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Old 8th Aug 2018, 5:38 pm   #61
John KC0G
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Re posts #54 and #59, John Wilson's review of the NRD-545 was in the July 1998 issue of short Wave Magazine.

And many thanks to the major contributors to this fascinating thread. It has been a real education.
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Old 8th Aug 2018, 7:04 pm   #62
Andrew2
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Thanks for the info, John. I think that one is missing from the American Radio History site.
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Old 9th Aug 2018, 1:54 am   #63
Damien VK3RX
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Quote:
Originally Posted by Radio1950 View Post
Damien,
In the interim.
Can you please have a look at the attached photo of my RX (in its original state, just a bit pulled apart), and check if your NRD515 has that same thin black earth wire and lug on the transformer mount screw?
<snip>
I only have the two pix I posted earlier that show the transformer area; the others I have are of the main board and filter area, all taken just before I sold the 515.
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Old 9th Aug 2018, 8:30 am   #64
Radio1950
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Sorry Damien,

You did say that in an earlier post, and I forgot.
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Old 12th Aug 2018, 7:53 am   #65
Radio1950
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Thanks again Jeremy for your analysis.

The Issues
I have been concentrating on three issues, the VCO spurious components around the centre frequency as seen in the demodulated 1090 Hz, the mains hum on the AF out, and the AF harmonic distortion.

By the way, the 1090 Hz is chosen to avoid possible aliasing in some Test Equipment, and to allow visual distinction on spectrum plots with any 1000Hz Ref signal from the Phase Detector and PPL/VCO. I should have used the standard 820Hz, but I am an old radar man, and old habits ...

VCO Sidebands
Without a Spectrum Analyser, nor an AF Wave Analyser, (not even an N&D Set!), I have been trying out new methods of visualising the VCO problem components, so that I can carry out live experiments and tweaks.

Plotting a new audio spectrum with Audacity is rather tiresome for the testing phase, as it is not live, and does not have sufficient frequency resolution, anyway, but is a good final guide.

I was using my Siemens D2055 Selective (ie Tuneable) Level Meter monitoring the RX IF Out at 455 KHz, and which has a 20 Hz filter. I was tuning off the centre frequency, and watching the sidebands, 15 dB down from centre, on the analogue meter.

I have also been observing the IF Out on a CRO, looking particularly at the spurious signals as showing as 1000 Hz AM.

I used IF Out, because it is before any final demodulation to AF, and should only contain VCO and mixer products, and perhaps a very little AM from the DC supplies.

The RX IF Out is useful.
I have measured its output at -3dBm75 ie in 75 ohms (abt 1.5 V pp) and measured its Zs at about 17 ohms. Deo gratia; JRC used an emitter follower here as driver.

To get an indicative value for the overall AM on the IF signal, and expressed in pseudo percent AM, I used my other 2022 sig gen at 455 KHz, and observed both sig gen and IF Out on a CRO.

Varying the 2022 AM mod level to get the same visual mod as the IF, I observe that there is about 2% AM, and, as it is a slightly distorted sine, there are other products.
There are measurement uncertainties here, especially at the bottom of the mod capabilities of the 2022, but it does provide a guide.
Reversing the 2018 and 2022 sig gen roles gave the same result.
Photo 1 shows this for RX1.

Interestingly, RX2 gives the same observed AM mod level, and with a similar mod wave shape on the IF.

No matter what I do, I cannot reduce the VCO spurious further, without more extensive mods or circuit change.
I again experimented with earth loops, and shielding, and seeing if any inductors etc were susceptible to mains effects.

Adding the extra 10M resistor across C316 in the loop filter area modifies the results, but is academic.
Removing that resistor, and adding another 4M7 capacitor, is little better.
The loop, as designed, is stable enough with no problems all through the VCO range.
I have restored my RX to standard.

All components here are original, except C83 0.47Mfd is now ceramic.
I think it is likely that circuit degradation which provokes VCO spurious will be limited to certain capacitors, and not semis, inductors nor the metal resistors. That is my experience.
I have replaced all relevant capacitors with new types.
RX2 has noticeable FM on its demod signals, as seen on a CRO.

Audio Distortion
I spent some time on the audio stages, to determine if the distortion was able to be improved, and to see if the distortion was from the VCO, the DC supplies, or from degraded components.

The Speaker Output has a very little more AM distortion (ie Harmonic Distortion) than the Line Out.
The AF Line Out looks very clean.

Unfortunately JRC has used a non-standard method to drive this output.
It is just fed from the wiper of a 5K trimpot off the AF driver output.

Its Zs is therefore variable. I found that the trimpot at 80% travel gave a useful compromise of AF level and Zs with approx -2dBm600 (approx 0.7V RMS, or 2.0 V pp) with a Zs approaching 600 ohms.

I cannot discern where the various components of the AF distortion are coming from, as the levels are too low.

I “see” on the CRO, and by experience, that the RX AF distortion on SSB is less than 4%.

I calculate the THD at 1.5% from the harmonic levels on the spectrum.

Improvement is not worthwhile. In professional coms specs (not HiFi) we normally used a 5% overall limit on RX distortion, and JRC specify less than 10%, so it is fine.
If my theory is correct, the contribution of VCO “noise” is small compared to audio non-linearities in the RX “back end” after the IF stages.

The FM component on the AF output, both Line Out and Speaker Output is now so low that only spectrum analysis reveals it.
It is not discernible on any other test equipment that I have, and certainly cannot be heard.

RX Setup for tests
10.505 MHz except as indicated, USB, BW 6KHz, Slow AGC, ANL OFF, RF Gain max, ATTN OFF, , PBT centre, Delta F OFF, VFO Int.
Signal Input level -53 dBm50, (500uV P.D.) mod off.


Recording For Analysis
I have upgraded my method of recording to WAV files.
I now use RX Line Out feeding a 600 ohm 2:1 shielded transformer (actually an old AF hybrid) and have an 8 dB resistive attenuator on the output, then feeding the MIC input of an AF to USB converter dongle into a (ASUS UX330) laptop using Audacity, and running on battery. Hopefully all this minimises unwanted signals.

Wrangler
The transformer has no bushes on the assembly screws.
Its copper strap is over the bobbin, but does not cut the flux, ie, it does not go through the core, but is wrapped over the top and bottom of the laminations.

I cannot see if it is bonded end to end, but I would think it is likely.

Also, hard to see in the photo, there is a ferrous metal strap going all around the laminations, at 90 deg to the windings, and bonded at the strap ends, on the bottom.
If I use a wire test loop fed to a CRO, there is just a little external field, and less than I would have expected.
There is no variation in the observed field as I approach the screws, but the methodology is not exact.

Jeremy,
The VCO drive swing is 5.50 V for freq xx.000KHz, 7.35 V for xx.500KHz, 11.9 V for xx.999KHz.
It swings 10 MHz from 24 to 34 MHz, then is divided by 10, as you are aware.

Some more WAV files as PDF for you, if you still have time, with details in file names. Next post due forum limitations.

Please put that Agilent analyser in your will for me please.

And some spectrum plots for everyone’s interest.

Temporary Break
I am approaching the end of this stage of my investigation.
We are going away on hols for a few weeks, away from the workshop at least.
I may have an update later, and with some maintenance tips specific to this RX, picked up along the way.


Many thanks to all contributors, especially Jeremy.
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Old 12th Aug 2018, 7:55 am   #66
Radio1950
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

WAV audio files
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Old 12th Aug 2018, 8:36 pm   #67
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Thanks I've had a quick look at the wav files and the biggest variation in PLL response appears to be between 10.995 and 10.500MHz.

There is quite a difference in loop response here and the loop BW is very narrow up at 10.995MHz as you can see in the plots below. I was expecting to see a big difference in loop response because of the huge variation in division ratio N and it looks like this is the case.

A modern PLL that uses a modern PLL chip can compensate for this because the charge pump current can be changed via the programming interface as the frequency is changed across the range. But back in the days of the NRD515 it wasn't so easy. Back in the early 1990s I helped design and develop a high performance (external) charge pump circuit for the Motorola MC145158-2 PLL chip. This used several BJTs and had very low noise, could run up at 28V and gave very low reference sidebands. At the time it was state of the art and completely transformed our PLL designs at work that used this chip.

There were other PLL chips we tried in that era such as the Plessey NJ882x series and these use two phase detectors. Inside these chips there was a classic digital PFD which was used as a coarse phase detector and this was disabled automatically once the chip neared phase lock. It then switched in a fine 'sample/hold' phase detector that was claimed to cure the issues with the dead zone and it gave much lower reference sidebands. But our own charge pump circuit total outclassed it in terms of lock time and phase noise and sideband suppression. It really was a bit special at the time and gave us an edge over our competitors.

However, the modern PLL chips have a similar circuit inside these days and the current is programmable. My old pump circuit was really good but it could only run at one current setting.

It looks like the 50Hz sidebands are much lower now on your NRD515 and if it sounds OK to the human ear then there probably isn't much point trying to polish it any further.

For the stuff I was designing the receiver had to be able to detect tiny signals next to really big ones so phase noise was critical, even very close to the carrier. The operator was an automated DSP system so it could detect stuff really close to a carrier in a tiny resolution bandwidth. It wasn't limited to the limits of the human ear so it was often looking >80dB down and I had to try and design for -80dBc reference sidebands on each synthesiser back in those days. We really did have to push the limits of the available technology at the time to achieve this.
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Old 8th Sep 2018, 1:53 am   #68
Radio1950
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

I have carried out some more investigations, and especially on the second receiver.
To recap, both NRD515 receivers have some FM on demodulated SSB signals, due to 50 Hz effects mainly on the 24 MHz PLL VCO.
It is not noticeable in normal reception operation.
Receiver 1 was affected worse than receiver 2.

The cause of this effect is leakage 50 Hz from the mains transformer assembly conducted into the earth system.

I have minimised the effect on both receivers to a very large degree.

The effect is repeatable.

It is related to small 50 Hz currents flowing from the transformer assembly via the mount screws into the chassis area.
The voltage is very low and is difficult to measure with certainty.

The effect can be completely eliminated by electrically insulating the transformer from the chassis, eg by just using a plastic plate and temporary cable ties for trial.

A permanent fix probably involves using an additional insulating plate under the transformer, enlarging the screw holes, and using bushes or non-metallic fasteners.

I didn’t want to do this on a heritage receiver.

Nylon screws could be used, but I don’t trust them to hold such a large mass long term, and I don’t want a transformer loose in the receiver if they break.

A very acceptable fix therefore is to relocate any cable lugs, and have tight mount screws.

Two possible original cable lugs under the rear mount screws may be found in receivers.
One is the DC PS earth lead, and the other is the shield from the AC feed to the S Meter lamp.
I did notice that these lugs seem to introduce a little extra FM, probably via the very slightly increased magnetic or electric path due to the lug itself.

Also, I noticed that the effect is increased if the mount screws were only loosely tightened.

The head of one of the transformer mount screws interferes with the bottom screen of the 70 MHz oscillator.
A rather disappointing find.
Receiver 2 FM was markedly affected by this, and more than Receiver 1.

Only one screw is implicated, the others are clear.

Fix this by removing the bottom PC board, remove the screw, and remove the captive spring washer from the screw. Reassemble with modified screw.
Photos assist location of this screw.


Steps to take to minimise this FM effect

Remove all earth cables and lugs from any of the mains transformer mount screws and relocate to eg the back panel.
Tighten the four transformer mount screws quite firmly. Use the original threaded plate, not new individual M4 nuts.
Check that all PCB mount screws are firm, including the Power Supply.
Fix the mains transformer mount screw touching the 70 MHz oscillator bottom screen.

Thanks to everyone who assisted.
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Old 8th Sep 2018, 12:20 pm   #69
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Quote:
Also, I noticed that the effect is increased if the mount screws were only loosely tightened.
I wonder if some of the effect could also be due to mechanical (50Hz) vibration in the transformer that causes microphony in the VCO? Sometimes it only take a tiny amount of 50Hz vibration from a transformer to make its way into a VCO. At work I used to test all my VCO designs on a vibrating table. It could be driven with various waveforms to replicate vibration in vehicles or from shocks etc.

I also had another look at your earlier VCO tuning data and 11.9V Vtune at xx.999MHz seems very close to the 12V supply limit. Maybe the VCO slug needs trimming slightly to give some margin here. When it gets this close to the supply voltage it can affect the loop response. It should tell you in the service manual what voltage is needed here and I think it would be best to adjust it. Obviously, take care not to crack the tuning slug as it could be very brittle. I would use a plastic trim tool here and not a hard tool.

Obviously, you have to make sure it doesn't get too close to the 3V Vtune clamp set by the zener diode when it is at the other end of the range. As I said before, I think the zener diode by the TC4016 PD switch IC is there for startup reliability. If the Vtune voltage could swing very low at startup I think the VCO frequency could get low enough to produce a (38MHz-VCO) mixer term that would be too high to get through the post mixer LPF in the PLL. This is the LPF shown in blue in the amended block diagram below. If this happens, the loop would be broken with no feedback signal to correct itself. So I think the clamp diode is there to provide a hard limit for how low the VCO can tune when starting up. This would ensure that the (38MHz-VCO) mixer term would always fall inside the range of the LPF.
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Old 11th Sep 2018, 6:13 am   #70
Radio1950
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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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Old 11th Sep 2018, 6:23 am   #71
Radio1950
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Some minor hints and suggestions for service on the NRD515.

Read the information on Walter G website. It has User and Service Manuals, photos, mods, hints and much other data, including some common problems.
https://people.zeelandnet.nl/wgeeraert/index515.htm

Many thanks to Walter.

Service Hints

Some of this is basic; ignore those sections as required.
Check the mains protective earth is sound, as this receiver has some interesting mains paths on PCBs.

All screws are either M3 or M4, with JIS head; use a Pozidriv #1 driver for M3, and a Phillips #2 for M4.
The knob retaining screws are M2.0 Allen.
A small handled 5.5mm AF nutdriver is useful for M3 nuts.

Check all PCB retaining screws are firm, and again after board removal. If you remove a board, check that the M3 standoffs are firm to chassis, with a 6 mm AF nutdriver.

Use a 10X CRO probe for monitoring signal levels or frequency.

Check all internal coax connectors for shield crimp integrity. Use a rotational rocking removal technique, no more than 30 degrees twist in one direction.
Consider using a wipe of the bare coax socket base with a small amount of lanoline or similar on a cotton bud, then wiping off.

The small Sumida transformers and tuneable inductors on the bottom “Oscillator” PCB are rare types and irreplaceable.
The ferrite slugs may be firm or stuck.
The top of the ferrite slugs is very, very fragile and cracks easily.
I use a 0.5ml hypodermic syringe with 0.25 mm needle diameter to carefully place micro-drops of silicone (ex spray can) at the top of the ferrite where it touches the coil former.
This may ease up the slug.
Use only if required, and use very sparingly.
You will need a magnifier of some kind to accurately place the drops.
Do not use anything which might affect the coil former.

You need an exact size tuning tool for the smaller ferrite slugs or you will crack the ferrite.
It should be new, without the inevitable furry edges of years of use.
This tool is best made from nylon etc to have some flexibility; do not use metal.

I have found a very good tool in the “Proskit Trimming Tool Set”, Part No1PK-A001, tool 0.8x1.3 mm on 3mm dia rod.

Gently attempt to rotate the slug, then rock it back and forth a little bit, to ease it up (and to distribute the silicone if used).
If the slug cannot be moved, I suggest leaving it alone, as it probably is on tune anyway, and you have little to gain, and a lot to lose if the ferrite cracks.
Some other owners have used alternative transformers, rewound for this receiver.

The Service Manual is good for general service and re-alignment.
I found no typos nor errors of adjustment flow and sequence.

Suggested additional text to reduce alignment confusion –
Page 21 – step b (2) add “on RX Unit” after TP18.
Page 21 – step c (1) add “on RX Unit” after TP12.
Page 22 – step d (2) add “on RX Unit” after TP7.

When you are satisfied with general receiver realignment, I suggest you finally adjust the various pots for demodulated carrier so that you get 1000 Hz on a frequency counter connected to Line or Spkr Output.

Your Digital Sig Gen and Frequency Counter are likely to be more accurate and stable than this receiver.

As you go through the adjustment of the various receiver oscillators, you will be a few Hz out here and there, and it either accumulates, or averages out.

Make your last adjustment of RV5 USB, RV6 LSB, RV7 RTTY, RV8 CW, for 1000 Hz with an injected signal of e.g. Fc +1KHz, or as required.
See Table 2 of the Service Manual on page 13 for the initial adjustment frequencies.

This will average out all slight errors, and should make your receiver accurate within 10 Hz of displayed dial frequency.
.
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