but not abnormally so.

I don’t consider 50 Hz vibrations to be a contributor to the problem.

I consider further improvement is not possible, nor worthwhile.

The academic trip has been interesting.


VCO Voltages in Service Manual

The 24 MHz VCO ( the 0 to 1 MHz tuning oscillator for each 1 MHz range) is adjusted with L28 (the VCO coil) to be 10.5 V at TP9 (the drive to the tuning diodes) at top of range, ie xx.999MHz.
The manual then says that the VCO voltage at range bottom ie xx.000MHz should be more than 3.0V.
In a previous investigation, I did offset L28 to see if the FM was reduced, but it was not affected.

VCO Startup and Running

Agree.
And ...
I was conscious of the fact that this was an early design, and made for a particular customer range, and at a price.
Over the time of all my FM investigation, I was looking for any arrangement of startup, tuning, range change, mode change, which might give tuning glitches. I have found none. The only thing which irritates some owners seems to be the tuning increment which is 100 Hz, and is relatively large compared to more modern receivers, or more professional contemporary receivers.

The receiver has a very good tuning “feel”, and it is better than my Icom 7600 in this regard.

Tuning Tools

Agree – see my hints post.
The situation with the tiny Sumida ferrite cores is that the indentation for the tool is relatively small in all axes, and this exacerbates the risk of cracking, especially when inappropriate tools are used.

Plot and WAV file

These are of the present situation, and Receiver 2 is almost identical.
One last recording and plot at mid tuning of 10.500 MHz.

As before, the pdf file is actually a 6 second WAV file ex Audacity. Rename it to view.
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